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This page lists diagnostic messages produced by the Dart analyzer, with details about what those messages mean and how you can fix your code. For more information about the analyzer, see Customizing static analysis.
This page uses the following terms:
A constant context is a region of code in which it isn‘t necessary to include the const keyword because it’s implied by the fact that everything in that region is required to be a constant. The following locations are constant contexts:
Everything inside a list, map or set literal that's prefixed by the const keyword. Example:
var l = const [/*constant context*/];
The arguments inside an invocation of a constant constructor. Example:
var p = const Point(/*constant context*/);
The initializer for a variable that's prefixed by the const keyword. Example:
const v = /*constant context*/;
Annotations
The expression in a case clause. Example:
void f(int e) { switch (e) { case /*constant context*/: break; } }
Definite assignment analysis is the process of determining, for each local variable at each point in the code, which of the following is true:
Definite assignment analysis helps find problems in code, such as places where a variable that might not have been assigned a value is being referenced, or places where a variable that can only be assigned a value one time is being assigned after it might already have been assigned a value.
For example, in the following code the variable s is definitely unassigned when it’s passed as an argument to print:
void f() { String s; print(s); }
But in the following code, the variable s is definitely assigned:
void f(String name) { String s = 'Hello $name!'; print(s); }
Definite assignment analysis can even tell whether a variable is definitely assigned (or unassigned) when there are multiple possible execution paths. In the following code the print function is called if execution goes through either the true or the false branch of the if statement, but because s is assigned no matter which branch is taken, it’s definitely assigned before it’s passed to print:
void f(String name, bool casual) { String s; if (casual) { s = 'Hi $name!'; } else { s = 'Hello $name!'; } print(s); }
In flow analysis, the end of the if statement is referred to as a join—a place where two or more execution paths merge back together. Where there's a join, the analysis says that a variable is definitely assigned if it’s definitely assigned along all of the paths that are merging, and definitely unassigned if it’s definitely unassigned along all of the paths.
Sometimes a variable is assigned a value on one path but not on another, in which case the variable might or might not have been assigned a value. In the following example, the true branch of the if statement might or might not be executed, so the variable might or might be assigned a value:
void f(String name, bool casual) { String s; if (casual) { s = 'Hi $name!'; } print(s); }
The same is true if there is a false branch that doesn’t assign a value to s.
The analysis of loops is a little more complicated, but it follows the same basic reasoning. For example, the condition in a while loop is always executed, but the body might or might not be. So just like an if statement, there's a join at the end of the while statement between the path in which the condition is true and the path in which the condition is false.
For additional details, see the specification of definite assignment.
A mixin application is the class created when a mixin is applied to a class. For example, consider the following declarations:
class A {} mixin M {} class B extends A with M {}
The class B is a subclass of the mixin application of M to A, sometimes nomenclated as A+M. The class A+M is a subclass of A and has members that are copied from M.
You can give an actual name to a mixin application by defining it as:
class A {} mixin M {} class A_M = A with M;
Given this declaration of A_M, the following declaration of B is equivalent to the declaration of B in the original example:
class B extends A_M {}
Override inference is the process by which any missing types in a method declaration are inferred based on the corresponding types from the method or methods that it overrides.
If a candidate method (the method that's missing type information) overrides a single inherited method, then the corresponding types from the overridden method are inferred. For example, consider the following code:
class A { int m(String s) => 0; } class B extends A { @override m(s) => 1; }
The declaration of m in B is a candidate because it's missing both the return type and the parameter type. Because it overrides a single method (the method m in A), the types from the overridden method will be used to infer the missing types and it will be as if the method in B had been declared as int m(String s) => 1;.
If a candidate method overrides multiple methods, and the function type one of those overridden methods, Ms, is a supertype of the function types of all of the other overridden methods, then Ms is used to infer the missing types. For example, consider the following code:
class A { int m(num n) => 0; } class B { num m(int i) => 0; } class C implements A, B { @override m(n) => 1; }
The declaration of m in C is a candidate for override inference because it's missing both the return type and the parameter type. It overrides both m in A and m in B, so we need to choose one of them from which the missing types can be inferred. But because the function type of m in A (int Function(num)) is a supertype of the function type of m in B (num Function(int)), the function in A is used to infer the missing types. The result is the same as declaring the method in C as int m(num n) => 1;.
It is an error if none of the overridden methods has a function type that is a supertype of all the other overridden methods.
A part file is a Dart source file that contains a part of directive.
A type is potentially non-nullable if it‘s either explicitly non-nullable or if it’s a type parameter.
A type is explicitly non-nullable if it is a type name that isn‘t followed by a question mark. Note that there are a few types that are always nullable, such as Null and dynamic, and that FutureOr is only non-nullable if it isn’t followed by a question mark and the type argument is non-nullable (such as FutureOr<String>).
Type parameters are potentially non-nullable because the actual runtime type (the type specified as a type argument) might be non-nullable. For example, given a declaration of class C<T> {}, the type C could be used with a non-nullable type argument as in C<int>.
A public library is a library that is located inside the package's lib directory but not inside the lib/src directory.
The analyzer produces the following diagnostics for code that doesn't conform to the language specification or that might work in unexpected ways.
Abstract fields can't have initializers.
The analyzer produces this diagnostic when a field that has the abstract modifier also has an initializer.
The following code produces this diagnostic because f is marked as abstract and has an initializer:
{% prettify dart tag=pre+code %} abstract class C { abstract int [!f!] = 0; } {% endprettify %}
The following code produces this diagnostic because f is marked as abstract and there's an initializer in the constructor:
{% prettify dart tag=pre+code %} abstract class C { abstract int f;
C() : [!f!] = 0; } {% endprettify %}
If the field must be abstract, then remove the initializer:
{% prettify dart tag=pre+code %} abstract class C { abstract int f; } {% endprettify %}
If the field isn't required to be abstract, then remove the keyword:
{% prettify dart tag=pre+code %} abstract class C { int f = 0; } {% endprettify %}
The {0} ‘{1}’ is always abstract in the supertype.
The analyzer produces this diagnostic when an inherited member is referenced using super, but there is no concrete implementation of the member in the superclass chain. Abstract members can't be invoked.
The following code produces this diagnostic because B doesn't inherit a concrete implementation of a:
{% prettify dart tag=pre+code %} abstract class A { int get a; } class B extends A { int get a => super.[!a!]; } {% endprettify %}
Remove the invocation of the abstract member, possibly replacing it with an invocation of a concrete member.
The name ‘{0}’ is defined in the libraries ‘{1}’ and ‘{2}’.
The analyzer produces this diagnostic when two or more export directives cause the same name to be exported from multiple libraries.
Given a file named a.dart containing
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
And a file named b.dart containing
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
The following code produces this diagnostic because the name C is being exported from both a.dart and b.dart:
{% prettify dart tag=pre+code %} export ‘a.dart’; export [!‘b.dart’!]; {% endprettify %}
If none of the names in one of the libraries needs to be exported, then remove the unnecessary export directives:
{% prettify dart tag=pre+code %} export ‘a.dart’; {% endprettify %}
If all of the export directives are needed, then hide the name in all except one of the directives:
{% prettify dart tag=pre+code %} export ‘a.dart’; export ‘b.dart’ hide C; {% endprettify %}
A member named ‘{0}’ is defined in {1}, and none are more specific.
When code refers to a member of an object (for example, o.m() or o.m or o[i]) where the static type of o doesn‘t declare the member (m or [], for example), then the analyzer tries to find the member in an extension. For example, if the member is m, then the analyzer looks for extensions that declare a member named m and have an extended type that the static type of o can be assigned to. When there’s more than one such extension in scope, the extension whose extended type is most specific is selected.
The analyzer produces this diagnostic when none of the extensions has an extended type that's more specific than the extended types of all of the other extensions, making the reference to the member ambiguous.
The following code produces this diagnostic because there's no way to choose between the member in E1 and the member in E2:
{% prettify dart tag=pre+code %} extension E1 on String { int get charCount => 1; }
extension E2 on String { int get charCount => 2; }
void f(String s) { print(s.[!charCount!]); } {% endprettify %}
If you don't need both extensions, then you can delete or hide one of them.
If you need both, then explicitly select the one you want to use by using an extension override:
{% prettify dart tag=pre+code %} extension E1 on String { int get charCount => length; }
extension E2 on String { int get charCount => length; }
void f(String s) { print(E2(s).charCount); } {% endprettify %}
The name ‘{0}’ is defined in the libraries {1}.
The analyzer produces this diagnostic when a name is referenced that is declared in two or more imported libraries.
Given a library (a.dart) that defines a class (C in this example):
{% prettify dart tag=pre+code %} class A {} class C {} {% endprettify %}
And a library (b.dart) that defines a different class with the same name:
{% prettify dart tag=pre+code %} class B {} class C {} {% endprettify %}
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} import ‘a.dart’; import ‘b.dart’;
void f([!C!] c1, [!C!] c2) {} {% endprettify %}
If any of the libraries aren't needed, then remove the import directives for them:
{% prettify dart tag=pre+code %} import ‘a.dart’;
void f(C c1, C c2) {} {% endprettify %}
If the name is still defined by more than one library, then add a hide clause to the import directives for all except one library:
{% prettify dart tag=pre+code %} import ‘a.dart’ hide C; import ‘b.dart’;
void f(C c1, C c2) {} {% endprettify %}
If you must be able to reference more than one of these types, then add a prefix to each of the import directives, and qualify the references with the appropriate prefix:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a; import ‘b.dart’ as b;
void f(a.C c1, b.C c2) {} {% endprettify %}
The literal can't be either a map or a set because it contains at least one literal map entry or a spread operator spreading a ‘Map’, and at least one element which is neither of these.
Because map and set literals use the same delimiters ({ and }), the analyzer looks at the type arguments and the elements to determine which kind of literal you meant. When there are no type arguments, then the analyzer uses the types of the elements. If all of the elements are literal map entries and all of the spread operators are spreading a Map then it‘s a Map. If none of the elements are literal map entries and all of the spread operators are spreading an Iterable, then it’s a Set. If neither of those is true then it's ambiguous.
The analyzer produces this diagnostic when at least one element is a literal map entry or a spread operator spreading a Map, and at least one element is neither of these, making it impossible for the analyzer to determine whether you are writing a map literal or a set literal.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} union(Map<String, String> a, List b, Map<String, String> c) => [!{...a, ...b, ...c}!]; {% endprettify %}
The list b can only be spread into a set, and the maps a and c can only be spread into a map, and the literal can't be both.
There are two common ways to fix this problem. The first is to remove all of the spread elements of one kind or another, so that the elements are consistent. In this case, that likely means removing the list and deciding what to do about the now unused parameter:
{% prettify dart tag=pre+code %} union(Map<String, String> a, List b, Map<String, String> c) => {...a, ...c}; {% endprettify %}
The second fix is to change the elements of one kind into elements that are consistent with the other elements. For example, you can add the elements of the list as keys that map to themselves:
{% prettify dart tag=pre+code %} union(Map<String, String> a, List b, Map<String, String> c) => {...a, for (String s in b) s: s, ...c}; {% endprettify %}
This literal must be either a map or a set, but the elements don't have enough information for type inference to work.
Because map and set literals use the same delimiters ({ and }), the analyzer looks at the type arguments and the elements to determine which kind of literal you meant. When there are no type arguments and all of the elements are spread elements (which are allowed in both kinds of literals) then the analyzer uses the types of the expressions that are being spread. If all of the expressions have the type Iterable, then it‘s a set literal; if they all have the type Map, then it’s a map literal.
This diagnostic is produced when none of the expressions being spread have a type that allows the analyzer to decide whether you were writing a map literal or a set literal.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} union(a, b) => [!{...a, ...b}!]; {% endprettify %}
The problem occurs because there are no type arguments, and there is no information about the type of either a or b.
There are three common ways to fix this problem. The first is to add type arguments to the literal. For example, if the literal is intended to be a map literal, you might write something like this:
{% prettify dart tag=pre+code %} union(a, b) => <String, String>{...a, ...b}; {% endprettify %}
The second fix is to add type information so that the expressions have either the type Iterable or the type Map. You can add an explicit cast or, in this case, add types to the declarations of the two parameters:
{% prettify dart tag=pre+code %} union(List a, List b) => {...a, ...b}; {% endprettify %}
The third fix is to add context information. In this case, that means adding a return type to the function:
{% prettify dart tag=pre+code %} Set union(a, b) => {...a, ...b}; {% endprettify %}
In other cases, you might add a type somewhere else. For example, say the original code looks like this:
{% prettify dart tag=pre+code %} union(a, b) { var x = [!{...a, ...b}!]; return x; } {% endprettify %}
You might add a type annotation on x, like this:
{% prettify dart tag=pre+code %} union(a, b) { Map<String, String> x = {...a, ...b}; return x; } {% endprettify %}
Fields in a struct class whose type is ‘Pointer’ shouldn't have any annotations.
The analyzer produces this diagnostic when a field that's declared in a subclass of Struct and has the type Pointer also has an annotation associated with it.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field p, which has the type Pointer and is declared in a subclass of Struct, has the annotation @Double():
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { [!@Double()!] external Pointer p; } {% endprettify %}
Remove the annotations from the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { external Pointer p; } {% endprettify %}
Argument ‘{0}’ must be a constant.
The analyzer produces this diagnostic when an invocation of either Pointer.asFunction or DynamicLibrary.lookupFunction has an isLeaf argument whose value isn't a constant expression.
The analyzer also produces this diagnostic when the value of the exceptionalReturn argument of Pointer.fromFunction.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the value of the isLeaf argument is a parameter, and hence isn't a constant:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
int Function(int) fromPointer( Pointer<NativeFunction<Int8 Function(Int8)>> p, bool isLeaf) { return p.asFunction(isLeaf: [!isLeaf!]); } {% endprettify %}
If there's a suitable constant that can be used, then replace the argument with a constant:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
const isLeaf = false;
int Function(int) fromPointer(Pointer<NativeFunction<Int8 Function(Int8)>> p) { return p.asFunction(isLeaf: isLeaf); } {% endprettify %}
If there isn't a suitable constant, then replace the argument with a boolean literal:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
int Function(int) fromPointer(Pointer<NativeFunction<Int8 Function(Int8)>> p) { return p.asFunction(isLeaf: true); } {% endprettify %}
The argument type ‘{0}’ can't be assigned to the parameter type ‘{1}’.
The analyzer produces this diagnostic when the static type of an argument can't be assigned to the static type of the corresponding parameter.
The following code produces this diagnostic because a num can't be assigned to a String:
{% prettify dart tag=pre+code %} String f(String x) => x; String g(num y) => f([!y!]); {% endprettify %}
If possible, rewrite the code so that the static type is assignable. In the example above you might be able to change the type of the parameter y:
{% prettify dart tag=pre+code %} String f(String x) => x; String g(String y) => f(y); {% endprettify %}
If that fix isn‘t possible, then add code to handle the case where the argument value isn’t the required type. One approach is to coerce other types to the required type:
{% prettify dart tag=pre+code %} String f(String x) => x; String g(num y) => f(y.toString()); {% endprettify %}
Another approach is to add explicit type tests and fallback code:
{% prettify dart tag=pre+code %} String f(String x) => x; String g(num y) => f(y is String ? y : ''); {% endprettify %}
If you believe that the runtime type of the argument will always be the same as the static type of the parameter, and you‘re willing to risk having an exception thrown at runtime if you’re wrong, then add an explicit cast:
{% prettify dart tag=pre+code %} String f(String x) => x; String g(num y) => f(y as String); {% endprettify %}
The argument type ‘{0}’ can't be assigned to the parameter type ‘{1} Function(Object)’ or ‘{1} Function(Object, StackTrace)’.
The analyzer produces this diagnostic when an invocation of Future.catchError has an argument that is a function whose parameters aren‘t compatible with the arguments that will be passed to the function when it’s invoked. The static type of the first argument to catchError is just Function, even though the function that is passed in is expected to have either a single parameter of type Object or two parameters of type Object and StackTrace.
The following code produces this diagnostic because the closure being passed to catchError doesn't take any parameters, but the function is required to take at least one parameter:
{% prettify dart tag=pre+code %} void f(Future f) { f.catchError([!() => 0!]); } {% endprettify %}
The following code produces this diagnostic because the closure being passed to catchError takes three parameters, but it can't have more than two required parameters:
{% prettify dart tag=pre+code %} void f(Future f) { f.catchError([!(one, two, three) => 0!]); } {% endprettify %}
The following code produces this diagnostic because even though the closure being passed to catchError takes one parameter, the closure doesn't have a type that is compatible with Object:
{% prettify dart tag=pre+code %} void f(Future f) { f.catchError([!(String error) => 0!]); } {% endprettify %}
Change the function being passed to catchError so that it has either one or two required parameters, and the parameters have the required types:
{% prettify dart tag=pre+code %} void f(Future f) { f.catchError((Object error) => 0); } {% endprettify %}
A redirecting constructor can't have an ‘assert’ initializer.
The analyzer produces this diagnostic when a redirecting constructor (a constructor that redirects to another constructor in the same class) has an assert in the initializer list.
The following code produces this diagnostic because the unnamed constructor is a redirecting constructor and also has an assert in the initializer list:
{% prettify dart tag=pre+code %} class C { C(int x) : [!assert(x > 0)!], this.name(); C.name() {} } {% endprettify %}
If the assert isn't needed, then remove it:
{% prettify dart tag=pre+code %} class C { C(int x) : this.name(); C.name() {} } {% endprettify %}
If the assert is needed, then convert the constructor into a factory constructor:
{% prettify dart tag=pre+code %} class C { factory C(int x) { assert(x > 0); return C.name(); } C.name() {} } {% endprettify %}
The asset directory ‘{0}’ doesn't exist.
The analyzer produces this diagnostic when an asset list contains a value referencing a directory that doesn't exist.
Assuming that the directory assets doesn‘t exist, the following code produces this diagnostic because it’s listed as a directory containing assets:
name: example flutter: assets: - assets/
If the path is correct, then create a directory at that path.
If the path isn't correct, then change the path to match the path of the directory containing the assets.
The asset file ‘{0}’ doesn't exist.
The analyzer produces this diagnostic when an asset list contains a value referencing a file that doesn't exist.
Assuming that the file doesNotExist.gif doesn‘t exist, the following code produces this diagnostic because it’s listed as an asset:
name: example flutter: assets: - doesNotExist.gif
If the path is correct, then create a file at that path.
If the path isn't correct, then change the path to match the path of the file containing the asset.
The value of the ‘asset’ field is expected to be a list of relative file paths.
The analyzer produces this diagnostic when the value of the asset key isn't a list.
The following code produces this diagnostic because the value of the assets key is a string when a list is expected:
name: example flutter: assets: assets/
Change the value of the asset list so that it's a list:
name: example flutter: assets: - assets/
Assets are required to be file paths (strings).
The analyzer produces this diagnostic when an asset list contains a value that isn't a string.
The following code produces this diagnostic because the asset list contains a map:
name: example flutter: assets: - image.gif: true
Change the asset list so that it only contains valid POSIX-style file paths:
name: example flutter: assets: - image.gif
‘{0}’ is marked ‘doNotStore’ and shouldn't be assigned to a field or top-level variable.
The analyzer produces this diagnostic when the value of a function (including methods and getters) that is explicitly or implicitly marked by the [doNotStore][meta-doNotStore] annotation is stored in either a field or top-level variable.
The following code produces this diagnostic because the value of the function f is being stored in the top-level variable x:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@doNotStore int f() => 1;
var x = [!f()!]; {% endprettify %}
Replace references to the field or variable with invocations of the function producing the value.
Constant variables can't be assigned a value.
The analyzer produces this diagnostic when it finds an assignment to a top-level variable, a static field, or a local variable that has the const modifier. The value of a compile-time constant can't be changed at runtime.
The following code produces this diagnostic because c is being assigned a value even though it has the const modifier:
{% prettify dart tag=pre+code %} const c = 0;
void f() { [!c!] = 1; print(c); } {% endprettify %}
If the variable must be assignable, then remove the const modifier:
{% prettify dart tag=pre+code %} var c = 0;
void f() { c = 1; print(c); } {% endprettify %}
If the constant shouldn't be changed, then either remove the assignment or use a local variable in place of references to the constant:
{% prettify dart tag=pre+code %} const c = 0;
void f() { var v = 1; print(v); } {% endprettify %}
‘{0}’ can‘t be used as a setter because it’s final.
The analyzer produces this diagnostic when it finds an invocation of a setter, but there's no setter because the field with the same name was declared to be final or const.
The following code produces this diagnostic because v is final:
{% prettify dart tag=pre+code %} class C { final v = 0; }
f(C c) { c.[!v!] = 1; } {% endprettify %}
If you need to be able to set the value of the field, then remove the modifier final from the field:
{% prettify dart tag=pre+code %} class C { int v = 0; }
f(C c) { c.v = 1; } {% endprettify %}
The final variable ‘{0}’ can only be set once.
The analyzer produces this diagnostic when a local variable that was declared to be final is assigned after it was initialized.
The following code produces this diagnostic because x is final, so it can't have a value assigned to it after it was initialized:
{% prettify dart tag=pre+code %} void f() { final x = 0; [!x!] = 3; print(x); } {% endprettify %}
Remove the keyword final, and replace it with var if there's no type annotation:
{% prettify dart tag=pre+code %} void f() { var x = 0; x = 3; print(x); } {% endprettify %}
There isn’t a setter named ‘{0}’ in class ‘{1}’.
The analyzer produces this diagnostic when a reference to a setter is found; there is no setter defined for the type; but there is a getter defined with the same name.
The following code produces this diagnostic because there is no setter named x in C, but there is a getter named x:
{% prettify dart tag=pre+code %} class C { int get x => 0; set y(int p) {} }
void f(C c) { c.[!x!] = 1; } {% endprettify %}
If you want to invoke an existing setter, then correct the name:
{% prettify dart tag=pre+code %} class C { int get x => 0; set y(int p) {} }
void f(C c) { c.y = 1; } {% endprettify %}
If you want to invoke the setter but it just doesn't exist yet, then declare it:
{% prettify dart tag=pre+code %} class C { int get x => 0; set x(int p) {} set y(int p) {} }
void f(C c) { c.x = 1; } {% endprettify %}
Functions can't be assigned a value.
The analyzer produces this diagnostic when the name of a function appears on the left-hand side of an assignment expression.
The following code produces this diagnostic because the assignment to the function f is invalid:
{% prettify dart tag=pre+code %} void f() {}
void g() { [!f!] = () {}; } {% endprettify %}
If the right-hand side should be assigned to something else, such as a local variable, then change the left-hand side:
{% prettify dart tag=pre+code %} void f() {}
void g() { var x = () {}; print(x); } {% endprettify %}
If the intent is to change the implementation of the function, then define a function-valued variable instead of a function:
{% prettify dart tag=pre+code %} void Function() f = () {};
void g() { f = () {}; } {% endprettify %}
Methods can't be assigned a value.
The analyzer produces this diagnostic when the target of an assignment is a method.
The following code produces this diagnostic because f can‘t be assigned a value because it’s a method:
{% prettify dart tag=pre+code %} class C { void f() {}
void g() { [!f!] = null; } } {% endprettify %}
Rewrite the code so that there isn't an assignment to a method.
Types can't be assigned a value.
The analyzer produces this diagnostic when the name of a type name appears on the left-hand side of an assignment expression.
The following code produces this diagnostic because the assignment to the class C is invalid:
{% prettify dart tag=pre+code %} class C {}
void f() { [!C!] = null; } {% endprettify %}
If the right-hand side should be assigned to something else, such as a local variable, then change the left-hand side:
{% prettify dart tag=pre+code %} void f() {}
void g() { var c = null; print(c); } {% endprettify %}
The async for-in loop can only be used in an async function.
The analyzer produces this diagnostic when an async for-in loop is found in a function or method whose body isn't marked as being either async or async*.
The following code produces this diagnostic because the body of f isn't marked as being either async or async*, but f contains an async for-in loop:
{% prettify dart tag=pre+code %} void f(list) { await for (var e [!in!] list) { print(e); } } {% endprettify %}
If the function should return a Future, then mark the body with async:
{% prettify dart tag=pre+code %} Future f(list) async { await for (var e in list) { print(e); } } {% endprettify %}
If the function should return a Stream of values, then mark the body with async*:
{% prettify dart tag=pre+code %} Stream f(list) async* { await for (var e in list) { print(e); } } {% endprettify %}
If the function should be synchronous, then remove the await before the loop:
{% prettify dart tag=pre+code %} void f(list) { for (var e in list) { print(e); } } {% endprettify %}
The ‘await’ expression can‘t be used in a ‘late’ local variable’s initializer.
The analyzer produces this diagnostic when a local variable that has the late modifier uses an await expression in the initializer.
The following code produces this diagnostic because an await expression is used in the initializer for v, a local variable that is marked late:
{% prettify dart tag=pre+code %} Future f() async { late var v = [!await!] 42; return v; } {% endprettify %}
If the initializer can be rewritten to not use await, then rewrite it:
{% prettify dart tag=pre+code %} Future f() async { late var v = 42; return v; } {% endprettify %}
If the initializer can't be rewritten, then remove the late modifier:
{% prettify dart tag=pre+code %} Future f() async { var v = await 42; return v; } {% endprettify %}
The body might complete normally, causing ‘null’ to be returned, but the return type, ‘{0}’, is a potentially non-nullable type.
The analyzer produces this diagnostic when a method or function has a return type that's potentially non-nullable but would implicitly return null if control reached the end of the function.
The following code produces this diagnostic because the method m has an implicit return of null inserted at the end of the method, but the method is declared to not return null:
{% prettify dart tag=pre+code %} class C { int [!m!](int t) { print(t); } } {% endprettify %}
The following code produces this diagnostic because the method m has an implicit return of null inserted at the end of the method, but because the class C can be instantiated with a non-nullable type argument, the method is effectively declared to not return null:
{% prettify dart tag=pre+code %} class C { T [!m!](T t) { print(t); } } {% endprettify %}
If there's a reasonable value that can be returned, then add a return statement at the end of the method:
{% prettify dart tag=pre+code %} class C { T m(T t) { print(t); return t; } } {% endprettify %}
If the method won't reach the implicit return, then add a throw at the end of the method:
{% prettify dart tag=pre+code %} class C { T m(T t) { print(t); throw ''; } } {% endprettify %}
If the method intentionally returns null at the end, then add an explicit return of null at the end of the method and change the return type so that it's valid to return null:
{% prettify dart tag=pre+code %} class C { T? m(T t) { print(t); return null; } } {% endprettify %}
This function has a nullable return type of ‘{0}’, but ends without returning a value.
The analyzer produces this diagnostic when a method or function can implicitly return null by falling off the end. While this is valid Dart code, it's better for the return of null to be explicit.
The following code produces this diagnostic because the function f implicitly returns null:
{% prettify dart tag=pre+code %} String? !f! {} {% endprettify %}
If the return of null is intentional, then make it explicit:
{% prettify dart tag=pre+code %} String? f() { return null; } {% endprettify %}
If the function should return a non-null value along that path, then add the missing return statement:
{% prettify dart tag=pre+code %} String? f() { return ''; } {% endprettify %}
A break label resolves to the ‘case’ or ‘default’ statement.
The analyzer produces this diagnostic when a break in a case clause inside a switch statement has a label that is associated with another case clause.
The following code produces this diagnostic because the label l is associated with the case clause for 0:
{% prettify dart tag=pre+code %} void f(int i) { switch (i) { l: case 0: break; case 1: break [!l!]; } } {% endprettify %}
If the intent is to transfer control to the statement after the switch, then remove the label from the break statement:
{% prettify dart tag=pre+code %} void f(int i) { switch (i) { case 0: break; case 1: break; } } {% endprettify %}
If the intent is to transfer control to a different case block, then use continue rather than break:
{% prettify dart tag=pre+code %} void f(int i) { switch (i) { l: case 0: break; case 1: continue l; } } {% endprettify %}
The built-in identifier ‘{0}’ can't be used as a type.
The analyzer produces this diagnostic when a built-in identifier is used where a type name is expected.
The following code produces this diagnostic because import can‘t be used as a type because it’s a built-in identifier:
{% prettify dart tag=pre+code %} [!import!] x; {% endprettify %}
Replace the built-in identifier with the name of a valid type:
{% prettify dart tag=pre+code %} List x; {% endprettify %}
The built-in identifier ‘{0}’ can't be used as a prefix name.
The built-in identifier ‘{0}’ can't be used as a type name.
The built-in identifier ‘{0}’ can't be used as a type parameter name.
The built-in identifier ‘{0}’ can't be used as a typedef name.
The built-in identifier ‘{0}’ can't be used as an extension name.
The analyzer produces this diagnostic when the name used in the declaration of a class, extension, mixin, typedef, type parameter, or import prefix is a built-in identifier. Built-in identifiers can’t be used to name any of these kinds of declarations.
The following code produces this diagnostic because mixin is a built-in identifier:
{% prettify dart tag=pre+code %} extension [!mixin!] on int {} {% endprettify %}
Choose a different name for the declaration.
The last statement of the ‘case’ should be ‘break’, ‘continue’, ‘rethrow’, ‘return’, or ‘throw’.
The analyzer produces this diagnostic when the last statement in a case block isn't one of the required terminators: break, continue, rethrow, return, or throw.
The following code produces this diagnostic because the case block ends with an assignment:
{% prettify dart tag=pre+code %} void f(int x) { switch (x) { [!case!] 0: x += 2; default: x += 1; } } {% endprettify %}
Add one of the required terminators:
{% prettify dart tag=pre+code %} void f(int x) { switch (x) { case 0: x += 2; break; default: x += 1; } } {% endprettify %}
The switch case expression type ‘{0}’ can't override the ‘==’ operator.
The analyzer produces this diagnostic when the type of the expression following the keyword case has an implementation of the == operator other than the one in Object.
The following code produces this diagnostic because the expression following the keyword case (C(0)) has the type C, and the class C overrides the == operator:
{% prettify dart tag=pre+code %} class C { final int value;
const C(this.value);
bool operator ==(Object other) { return false; } }
void f(C c) { switch (c) { case [!C(0)!]: break; } } {% endprettify %}
If there isn't a strong reason not to do so, then rewrite the code to use an if-else structure:
{% prettify dart tag=pre+code %} class C { final int value;
const C(this.value);
bool operator ==(Object other) { return false; } }
void f(C c) { if (c == C(0)) { // ... } } {% endprettify %}
If you can‘t rewrite the switch statement and the implementation of == isn’t necessary, then remove it:
{% prettify dart tag=pre+code %} class C { final int value;
const C(this.value); }
void f(C c) { switch (c) { case C(0): break; } } {% endprettify %}
If you can‘t rewrite the switch statement and you can’t remove the definition of ==, then find some other value that can be used to control the switch:
{% prettify dart tag=pre+code %} class C { final int value;
const C(this.value);
bool operator ==(Object other) { return false; } }
void f(C c) { switch (c.value) { case 0: break; } } {% endprettify %}
The switch case expression type ‘{0}’ must be a subtype of the switch expression type ‘{1}’.
The analyzer produces this diagnostic when the expression following case in a switch statement has a static type that isn't a subtype of the static type of the expression following switch.
The following code produces this diagnostic because 1 is an int, which isn't a subtype of String (the type of s):
{% prettify dart tag=pre+code %} void f(String s) { switch (s) { case [!1!]: break; } } {% endprettify %}
If the value of the case expression is wrong, then change the case expression so that it has the required type:
{% prettify dart tag=pre+code %} void f(String s) { switch (s) { case ‘1’: break; } } {% endprettify %}
If the value of the case expression is correct, then change the switch expression to have the required type:
{% prettify dart tag=pre+code %} void f(int s) { switch (s) { case 1: break; } } {% endprettify %}
The name ‘{0}’ isn‘t a type, so it can’t be used in an ‘as’ expression.
The analyzer produces this diagnostic when the name following the as in a cast expression is defined to be something other than a type.
The following code produces this diagnostic because x is a variable, not a type:
{% prettify dart tag=pre+code %} num x = 0; int y = x as [!x!]; {% endprettify %}
Replace the name with the name of a type:
{% prettify dart tag=pre+code %} num x = 0; int y = x as int; {% endprettify %}
Constant values from a deferred library can't be used as keys in a ‘const’ map literal.
Constant values from a deferred library can't be used as values in a ‘const’ list literal.
Constant values from a deferred library can't be used as values in a ‘const’ map literal.
Constant values from a deferred library can't be used as values in a ‘const’ set literal.
The analyzer produces this diagnostic when a collection literal that is either explicitly (because it‘s prefixed by the const keyword) or implicitly (because it appears in a constant context) a constant contains a value that is declared in a library that is imported using a deferred import. Constants are evaluated at compile time, and values from deferred libraries aren’t available at compile time.
For more information, see the language tour's coverage of deferred loading.
Given a file (a.dart) that defines the constant zero:
{% prettify dart tag=pre+code %} const zero = 0; {% endprettify %}
The following code produces this diagnostic because the constant list literal contains a.zero, which is imported using a deferred import:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a;
var l = const [[!a.zero!]]; {% endprettify %}
If the collection literal isn't required to be constant, then remove the const keyword:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a;
var l = [a.zero]; {% endprettify %}
If the collection is required to be constant and the imported constant must be referenced, then remove the keyword deferred from the import:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a;
var l = const [a.zero]; {% endprettify %}
If you don't need to reference the constant, then replace it with a suitable value:
{% prettify dart tag=pre+code %} var l = const [0]; {% endprettify %}
‘{0}’ must have a method body because ‘{1}’ isn't abstract.
The analyzer produces this diagnostic when a member of a concrete class is found that doesn‘t have a concrete implementation. Concrete classes aren’t allowed to contain abstract members.
The following code produces this diagnostic because m is an abstract method but C isn't an abstract class:
{% prettify dart tag=pre+code %} class C { [!void m();!] } {% endprettify %}
If it's valid to create instances of the class, provide an implementation for the member:
{% prettify dart tag=pre+code %} class C { void m() {} } {% endprettify %}
If it isn't valid to create instances of the class, mark the class as being abstract:
{% prettify dart tag=pre+code %} abstract class C { void m(); } {% endprettify %}
‘{0}’ can't be used to name both a constructor and a static field in this class.
‘{0}’ can't be used to name both a constructor and a static getter in this class.
‘{0}’ can't be used to name both a constructor and a static method in this class.
‘{0}’ can't be used to name both a constructor and a static setter in this class.
The analyzer produces this diagnostic when a named constructor and either a static method or static field have the same name. Both are accessed using the name of the class, so having the same name makes the reference ambiguous.
The following code produces this diagnostic because the static field foo and the named constructor foo have the same name:
{% prettify dart tag=pre+code %} class C { C.!foo!; static int foo = 0; } {% endprettify %}
The following code produces this diagnostic because the static method foo and the named constructor foo have the same name:
{% prettify dart tag=pre+code %} class C { C.!foo!; static void foo() {} } {% endprettify %}
Rename either the member or the constructor.
The class ‘{0}’ can't implement both ‘{1}’ and ‘{2}’ because the type arguments are different.
The analyzer produces this diagnostic when a class attempts to implement a generic interface multiple times, and the values of the type arguments aren't the same.
The following code produces this diagnostic because C is defined to implement both I<int> (because it extends A) and I<String> (because it implementsB), but int and String aren't the same type:
{% prettify dart tag=pre+code %} class I {} class A implements I {} class B implements I {} class [!C!] extends A implements B {} {% endprettify %}
Rework the type hierarchy to avoid this situation. For example, you might make one or both of the inherited types generic so that C can specify the same type for both type arguments:
{% prettify dart tag=pre+code %} class I {} class A implements I {} class B implements I {} class C extends A implements B {} {% endprettify %}
‘{0}’ can't be used to name both a type variable and the class in which the type variable is defined.
‘{0}’ can't be used to name both a type variable and the enum in which the type variable is defined.
‘{0}’ can't be used to name both a type variable and the extension in which the type variable is defined.
‘{0}’ can't be used to name both a type variable and the mixin in which the type variable is defined.
The analyzer produces this diagnostic when a class, mixin, or extension declaration declares a type parameter with the same name as the class, mixin, or extension that declares it.
The following code produces this diagnostic because the type parameter C has the same name as the class C of which it's a part:
{% prettify dart tag=pre+code %} class C<[!C!]> {} {% endprettify %}
Rename either the type parameter, or the class, mixin, or extension:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
‘{0}’ can't be used to name both a type variable and a member in this class.
‘{0}’ can't be used to name both a type variable and a member in this enum.
‘{0}’ can't be used to name both a type variable and a member in this extension.
‘{0}’ can't be used to name both a type variable and a member in this mixin.
The analyzer produces this diagnostic when a class, mixin, or extension declaration declares a type parameter with the same name as one of the members of the class, mixin, or extension that declares it.
The following code produces this diagnostic because the type parameter T has the same name as the field T:
{% prettify dart tag=pre+code %} class C<[!T!]> { int T = 0; } {% endprettify %}
Rename either the type parameter or the member with which it conflicts:
{% prettify dart tag=pre+code %} class C { int total = 0; } {% endprettify %}
A value of type ‘{0}’ can't be assigned to a parameter of type ‘{1}’ in a const constructor.
The analyzer produces this diagnostic when the runtime type of a constant value can‘t be assigned to the static type of a constant constructor’s parameter.
The following code produces this diagnostic because the runtime type of i is int, which can't be assigned to the static type of s:
{% prettify dart tag=pre+code %} class C { final String s;
const C(this.s); }
const dynamic i = 0;
void f() { const C([!i!]); } {% endprettify %}
Pass a value of the correct type to the constructor:
{% prettify dart tag=pre+code %} class C { final String s;
const C(this.s); }
const dynamic i = 0;
void f() { const C(‘$i’); } {% endprettify %}
Can't define the ‘const’ constructor because the field ‘{0}’ is initialized with a non-constant value.
The analyzer produces this diagnostic when a constructor has the keyword const, but a field in the class is initialized to a non-constant value.
The following code produces this diagnostic because the field s is initialized to a non-constant value:
{% prettify dart tag=pre+code %} class C { final String s = 3.toString(); [!const!] C(); } {% endprettify %}
If the field can be initialized to a constant value, then change the initializer to a constant expression:
{% prettify dart tag=pre+code %} class C { final String s = ‘3’; const C(); } {% endprettify %}
If the field can't be initialized to a constant value, then remove the keyword const from the constructor:
{% prettify dart tag=pre+code %} class C { final String s = 3.toString(); C(); } {% endprettify %}
A constant constructor can't call a non-constant super constructor of ‘{0}’.
The analyzer produces this diagnostic when a constructor that is marked as const invokes a constructor from its superclass that isn't marked as const.
The following code produces this diagnostic because the const constructor in B invokes the constructor nonConst from the class A, and the superclass constructor isn't a const constructor:
{% prettify dart tag=pre+code %} class A { const A(); A.nonConst(); }
class B extends A { const B() : [!super.nonConst()!]; } {% endprettify %}
If it isn't essential to invoke the superclass constructor that is currently being invoked, then invoke a constant constructor from the superclass:
{% prettify dart tag=pre+code %} class A { const A(); A.nonConst(); }
class B extends A { const B() : super(); } {% endprettify %}
If it's essential that the current constructor be invoked and if you can modify it, then add const to the constructor in the superclass:
{% prettify dart tag=pre+code %} class A { const A(); const A.nonConst(); }
class B extends A { const B() : super.nonConst(); } {% endprettify %}
If it‘s essential that the current constructor be invoked and you can’t modify it, then remove const from the constructor in the subclass:
{% prettify dart tag=pre+code %} class A { const A(); A.nonConst(); }
class B extends A { B() : super.nonConst(); } {% endprettify %}
Can't define a const constructor for a class with non-final fields.
The analyzer produces this diagnostic when a constructor is marked as a const constructor, but the constructor is defined in a class that has at least one non-final instance field (either directly or by inheritance).
The following code produces this diagnostic because the field x isn't final:
{% prettify dart tag=pre+code %} class C { int x;
const !C!; } {% endprettify %}
If it's possible to mark all of the fields as final, then do so:
{% prettify dart tag=pre+code %} class C { final int x;
const C(this.x); } {% endprettify %}
If it isn't possible to mark all of the fields as final, then remove the keyword const from the constructor:
{% prettify dart tag=pre+code %} class C { int x;
C(this.x); } {% endprettify %}
Deferred classes can't be created with ‘const’.
The analyzer produces this diagnostic when a class from a library that is imported using a deferred import is used to create a const object. Constants are evaluated at compile time, and classes from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
The following code produces this diagnostic because it attempts to create a const instance of a class from a deferred library:
{% prettify dart tag=pre+code %} import ‘dart:convert’ deferred as convert;
const json2 = [!convert.JsonCodec()!]; {% endprettify %}
If the object isn't required to be a constant, then change the code so that a non-constant instance is created:
{% prettify dart tag=pre+code %} import ‘dart:convert’ deferred as convert;
final json2 = convert.JsonCodec(); {% endprettify %}
If the object must be a constant, then remove deferred from the import directive:
{% prettify dart tag=pre+code %} import ‘dart:convert’ as convert;
const json2 = convert.JsonCodec(); {% endprettify %}
Const variables must be initialized with a constant value.
The analyzer produces this diagnostic when a value that isn‘t statically known to be a constant is assigned to a variable that’s declared to be a const variable.
The following code produces this diagnostic because x isn't declared to be const:
{% prettify dart tag=pre+code %} var x = 0; const y = [!x!]; {% endprettify %}
If the value being assigned can be declared to be const, then change the declaration:
{% prettify dart tag=pre+code %} const x = 0; const y = x; {% endprettify %}
If the value can't be declared to be const, then remove the const modifier from the variable, possibly using final in its place:
{% prettify dart tag=pre+code %} var x = 0; final y = x; {% endprettify %}
Constant values from a deferred library can't be used to initialize a ‘const’ variable.
The analyzer produces this diagnostic when a const variable is initialized using a const variable from a library that is imported using a deferred import. Constants are evaluated at compile time, and values from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
The following code produces this diagnostic because the variable pi is being initialized using the constant math.pi from the library dart:math, and dart:math is imported as a deferred library:
{% prettify dart tag=pre+code %} import ‘dart:math’ deferred as math;
const pi = [!math.pi!]; {% endprettify %}
If you need to reference the value of the constant from the imported library, then remove the keyword deferred:
{% prettify dart tag=pre+code %} import ‘dart:math’ as math;
const pi = math.pi; {% endprettify %}
If you don't need to reference the imported constant, then remove the reference:
{% prettify dart tag=pre+code %} const pi = 3.14; {% endprettify %}
Only static fields can be declared as const.
The analyzer produces this diagnostic when an instance field is marked as being const.
The following code produces this diagnostic because f is an instance field:
{% prettify dart tag=pre+code %} class C { [!const!] int f = 3; } {% endprettify %}
If the field needs to be an instance field, then remove the keyword const, or replace it with final:
{% prettify dart tag=pre+code %} class C { final int f = 3; } {% endprettify %}
If the field really should be a const field, then make it a static field:
{% prettify dart tag=pre+code %} class C { static const int f = 3; } {% endprettify %}
The type of a key in a constant map can't override the ‘==’ operator, but the class ‘{0}’ does.
The analyzer produces this diagnostic when the class of object used as a key in a constant map literal implements the == operator. The implementation of constant maps uses the == operator, so any implementation other than the one inherited from Object requires executing arbitrary code at compile time, which isn't supported.
The following code produces this diagnostic because the constant map contains a key whose type is C, and the class C overrides the implementation of ==:
{% prettify dart tag=pre+code %} class C { const C();
bool operator ==(Object other) => true; }
const map = {[!C()!] : 0}; {% endprettify %}
If you can remove the implementation of == from the class, then do so:
{% prettify dart tag=pre+code %} class C { const C(); }
const map = {C() : 0}; {% endprettify %}
If you can't remove the implementation of == from the class, then make the map be non-constant:
{% prettify dart tag=pre+code %} class C { const C();
bool operator ==(Object other) => true; }
final map = {C() : 0}; {% endprettify %}
The constant ‘{0}’ must be initialized.
The analyzer produces this diagnostic when a variable that is declared to be a constant doesn't have an initializer.
The following code produces this diagnostic because c isn't initialized:
{% prettify dart tag=pre+code %} const [!c!]; {% endprettify %}
Add an initializer:
{% prettify dart tag=pre+code %} const c = ‘c’; {% endprettify %}
The type of an element in a constant set can't override the ‘==’ operator, but the type ‘{0}’ does.
The analyzer produces this diagnostic when the class of object used as an element in a constant set literal implements the == operator. The implementation of constant sets uses the == operator, so any implementation other than the one inherited from Object requires executing arbitrary code at compile time, which isn't supported.
The following code produces this diagnostic because the constant set contains an element whose type is C, and the class C overrides the implementation of ==:
{% prettify dart tag=pre+code %} class C { const C();
bool operator ==(Object other) => true; }
const set = {[!C()!]}; {% endprettify %}
If you can remove the implementation of == from the class, then do so:
{% prettify dart tag=pre+code %} class C { const C(); }
const set = {C()}; {% endprettify %}
If you can't remove the implementation of == from the class, then make the set be non-constant:
{% prettify dart tag=pre+code %} class C { const C();
bool operator ==(Object other) => true; }
final set = {C()}; {% endprettify %}
A list or a set is expected in this spread.
The analyzer produces this diagnostic when the expression of a spread operator in a constant list or set evaluates to something other than a list or a set.
The following code produces this diagnostic because the value of list1 is null, which is neither a list nor a set:
{% prettify dart tag=pre+code %} const List list1 = null; const List list2 = [...[!list1!]]; {% endprettify %}
Change the expression to something that evaluates to either a constant list or a constant set:
{% prettify dart tag=pre+code %} const List list1 = []; const List list2 = [...list1]; {% endprettify %}
A map is expected in this spread.
The analyzer produces this diagnostic when the expression of a spread operator in a constant map evaluates to something other than a map.
The following code produces this diagnostic because the value of map1 is null, which isn't a map:
{% prettify dart tag=pre+code %} const Map<String, int> map1 = null; const Map<String, int> map2 = {...[!map1!]}; {% endprettify %}
Change the expression to something that evaluates to a constant map:
{% prettify dart tag=pre+code %} const Map<String, int> map1 = {}; const Map<String, int> map2 = {...map1}; {% endprettify %}
The constructor being called isn't a const constructor.
The analyzer produces this diagnostic when the keyword const is used to invoke a constructor that isn't marked with const.
The following code produces this diagnostic because the constructor in A isn't a const constructor:
{% prettify dart tag=pre+code %} class A { A(); }
A f() => [!const!] A(); {% endprettify %}
If it's desirable and possible to make the class a constant class (by making all of the fields of the class, including inherited fields, final), then add the keyword const to the constructor:
{% prettify dart tag=pre+code %} class A { const A(); }
A f() => const A(); {% endprettify %}
Otherwise, remove the keyword const:
{% prettify dart tag=pre+code %} class A { A(); }
A f() => A(); {% endprettify %}
Arguments of a constant creation must be constant expressions.
The analyzer produces this diagnostic when a const constructor is invoked with an argument that isn't a constant expression.
The following code produces this diagnostic because i isn't a constant:
{% prettify dart tag=pre+code %} class C { final int i; const C(this.i); } C f(int i) => const C([!i!]); {% endprettify %}
Either make all of the arguments constant expressions, or remove the const keyword to use the non-constant form of the constructor:
{% prettify dart tag=pre+code %} class C { final int i; const C(this.i); } C f(int i) => C(i); {% endprettify %}
A constant constructor tearoff can't use a type parameter as a type argument.
A constant creation can't use a type parameter as a type argument.
A constant function tearoff can't use a type parameter as a type argument.
The analyzer produces this diagnostic when a type parameter is used as a type argument in a const invocation of a constructor. This isn‘t allowed because the value of the type parameter (the actual type that will be used at runtime) can’t be known at compile time.
The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant:
{% prettify dart tag=pre+code %} class C { const C(); }
C newC() => const C<[!T!]>(); {% endprettify %}
If the type that will be used for the type parameter can be known at compile time, then remove the use of the type parameter:
{% prettify dart tag=pre+code %} class C { const C(); }
C newC() => const C(); {% endprettify %}
If the type that will be used for the type parameter can't be known until runtime, then remove the keyword const:
{% prettify dart tag=pre+code %} class C { const C(); }
C newC() => C(); {% endprettify %}
A continue label resolves to a switch statement, but the label must be on a loop or a switch member.
The analyzer produces this diagnostic when the label in a continue statement resolves to a label on a switch statement.
The following code produces this diagnostic because the label l, used to label a switch statement, is used in the continue statement:
{% prettify dart tag=pre+code %} void f(int i) { l: switch (i) { case 0: continue [!l!]; } } {% endprettify %}
Find a different way to achieve the control flow you need; for example, by introducing a loop that re-executes the switch statement.
Subclasses of ‘Struct’ and ‘Union’ are backed by native memory, and can't be instantiated by a generative constructor.
The analyzer produces this diagnostic when a subclass of either Struct or Union is instantiated using a generative constructor.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class C is being instantiated using a generative constructor:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int32() external int a; }
void f() { !C!; } {% endprettify %}
If you need to allocate the structure described by the class, then use the ffi package to do so:
{% prettify dart tag=pre+code %} import ‘dart:ffi’; import ‘package:ffi/ffi.dart’;
class C extends Struct { @Int32() external int a; }
void f() { final pointer = calloc.allocate(4); final c = pointer.ref; print(c); calloc.free(pointer); } {% endprettify %}
The name ‘{0}’ isn't a class.
The analyzer produces this diagnostic when an instance creation using either new or const specifies a name that isn't defined as a class.
The following code produces this diagnostic because f is a function rather than a class:
{% prettify dart tag=pre+code %} int f() => 0;
void g() { new !f!; } {% endprettify %}
If a class should be created, then replace the invalid name with the name of a valid class:
{% prettify dart tag=pre+code %} int f() => 0;
void g() { new Object(); } {% endprettify %}
If the name is the name of a function and you want that function to be invoked, then remove the new or const keyword:
{% prettify dart tag=pre+code %} int f() => 0;
void g() { f(); } {% endprettify %}
Dead code.
The analyzer produces this diagnostic when code is found that won't be executed because execution will never reach the code.
The following code produces this diagnostic because the invocation of print occurs after the function has returned:
{% prettify dart tag=pre+code %} void f() { return; [!print(‘here’);!] } {% endprettify %}
If the code isn't needed, then remove it:
{% prettify dart tag=pre+code %} void f() { return; } {% endprettify %}
If the code needs to be executed, then either move the code to a place where it will be executed:
{% prettify dart tag=pre+code %} void f() { print(‘here’); return; } {% endprettify %}
Or, rewrite the code before it, so that it can be reached:
{% prettify dart tag=pre+code %} void f({bool skipPrinting = true}) { if (skipPrinting) { return; } print(‘here’); } {% endprettify %}
Dead code: Catch clauses after a ‘catch (e)’ or an ‘on Object catch (e)’ are never reached.
The analyzer produces this diagnostic when a catch clause is found that can't be executed because it’s after a catch clause of the form catch (e) or on Object catch (e). The first catch clause that matches the thrown object is selected, and both of those forms will match any object, so no catch clauses that follow them will be selected.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} void f() { try { } catch (e) { } [!on String { }!] } {% endprettify %}
If the clause should be selectable, then move the clause before the general clause:
{% prettify dart tag=pre+code %} void f() { try { } on String { } catch (e) { } } {% endprettify %}
If the clause doesn't need to be selectable, then remove it:
{% prettify dart tag=pre+code %} void f() { try { } catch (e) { } } {% endprettify %}
Dead code: This on-catch block won’t be executed because ‘{0}’ is a subtype of ‘{1}’ and hence will have been caught already.
The analyzer produces this diagnostic when a catch clause is found that can‘t be executed because it is after a catch clause that catches either the same type or a supertype of the clause’s type. The first catch clause that matches the thrown object is selected, and the earlier clause always matches anything matchable by the highlighted clause, so the highlighted clause will never be selected.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} void f() { try { } on num { } [!on int { }!] } {% endprettify %}
If the clause should be selectable, then move the clause before the general clause:
{% prettify dart tag=pre+code %} void f() { try { } on int { } on num { } } {% endprettify %}
If the clause doesn't need to be selectable, then remove it:
{% prettify dart tag=pre+code %} void f() { try { } on num { } } {% endprettify %}
The left operand can't be null, so the right operand is never executed.
The analyzer produces this diagnostic in two cases.
The first is when the left operand of an ?? operator can‘t be null. The right operand is only evaluated if the left operand has the value null, and because the left operand can’t be null, the right operand is never evaluated.
The second is when the left-hand side of an assignment using the ??= operator can‘t be null. The right-hand side is only evaluated if the left-hand side has the value null, and because the left-hand side can’t be null, the right-hand side is never evaluated.
The following code produces this diagnostic because x can't be null:
{% prettify dart tag=pre+code %} int f(int x) { return x ?? [!0!]; } {% endprettify %}
The following code produces this diagnostic because f can't be null:
{% prettify dart tag=pre+code %} class C { int f = -1;
void m(int x) { f ??= [!x!]; } } {% endprettify %}
If the diagnostic is reported for an ?? operator, then remove the ?? operator and the right operand:
{% prettify dart tag=pre+code %} int f(int x) { return x; } {% endprettify %}
If the diagnostic is reported for an assignment, and the assignment isn't needed, then remove the assignment:
{% prettify dart tag=pre+code %} class C { int f = -1;
void m(int x) { } } {% endprettify %}
If the assignment is needed, but should be based on a different condition, then rewrite the code to use = and the different condition:
{% prettify dart tag=pre+code %} class C { int f = -1;
void m(int x) { if (f < 0) { f = x; } } } {% endprettify %}
The default ‘List’ constructor isn't available when null safety is enabled.
The analyzer produces this diagnostic when it finds a use of the default constructor for the class List in code that has opted in to null safety.
Assuming the following code is opted in to null safety, it produces this diagnostic because it uses the default List constructor:
{% prettify dart tag=pre+code %} var l = !List!; {% endprettify %}
If no initial size is provided, then convert the code to use a list literal:
{% prettify dart tag=pre+code %} var l = []; {% endprettify %}
If an initial size needs to be provided and there is a single reasonable initial value for the elements, then use List.filled:
{% prettify dart tag=pre+code %} var l = List.filled(3, 0); {% endprettify %}
If an initial size needs to be provided but each element needs to be computed, then use List.generate:
{% prettify dart tag=pre+code %} var l = List.generate(3, (i) => i); {% endprettify %}
Parameters in a function type can't have default values.
The analyzer produces this diagnostic when a function type associated with a parameter includes optional parameters that have a default value. This isn‘t allowed because the default values of parameters aren’t part of the function‘s type, and therefore including them doesn’t provide any value.
The following code produces this diagnostic because the parameter p has a default value even though it's part of the type of the parameter g:
{% prettify dart tag=pre+code %} void f(void Function([int p [!=!] 0]) g) { } {% endprettify %}
Remove the default value from the function-type's parameter:
{% prettify dart tag=pre+code %} void f(void Function([int p]) g) { } {% endprettify %}
Default values aren't allowed in factory constructors that redirect to another constructor.
The analyzer produces this diagnostic when a factory constructor that redirects to another constructor specifies a default value for an optional parameter.
The following code produces this diagnostic because the factory constructor in A has a default value for the optional parameter x:
{% prettify dart tag=pre+code %} class A { factory A([int [!x!] = 0]) = B; }
class B implements A { B([int x = 1]) {} } {% endprettify %}
Remove the default value from the factory constructor:
{% prettify dart tag=pre+code %} class A { factory A([int x]) = B; }
class B implements A { B([int x = 1]) {} } {% endprettify %}
Note that this fix might change the value used when the optional parameter is omitted. If that happens, and if that change is a problem, then consider making the optional parameter a required parameter in the factory method:
{% prettify dart tag=pre+code %} class A { factory A(int x) = B; }
class B implements A { B([int x = 1]) {} } {% endprettify %}
Required named parameters can't have a default value.
The analyzer produces this diagnostic when a named parameter has both the required modifier and a default value. If the parameter is required, then a value for the parameter is always provided at the call sites, so the default value can never be used.
The following code generates this diagnostic:
{% prettify dart tag=pre+code %} void log({required String [!message!] = ‘no message’}) {} {% endprettify %}
If the parameter is really required, then remove the default value:
{% prettify dart tag=pre+code %} void log({required String message}) {} {% endprettify %}
If the parameter isn't always required, then remove the required modifier:
{% prettify dart tag=pre+code %} void log({String message = ‘no message’}) {} {% endprettify %}
Imports of deferred libraries must hide all extensions.
The analyzer produces this diagnostic when a library that is imported using a deferred import declares an extension that is visible in the importing library. Extension methods are resolved at compile time, and extensions from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
Given a file (a.dart) that defines a named extension:
{% prettify dart tag=pre+code %} class C {}
extension E on String { int get size => length; } {% endprettify %}
The following code produces this diagnostic because the named extension is visible to the library:
{% prettify dart tag=pre+code %} import [!‘a.dart’!] deferred as a;
void f() { a.C(); } {% endprettify %}
If the library must be imported as deferred, then either add a show clause listing the names being referenced or add a hide clause listing all of the named extensions. Adding a show clause would look like this:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a show C;
void f() { a.C(); } {% endprettify %}
Adding a hide clause would look like this:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a hide E;
void f() { a.C(); } {% endprettify %}
With the first fix, the benefit is that if new extensions are added to the imported library, then the extensions won't cause a diagnostic to be generated.
If the library doesn't need to be imported as deferred, or if you need to make use of the extension method declared in it, then remove the keyword deferred:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a;
void f() { a.C(); } {% endprettify %}
The late local variable ‘{0}’ is definitely unassigned at this point.
The analyzer produces this diagnostic when definite assignment analysis shows that a local variable that's marked as late is read before being assigned.
The following code produces this diagnostic because x wasn't assigned a value before being read:
{% prettify dart tag=pre+code %} void f(bool b) { late int x; print([!x!]); } {% endprettify %}
Assign a value to the variable before reading from it:
{% prettify dart tag=pre+code %} void f(bool b) { late int x; x = b ? 1 : 0; print(x); } {% endprettify %}
The value of the ‘{0}’ field is expected to be a map.
The analyzer produces this diagnostic when the value of either the dependencies or dev_dependencies key isn't a map.
The following code produces this diagnostic because the value of the top-level dependencies key is a list:
name: example dependencies: - meta
Use a map as the value of the dependencies key:
name: example dependencies: meta: ^1.0.2
The ‘{0}’ field is no longer used and can be removed.
The analyzer produces this diagnostic when a key is used in a pubspec.yaml file that was deprecated. Unused keys take up space and might imply semantics that are no longer valid.
The following code produces this diagnostic because the author key is no longer being used:
{% prettify dart tag=pre+code %} name: example author: ‘Dash’ {% endprettify %}
Remove the deprecated key:
{% prettify dart tag=pre+code %} name: example {% endprettify %}
‘{0}’ is deprecated and shouldn't be used.
‘{0}’ is deprecated and shouldn't be used. {1}.
The analyzer produces this diagnostic when a deprecated library or class member is used in a different package.
If the method m in the class C is annotated with @deprecated, then the following code produces this diagnostic:
{% prettify dart tag=pre+code %} void f(C c) { c.!m!; } {% endprettify %}
The documentation for declarations that are annotated with @deprecated should indicate what code to use in place of the deprecated code.
‘{0}’ is deprecated and shouldn't be used.
‘{0}’ is deprecated and shouldn't be used. {1}.
The analyzer produces this diagnostic when a deprecated library member or class member is used in the same package in which it's declared.
The following code produces this diagnostic because x is deprecated:
{% prettify dart tag=pre+code %} @deprecated var x = 0; var y = [!x!]; {% endprettify %}
The fix depends on what's been deprecated and what the replacement is. The documentation for deprecated declarations should indicate what code to use in place of the deprecated code.
Using the ‘new’ keyword in a comment reference is deprecated.
The analyzer produces this diagnostic when a comment reference (the name of a declaration enclosed in square brackets in a documentation comment) uses the keyword new to refer to a constructor. This form is deprecated.
The following code produces this diagnostic because the unnamed constructor is being referenced using new C:
{% prettify dart tag=pre+code %} /// See [[!new!] C]. class C { C(); } {% endprettify %}
The following code produces this diagnostic because the constructor named c is being referenced using new C.c:
{% prettify dart tag=pre+code %} /// See [[!new!] C.c]. class C { C.c(); } {% endprettify %}
If you're referencing a named constructor, then remove the keyword new:
{% prettify dart tag=pre+code %} /// See [C.c]. class C { C.c(); } {% endprettify %}
If you're referencing the unnamed constructor, then remove the keyword new and append .new after the class name:
{% prettify dart tag=pre+code %} /// See [C.new]. class C { C.c(); } {% endprettify %}
Extending ‘Function’ is deprecated.
Implementing ‘Function’ has no effect.
Mixing in ‘Function’ is deprecated.
The analyzer produces this diagnostic when the class Function is used in either the extends, implements, or with clause of a class or mixin. Using the class Function in this way has no semantic value, so it's effectively dead code.
The following code produces this diagnostic because Function is used as the superclass of F:
{% prettify dart tag=pre+code %} class F extends [!Function!] {} {% endprettify %}
Remove the class Function from whichever clause it's in, and remove the whole clause if Function is the only type in the clause:
{% prettify dart tag=pre+code %} class F {} {% endprettify %}
Only a generic type, generic function, generic instance method, or generic constructor can have type arguments.
The analyzer produces this diagnostic when an expression with a value that is anything other than one of the allowed kinds of values is followed by type arguments. The allowed kinds of values are:
The following code produces this diagnostic because i is a top-level variable, which isn't one of the allowed cases:
{% prettify dart tag=pre+code %} int i = 1;
void f() { print([!i!]); } {% endprettify %}
If the referenced value is correct, then remove the type arguments:
{% prettify dart tag=pre+code %} int i = 1;
void f() { print(i); } {% endprettify %}
The constructor with name ‘{0}’ is already defined.
The unnamed constructor is already defined.
The analyzer produces this diagnostic when a class declares more than one unnamed constructor or when it declares more than one constructor with the same name.
The following code produces this diagnostic because there are two declarations for the unnamed constructor:
{% prettify dart tag=pre+code %} class C { C();
!C!; } {% endprettify %}
The following code produces this diagnostic because there are two declarations for the constructor named m:
{% prettify dart tag=pre+code %} class C { C.m();
!C.m!; } {% endprettify %}
If there are multiple unnamed constructors and all of the constructors are needed, then give all of them, or all except one of them, a name:
{% prettify dart tag=pre+code %} class C { C();
C.n(); } {% endprettify %}
If there are multiple unnamed constructors and all except one of them are unneeded, then remove the constructors that aren't needed:
{% prettify dart tag=pre+code %} class C { C(); } {% endprettify %}
If there are multiple named constructors and all of the constructors are needed, then rename all except one of them:
{% prettify dart tag=pre+code %} class C { C.m();
C.n(); } {% endprettify %}
If there are multiple named constructors and all except one of them are unneeded, then remove the constructorsthat aren't needed:
{% prettify dart tag=pre+code %} class C { C.m(); } {% endprettify %}
The name ‘{0}’ is already defined.
The analyzer produces this diagnostic when a name is declared, and there is a previous declaration with the same name in the same scope.
The following code produces this diagnostic because the name x is declared twice:
{% prettify dart tag=pre+code %} int x = 0; int [!x!] = 1; {% endprettify %}
Choose a different name for one of the declarations.
{% prettify dart tag=pre+code %} int x = 0; int y = 1; {% endprettify %}
The field ‘{0}’ can't be initialized by multiple parameters in the same constructor.
The analyzer produces this diagnostic when there‘s more than one initializing formal parameter for the same field in a constructor’s parameter list. It isn't useful to assign a value that will immediately be overwritten.
The following code produces this diagnostic because this.f appears twice in the parameter list:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f, this.[!f!]) {} } {% endprettify %}
Remove one of the initializing formal parameters:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f) {} } {% endprettify %}
Duplicate hidden name.
The analyzer produces this diagnostic when a name occurs multiple times in a hide clause. Repeating the name is unnecessary.
The following code produces this diagnostic because the name min is hidden more than once:
{% prettify dart tag=pre+code %} import ‘dart:math’ hide min, [!min!];
var x = pi; {% endprettify %}
If the name was mistyped in one or more places, then correct the mistyped names:
{% prettify dart tag=pre+code %} import ‘dart:math’ hide max, min;
var x = pi; {% endprettify %}
If the name wasn't mistyped, then remove the unnecessary name from the list:
{% prettify dart tag=pre+code %} import ‘dart:math’ hide min;
var x = pi; {% endprettify %}
The diagnostic ‘{0}’ doesn‘t need to be ignored here because it’s already being ignored.
The analyzer produces this diagnostic when a diagnostic name appears in an ignore comment, but the diagnostic is already being ignored, either because it's already included in the same ignore comment or because it appears in an ignore-in-file comment.
The following code produces this diagnostic because the diagnostic named unused_local_variable is already being ignored for the whole file so it doesn't need to be ignored on a specific line:
{% prettify dart tag=pre+code %} // ignore_for_file: unused_local_variable void f() { // ignore: [!unused_local_variable!] var x = 0; } {% endprettify %}
The following code produces this diagnostic because the diagnostic named unused_local_variable is being ignored twice on the same line:
{% prettify dart tag=pre+code %} void f() { // ignore: unused_local_variable, [!unused_local_variable!] var x = 0; } {% endprettify %}
Remove the ignore comment, or remove the unnecessary diagnostic name if the ignore comment is ignoring more than one diagnostic:
{% prettify dart tag=pre+code %} // ignore_for_file: unused_local_variable void f() { var x = 0; } {% endprettify %}
Duplicate import.
The analyzer produces this diagnostic when an import directive is found that is the same as an import before it in the file. The second import doesn’t add value and should be removed.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’; import [!‘package:meta/meta.dart’!];
@sealed class C {} {% endprettify %}
Remove the unnecessary import:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@sealed class C {} {% endprettify %}
The argument for the named parameter ‘{0}’ was already specified.
The analyzer produces this diagnostic when an invocation has two or more named arguments that have the same name.
The following code produces this diagnostic because there are two arguments with the name a:
{% prettify dart tag=pre+code %} void f(C c) { c.m(a: 0, [!a!]: 1); }
class C { void m({int a, int b}) {} } {% endprettify %}
If one of the arguments should have a different name, then change the name:
{% prettify dart tag=pre+code %} void f(C c) { c.m(a: 0, b: 1); }
class C { void m({int a, int b}) {} } {% endprettify %}
If one of the arguments is wrong, then remove it:
{% prettify dart tag=pre+code %} void f(C c) { c.m(a: 1); }
class C { void m({int a, int b}) {} } {% endprettify %}
The library already contains a part with the URI ‘{0}’.
The analyzer produces this diagnostic when a single file is referenced in multiple part directives.
Given a file named part.dart containing
{% prettify dart tag=pre+code %} part of lib; {% endprettify %}
The following code produces this diagnostic because the file part.dart is included multiple times:
{% prettify dart tag=pre+code %} library lib;
part ‘part.dart’; part [!‘part.dart’!]; {% endprettify %}
Remove all except the first of the duplicated part directives:
{% prettify dart tag=pre+code %} library lib;
part ‘part.dart’; {% endprettify %}
Duplicate shown name.
The analyzer produces this diagnostic when a name occurs multiple times in a show clause. Repeating the name is unnecessary.
The following code produces this diagnostic because the name min is shown more than once:
{% prettify dart tag=pre+code %} import ‘dart:math’ show min, [!min!];
var x = min(2, min(0, 1)); {% endprettify %}
If the name was mistyped in one or more places, then correct the mistyped names:
{% prettify dart tag=pre+code %} import ‘dart:math’ show max, min;
var x = max(2, min(0, 1)); {% endprettify %}
If the name wasn't mistyped, then remove the unnecessary name from the list:
{% prettify dart tag=pre+code %} import ‘dart:math’ show min;
var x = min(2, min(0, 1)); {% endprettify %}
The class ‘{0}’ can‘t be empty because it’s a subclass of ‘{1}’.
The analyzer produces this diagnostic when a subclass of Struct or Union doesn‘t have any fields. Having an empty Struct or Union isn’t supported.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class C, which extends Struct, doesn't declare any fields:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class [!C!] extends Struct {} {% endprettify %}
If the class is intended to be a struct, then declare one or more fields:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int32() external int x; } {% endprettify %}
If the class is intended to be used as a type argument to Pointer, then make it a subclass of Opaque:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Opaque {} {% endprettify %}
If the class isn't intended to be a struct, then remove or change the extends clause:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
The name of the enum constant can‘t be the same as the enum’s name.
The analyzer produces this diagnostic when an enum constant has the same name as the enum in which it's declared.
The following code produces this diagnostic because the enum constant E has the same name as the enclosing enum E:
{% prettify dart tag=pre+code %} enum E { [!E!] } {% endprettify %}
If the name of the enum is correct, then rename the constant:
{% prettify dart tag=pre+code %} enum E { e } {% endprettify %}
If the name of the constant is correct, then rename the enum:
{% prettify dart tag=pre+code %} enum F { E } {% endprettify %}
Two elements in a constant set literal can't be equal.
The analyzer produces this diagnostic when two elements in a constant set literal have the same value. The set can only contain each value once, which means that one of the values is unnecessary.
The following code produces this diagnostic because the string 'a' is specified twice:
{% prettify dart tag=pre+code %} const Set set = {‘a’, [!‘a’!]}; {% endprettify %}
Remove one of the duplicate values:
{% prettify dart tag=pre+code %} const Set set = {‘a’}; {% endprettify %}
Note that literal sets preserve the order of their elements, so the choice of which element to remove might affect the order in which elements are returned by an iterator.
Two elements in a set literal shouldn't be equal.
The analyzer produces this diagnostic when an element in a non-constant set is the same as a previous element in the same set. If two elements are the same, then the second value is ignored, which makes having both elements pointless and likely signals a bug.
The following code produces this diagnostic because the element 1 appears twice:
{% prettify dart tag=pre+code %} const a = 1; const b = 1; var s = {a, [!b!]}; {% endprettify %}
If both elements should be included in the set, then change one of the elements:
{% prettify dart tag=pre+code %} const a = 1; const b = 2; var s = {a, b}; {% endprettify %}
If only one of the elements is needed, then remove the one that isn't needed:
{% prettify dart tag=pre+code %} const a = 1; var s = {a}; {% endprettify %}
Note that literal sets preserve the order of their elements, so the choice of which element to remove might affect the order in which elements are returned by an iterator.
Two keys in a constant map literal can't be equal.
The analyzer produces this diagnostic when a key in a constant map is the same as a previous key in the same map. If two keys are the same, then the second value would overwrite the first value, which makes having both pairs pointless.
The following code produces this diagnostic because the key 1 is used twice:
{% prettify dart tag=pre+code %} const map = <int, String>{1: ‘a’, 2: ‘b’, [!1!]: ‘c’, 4: ‘d’}; {% endprettify %}
If both entries should be included in the map, then change one of the keys to be different:
{% prettify dart tag=pre+code %} const map = <int, String>{1: ‘a’, 2: ‘b’, 3: ‘c’, 4: ‘d’}; {% endprettify %}
If only one of the entries is needed, then remove the one that isn't needed:
{% prettify dart tag=pre+code %} const map = <int, String>{1: ‘a’, 2: ‘b’, 4: ‘d’}; {% endprettify %}
Note that literal maps preserve the order of their entries, so the choice of which entry to remove might affect the order in which keys and values are returned by an iterator.
Two keys in a map literal shouldn't be equal.
The analyzer produces this diagnostic when a key in a non-constant map is the same as a previous key in the same map. If two keys are the same, then the second value overwrites the first value, which makes having both pairs pointless and likely signals a bug.
The following code produces this diagnostic because the keys a and b have the same value:
{% prettify dart tag=pre+code %} const a = 1; const b = 1; var m = <int, String>{a: ‘a’, [!b!]: ‘b’}; {% endprettify %}
If both entries should be included in the map, then change one of the keys:
{% prettify dart tag=pre+code %} const a = 1; const b = 2; var m = <int, String>{a: ‘a’, b: ‘b’}; {% endprettify %}
If only one of the entries is needed, then remove the one that isn't needed:
{% prettify dart tag=pre+code %} const a = 1; var m = <int, String>{a: ‘a’}; {% endprettify %}
Note that literal maps preserve the order of their entries, so the choice of which entry to remove might affect the order in which the keys and values are returned by an iterator.
List literals require one type argument or none, but {0} found.
The analyzer produces this diagnostic when a list literal has more than one type argument.
The following code produces this diagnostic because the list literal has two type arguments when it can have at most one:
{% prettify dart tag=pre+code %} var l = [!<int, int>!][]; {% endprettify %}
Remove all except one of the type arguments:
{% prettify dart tag=pre+code %} var l = []; {% endprettify %}
Set literals require one type argument or none, but {0} were found.
The analyzer produces this diagnostic when a set literal has more than one type argument.
The following code produces this diagnostic because the set literal has three type arguments when it can have at most one:
{% prettify dart tag=pre+code %} var s = [!<int, String, int>!]{0, ‘a’, 1}; {% endprettify %}
Remove all except one of the type arguments:
{% prettify dart tag=pre+code %} var s = {0, 1}; {% endprettify %}
Map literals require two type arguments or none, but {0} found.
The analyzer produces this diagnostic when a map literal has either one or more than two type arguments.
The following code produces this diagnostic because the map literal has three type arguments when it can have either two or zero:
{% prettify dart tag=pre+code %} var m = [!<int, String, int>!]{}; {% endprettify %}
Remove all except two of the type arguments:
{% prettify dart tag=pre+code %} var m = <int, String>{}; {% endprettify %}
The library ‘{0}’ is internal and can't be exported.
The analyzer produces this diagnostic when it finds an export whose dart: URI references an internal library.
The following code produces this diagnostic because _interceptors is an internal library:
{% prettify dart tag=pre+code %} export [!‘dart:_interceptors’!]; {% endprettify %}
Remove the export directive.
The symbol ‘{0}’ is defined in a legacy library, and can't be re-exported from a library with null safety enabled.
The analyzer produces this diagnostic when a library that was opted in to null safety exports another library, and the exported library is opted out of null safety.
Given a library that is opted out of null safety:
{% prettify dart tag=pre+code %} // @dart = 2.8 String s; {% endprettify %}
The following code produces this diagnostic because it's exporting symbols from an opted-out library:
{% prettify dart tag=pre+code %} export [!‘optedOut.dart’!];
class C {} {% endprettify %}
If you‘re able to do so, migrate the exported library so that it doesn’t need to opt out:
{% prettify dart tag=pre+code %} String? s; {% endprettify %}
If you can't migrate the library, then remove the export:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
If the exported library (the one that is opted out) itself exports an opted-in library, then it's valid for your library to indirectly export the symbols from the opted-in library. You can do so by adding a hide combinator to the export directive in your library that hides all of the names declared in the opted-out library.
The exported library ‘{0}’ can't have a part-of directive.
The analyzer produces this diagnostic when an export directive references a part rather than a library.
Given a file named part.dart containing
{% prettify dart tag=pre+code %} part of lib; {% endprettify %}
The following code produces this diagnostic because the file part.dart is a part, and only libraries can be exported:
{% prettify dart tag=pre+code %} library lib;
export [!‘part.dart’!]; {% endprettify %}
Either remove the export directive, or change the URI to be the URI of the library containing the part.
Expressions can't be used in a map literal.
The analyzer produces this diagnostic when the analyzer finds an expression, rather than a map entry, in what appears to be a map literal.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} var map = <String, int>{‘a’: 0, ‘b’: 1, [!‘c’!]}; {% endprettify %}
If the expression is intended to compute either a key or a value in an entry, fix the issue by replacing the expression with the key or the value. For example:
{% prettify dart tag=pre+code %} var map = <String, int>{‘a’: 0, ‘b’: 1, ‘c’: 2}; {% endprettify %}
Classes can only extend other classes.
The analyzer produces this diagnostic when an extends clause contains a name that is declared to be something other than a class.
The following code produces this diagnostic because f is declared to be a function:
{% prettify dart tag=pre+code %} void f() {}
class C extends [!f!] {} {% endprettify %}
If you want the class to extend a class other than Object, then replace the name in the extends clause with the name of that class:
{% prettify dart tag=pre+code %} void f() {}
class C extends B {}
class B {} {% endprettify %}
If you want the class to extend Object, then remove the extends clause:
{% prettify dart tag=pre+code %} void f() {}
class C {} {% endprettify %}
Extension ‘{0}’ can't be used as an expression.
The analyzer produces this diagnostic when the name of an extension is used in an expression other than in an extension override or to qualify an access to a static member of the extension. Because classes define a type, the name of a class can be used to refer to the instance of Type representing the type of the class. Extensions, on the other hand, don‘t define a type and can’t be used as a type literal.
The following code produces this diagnostic because E is an extension:
{% prettify dart tag=pre+code %} extension E on int { static String m() => ''; }
var x = [!E!]; {% endprettify %}
Replace the name of the extension with a name that can be referenced, such as a static member defined on the extension:
{% prettify dart tag=pre+code %} extension E on int { static String m() => ''; }
var x = E.m(); {% endprettify %}
An extension can't define static member ‘{0}’ and an instance member with the same name.
The analyzer produces this diagnostic when an extension declaration contains both an instance member and a static member that have the same name. The instance member and the static member can‘t have the same name because it’s unclear which member is being referenced by an unqualified use of the name within the body of the extension.
The following code produces this diagnostic because the name a is being used for two different members:
{% prettify dart tag=pre+code %} extension E on Object { int get a => 0; static int !a! => 0; } {% endprettify %}
Rename or remove one of the members:
{% prettify dart tag=pre+code %} extension E on Object { int get a => 0; static int b() => 0; } {% endprettify %}
Extensions can't declare abstract members.
The analyzer produces this diagnostic when an abstract declaration is declared in an extension. Extensions can declare only concrete members.
The following code produces this diagnostic because the method a doesn't have a body:
{% prettify dart tag=pre+code %} extension E on String { int !a!; } {% endprettify %}
Either provide an implementation for the member or remove it.
Extensions can't declare constructors.
The analyzer produces this diagnostic when a constructor declaration is found in an extension. It isn‘t valid to define a constructor because extensions aren’t classes, and it isn't possible to create an instance of an extension.
The following code produces this diagnostic because there is a constructor declaration in E:
{% prettify dart tag=pre+code %} extension E on String { !E! : super(); } {% endprettify %}
Remove the constructor or replace it with a static method.
Extensions can't declare instance fields
The analyzer produces this diagnostic when an instance field declaration is found in an extension. It isn't valid to define an instance field because extensions can only add behavior, not state.
The following code produces this diagnostic because s is an instance field:
{% prettify dart tag=pre+code %} extension E on String { String [!s!]; } {% endprettify %}
Remove the field, make it a static field, or convert it to be a getter, setter, or method.
Extensions can't declare members with the same name as a member declared by ‘Object’.
The analyzer produces this diagnostic when an extension declaration declares a member with the same name as a member declared in the class Object. Such a member can never be used because the member in Object is always found first.
The following code produces this diagnostic because toString is defined by Object:
{% prettify dart tag=pre+code %} extension E on String { String !toString! => this; } {% endprettify %}
Remove the member or rename it so that the name doesn't conflict with the member in Object:
{% prettify dart tag=pre+code %} extension E on String { String displayString() => this; } {% endprettify %}
An extension override can't be used to access a static member from an extension.
The analyzer produces this diagnostic when an extension override is the receiver of the invocation of a static member. Similar to static members in classes, the static members of an extension should be accessed using the name of the extension, not an extension override.
The following code produces this diagnostic because m is static:
{% prettify dart tag=pre+code %} extension E on String { static void m() {} }
void f() { E('').!m!; } {% endprettify %}
Replace the extension override with the name of the extension:
{% prettify dart tag=pre+code %} extension E on String { static void m() {} }
void f() { E.m(); } {% endprettify %}
The type of the argument to the extension override ‘{0}’ isn't assignable to the extended type ‘{1}’.
The analyzer produces this diagnostic when the argument to an extension override isn't assignable to the type being extended by the extension.
The following code produces this diagnostic because 3 isn't a String:
{% prettify dart tag=pre+code %} extension E on String { void method() {} }
void f() { E([!3!]).method(); } {% endprettify %}
If you're using the correct extension, then update the argument to have the correct type:
{% prettify dart tag=pre+code %} extension E on String { void method() {} }
void f() { E(3.toString()).method(); } {% endprettify %}
If there‘s a different extension that’s valid for the type of the argument, then either replace the name of the extension or unwrap the argument so that the correct extension is found.
An extension override can only be used to access instance members.
The analyzer produces this diagnostic when an extension override is found that isn‘t being used to access one of the members of the extension. The extension override syntax doesn’t have any runtime semantics; it only controls which member is selected at compile time.
The following code produces this diagnostic because E(i) isn't an expression:
{% prettify dart tag=pre+code %} extension E on int { int get a => 0; }
void f(int i) { print([!E(i)!]); } {% endprettify %}
If you want to invoke one of the members of the extension, then add the invocation:
{% prettify dart tag=pre+code %} extension E on int { int get a => 0; }
void f(int i) { print(E(i).a); } {% endprettify %}
If you don't want to invoke a member, then unwrap the argument:
{% prettify dart tag=pre+code %} extension E on int { int get a => 0; }
void f(int i) { print(i); } {% endprettify %}
Extension overrides have no value so they can't be used as the receiver of a cascade expression.
The analyzer produces this diagnostic when an extension override is used as the receiver of a cascade expression. The value of a cascade expression e..m is the value of the receiver e, but extension overrides aren‘t expressions and don’t have a value.
The following code produces this diagnostic because E(3) isn't an expression:
{% prettify dart tag=pre+code %} extension E on int { void m() {} } f() { !E!..m(); } {% endprettify %}
Use . rather than ..:
{% prettify dart tag=pre+code %} extension E on int { void m() {} } f() { E(3).m(); } {% endprettify %}
If there are multiple cascaded accesses, you'll need to duplicate the extension override for each one.
External fields can't have initializers.
External variables can't have initializers.
The analyzer produces this diagnostic when a field or variable marked with the keyword external has an initializer, or when an external field is initialized in a constructor.
The following code produces this diagnostic because the external field x is assigned a value in an initializer:
{% prettify dart tag=pre+code %} class C { external int x; C() : [!x!] = 0; } {% endprettify %}
The following code produces this diagnostic because the external field x has an initializer:
{% prettify dart tag=pre+code %} class C { external final int [!x!] = 0; } {% endprettify %}
The following code produces this diagnostic because the external top level variable x has an initializer:
{% prettify dart tag=pre+code %} external final int [!x!] = 0; {% endprettify %}
Remove the initializer:
{% prettify dart tag=pre+code %} class C { external final int x; } {% endprettify %}
Fields in a struct class must have exactly one annotation indicating the native type.
The analyzer produces this diagnostic when a field in a subclass of Struct has more than one annotation describing the native type of the field.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field x has two annotations describing the native type of the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int32() [!@Int16()!] external int x; } {% endprettify %}
Remove all but one of the annotations:
{% prettify dart tag=pre+code %} import ‘dart:ffi’; class C extends Struct { @Int32() external int x; } {% endprettify %}
Too many positional arguments: {0} expected, but {1} found.
The analyzer produces this diagnostic when a method or function invocation has more positional arguments than the method or function allows.
The following code produces this diagnostic because f defines 2 parameters but is invoked with 3 arguments:
{% prettify dart tag=pre+code %} void f(int a, int b) {} void g() { f(1, 2, [!3!]); } {% endprettify %}
Remove the arguments that don't correspond to parameters:
{% prettify dart tag=pre+code %} void f(int a, int b) {} void g() { f(1, 2); } {% endprettify %}
Too many positional arguments: {0} expected, but {1} found.
The analyzer produces this diagnostic when a method or function invocation has more positional arguments than the method or function allows, but the method or function defines named parameters.
The following code produces this diagnostic because f defines 2 positional parameters but has a named parameter that could be used for the third argument:
{% prettify dart tag=pre+code %} void f(int a, int b, {int c}) {} void g() { f(1, 2, [!3!]); } {% endprettify %}
If some of the arguments should be values for named parameters, then add the names before the arguments:
{% prettify dart tag=pre+code %} void f(int a, int b, {int c}) {} void g() { f(1, 2, c: 3); } {% endprettify %}
Otherwise, remove the arguments that don't correspond to positional parameters:
{% prettify dart tag=pre+code %} void f(int a, int b, {int c}) {} void g() { f(1, 2); } {% endprettify %}
‘Array’s must have exactly one ‘Array’ annotation.
The analyzer produces this diagnostic when a field in a subclass of Struct has more than one annotation describing the size of the native array.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field a0 has two annotations that specify the size of the native array:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(4) [!@Array(8)!] external Array a0; } {% endprettify %}
Remove all but one of the annotations:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(8) external Array a0; } {% endprettify %}
The field ‘{0}’ can't be initialized twice in the same constructor.
The analyzer produces this diagnostic when the initializer list of a constructor initializes a field more than once. There is no value to allow both initializers because only the last value is preserved.
The following code produces this diagnostic because the field f is being initialized twice:
{% prettify dart tag=pre+code %} class C { int f;
C() : f = 0, [!f!] = 1; } {% endprettify %}
Remove one of the initializers:
{% prettify dart tag=pre+code %} class C { int f;
C() : f = 0; } {% endprettify %}
Fields can't be initialized in the constructor if they are final and were already initialized at their declaration.
The analyzer produces this diagnostic when a final field is initialized in both the declaration of the field and in an initializer in a constructor. Final fields can only be assigned once, so it can't be initialized in both places.
The following code produces this diagnostic because f is :
{% prettify dart tag=pre+code %} class C { final int f = 0; C() : [!f!] = 1; } {% endprettify %}
If the initialization doesn't depend on any values passed to the constructor, and if all of the constructors need to initialize the field to the same value, then remove the initializer from the constructor:
{% prettify dart tag=pre+code %} class C { final int f = 0; C(); } {% endprettify %}
If the initialization depends on a value passed to the constructor, or if different constructors need to initialize the field differently, then remove the initializer in the field's declaration:
{% prettify dart tag=pre+code %} class C { final int f; C() : f = 1; } {% endprettify %}
Fields can't be initialized in both the parameter list and the initializers.
The analyzer produces this diagnostic when a field is initialized in both the parameter list and in the initializer list of a constructor.
The following code produces this diagnostic because the field f is initialized both by an initializing formal parameter and in the initializer list:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f) : [!f!] = 0; } {% endprettify %}
If the field should be initialized by the parameter, then remove the initialization in the initializer list:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f); } {% endprettify %}
If the field should be initialized in the initializer list and the parameter isn't needed, then remove the parameter:
{% prettify dart tag=pre+code %} class C { int f;
C() : f = 0; } {% endprettify %}
If the field should be initialized in the initializer list and the parameter is needed, then make it a normal parameter:
{% prettify dart tag=pre+code %} class C { int f;
C(int g) : f = g * 2; } {% endprettify %}
Initializing formal parameters can't be used in factory constructors.
The analyzer produces this diagnostic when a factory constructor has an initializing formal parameter. Factory constructors can't assign values to fields because no instance is created; hence, there is no field to assign.
The following code produces this diagnostic because the factory constructor uses an initializing formal parameter:
{% prettify dart tag=pre+code %} class C { int? f;
factory C([!this.f!]) => throw 0; } {% endprettify %}
Replace the initializing formal parameter with a normal parameter:
{% prettify dart tag=pre+code %} class C { int? f;
factory C(int f) => throw 0; } {% endprettify %}
Constructors in subclasses of ‘Struct’ and ‘Union’ can't have field initializers.
The analyzer produces this diagnostic when a constructor in a subclass of either Struct or Union has one or more field initializers.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class C has a constructor with an initializer for the field f:
{% prettify dart tag=pre+code %} // @dart = 2.9 import ‘dart:ffi’;
class C extends Struct { @Int32() int f;
C() : [!f = 0!]; } {% endprettify %}
Remove the field initializer:
{% prettify dart tag=pre+code %} // @dart = 2.9 import ‘dart:ffi’;
class C extends Struct { @Int32() int f;
C(); } {% endprettify %}
The initializer type ‘{0}’ can't be assigned to the field type ‘{1}’ in a const constructor.
The initializer type ‘{0}’ can't be assigned to the field type ‘{1}’.
The analyzer produces this diagnostic when the initializer list of a constructor initializes a field to a value that isn't assignable to the field.
The following code produces this diagnostic because 0 has the type int, and an int can't be assigned to a field of type String:
{% prettify dart tag=pre+code %} class C { String s;
C() : s = [!0!]; } {% endprettify %}
If the type of the field is correct, then change the value assigned to it so that the value has a valid type:
{% prettify dart tag=pre+code %} class C { String s;
C() : s = ‘0’; } {% endprettify %}
If the type of the value is correct, then change the type of the field to allow the assignment:
{% prettify dart tag=pre+code %} class C { int s;
C() : s = 0; } {% endprettify %}
Field formal parameters can only be used in a constructor.
Initializing formal parameters can only be used in constructors.
The analyzer produces this diagnostic when an initializing formal parameter is used in the parameter list for anything other than a constructor.
The following code produces this diagnostic because the initializing formal parameter this.x is being used in the method m:
{% prettify dart tag=pre+code %} class A { int x = 0;
m([[!this.x!] = 0]) {} } {% endprettify %}
Replace the initializing formal parameter with a normal parameter and assign the field within the body of the method:
{% prettify dart tag=pre+code %} class A { int x = 0;
m([int x = 0]) { this.x = x; } } {% endprettify %}
The redirecting constructor can't have a field initializer.
The analyzer produces this diagnostic when a redirecting constructor initializes a field in the object. This isn‘t allowed because the instance that has the field hasn’t been created at the point at which it should be initialized.
The following code produces this diagnostic because the constructor C.zero, which redirects to the constructor C, has an initializing formal parameter that initializes the field f:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f);
C.zero([!this.f!]) : this(f); } {% endprettify %}
The following code produces this diagnostic because the constructor C.zero, which redirects to the constructor C, has an initializer that initializes the field f:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f);
C.zero() : [!f = 0!], this(1); } {% endprettify %}
If the initialization is done by an initializing formal parameter, then use a normal parameter:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f);
C.zero(int f) : this(f); } {% endprettify %}
If the initialization is done in an initializer, then remove the initializer:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f);
C.zero() : this(0); } {% endprettify %}
The parameter type ‘{0}’ is incompatible with the field type ‘{1}’.
The analyzer produces this diagnostic when the type of an initializing formal parameter isn't assignable to the type of the field being initialized.
The following code produces this diagnostic because the initializing formal parameter has the type String, but the type of the field is int. The parameter must have a type that is a subtype of the field's type.
{% prettify dart tag=pre+code %} class C { int f;
C([!String this.f!]); } {% endprettify %}
If the type of the field is incorrect, then change the type of the field to match the type of the parameter, and consider removing the type from the parameter:
{% prettify dart tag=pre+code %} class C { String f;
C(this.f); } {% endprettify %}
If the type of the parameter is incorrect, then remove the type of the parameter:
{% prettify dart tag=pre+code %} class C { int f;
C(this.f); } {% endprettify %}
If the types of both the field and the parameter are correct, then use an initializer rather than an initializing formal parameter to convert the parameter value into a value of the correct type:
{% prettify dart tag=pre+code %} class C { int f;
C(String s) : f = int.parse(s); } {% endprettify %}
Fields in subclasses of ‘Struct’ and ‘Union’ can't have initializers.
The analyzer produces this diagnostic when a field in a subclass of Struct has an initializer.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field p has an initializer:
{% prettify dart tag=pre+code %} // @dart = 2.9 import ‘dart:ffi’;
class C extends Struct { Pointer [!p!] = nullptr; } {% endprettify %}
Remove the initializer:
{% prettify dart tag=pre+code %} // @dart = 2.9 import ‘dart:ffi’;
class C extends Struct { Pointer p; } {% endprettify %}
Fields of ‘Struct’ and ‘Union’ subclasses must be marked external.
The analyzer produces this diagnostic when a field in a subclass of either Struct or Union isn't marked as being external.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field a isn't marked as being external:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int16() int [!a!]; } {% endprettify %}
Add the required external modifier:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int16() external int a; } {% endprettify %}
‘{0}’ is final and was given a value when it was declared, so it can't be set to a new value.
The analyzer produces this diagnostic when a final field is initialized twice: once where it‘s declared and once by a constructor’s parameter.
The following code produces this diagnostic because the field f is initialized twice:
{% prettify dart tag=pre+code %} class C { final int f = 0;
C(this.[!f!]); } {% endprettify %}
If the field should have the same value for all instances, then remove the initialization in the parameter list:
{% prettify dart tag=pre+code %} class C { final int f = 0;
C(); } {% endprettify %}
If the field can have different values in different instances, then remove the initialization in the declaration:
{% prettify dart tag=pre+code %} class C { final int f;
C(this.f); } {% endprettify %}
The final variable ‘{0}’ must be initialized.
The analyzer produces this diagnostic when a final field or variable isn't initialized.
The following code produces this diagnostic because x doesn't have an initializer:
{% prettify dart tag=pre+code %} final [!x!]; {% endprettify %}
For variables and static fields, you can add an initializer:
{% prettify dart tag=pre+code %} final x = 0; {% endprettify %}
For instance fields, you can add an initializer as shown in the previous example, or you can initialize the field in every constructor. You can initialize the field by using an initializing formal parameter:
{% prettify dart tag=pre+code %} class C { final int x; C(this.x); } {% endprettify %}
You can also initialize the field by using an initializer in the constructor:
{% prettify dart tag=pre+code %} class C { final int x; C(int y) : x = y * 2; } {% endprettify %}
All final variables must be initialized, but ‘{0}’ and ‘{1}’ aren't.
All final variables must be initialized, but ‘{0}’ isn't.
All final variables must be initialized, but ‘{0}’, ‘{1}’, and {2} others aren't.
The analyzer produces this diagnostic when a class defines one or more final instance fields without initializers and has at least one constructor that doesn‘t initialize those fields. All final instance fields must be initialized when the instance is created, either by the field’s initializer or by the constructor.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} class C { final String value;
!C!; } {% endprettify %}
If the value should be passed in to the constructor directly, then use an initializing formal parameter to initialize the field value:
{% prettify dart tag=pre+code %} class C { final String value;
C(this.value); } {% endprettify %}
If the value should be computed indirectly from a value provided by the caller, then add a parameter and include an initializer:
{% prettify dart tag=pre+code %} class C { final String value;
C(Object o) : value = o.toString(); } {% endprettify %}
If the value of the field doesn't depend on values that can be passed to the constructor, then add an initializer for the field as part of the field declaration:
{% prettify dart tag=pre+code %} class C { final String value = '';
C(); } {% endprettify %}
If the value of the field doesn't depend on values that can be passed to the constructor but different constructors need to initialize it to different values, then add an initializer for the field in the initializer list:
{% prettify dart tag=pre+code %} class C { final String value;
C() : value = '';
C.named() : value = ‘c’; } {% endprettify %}
However, if the value is the same for all instances, then consider using a static field instead of an instance field:
{% prettify dart tag=pre+code %} class C { static const String value = '';
C(); } {% endprettify %}
The value of the ‘flutter’ field is expected to be a map.
The analyzer produces this diagnostic when the value of the flutter key isn't a map.
The following code produces this diagnostic because the value of the top-level flutter key is a string:
name: example flutter: true
If you need to specify Flutter-specific options, then change the value to be a map:
name: example flutter: uses-material-design: true
If you don't need to specify Flutter-specific options, then remove the flutter key:
name: example
The type ‘{0}’ used in the ‘for’ loop must implement ‘{1}’ with a type argument that can be assigned to ‘{2}’.
The analyzer produces this diagnostic when the Iterable or Stream in a for-in loop has an element type that can't be assigned to the loop variable.
The following code produces this diagnostic because <String>[] has an element type of String, and String can't be assigned to the type of e (int):
{% prettify dart tag=pre+code %} void f() { for (int e in [![]!]) { print(e); } } {% endprettify %}
If the type of the loop variable is correct, then update the type of the iterable:
{% prettify dart tag=pre+code %} void f() { for (int e in []) { print(e); } } {% endprettify %}
If the type of the iterable is correct, then update the type of the loop variable:
{% prettify dart tag=pre+code %} void f() { for (String e in []) { print(e); } } {% endprettify %}
The type ‘{0}’ used in the ‘for’ loop must implement {1}.
The analyzer produces this diagnostic when the expression following in in a for-in loop has a type that isn't a subclass of Iterable.
The following code produces this diagnostic because m is a Map, and Map isn't a subclass of Iterable:
{% prettify dart tag=pre+code %} void f(Map<String, String> m) { for (String s in [!m!]) { print(s); } } {% endprettify %}
Replace the expression with one that produces an iterable value:
{% prettify dart tag=pre+code %} void f(Map<String, String> m) { for (String s in m.values) { print(s); } } {% endprettify %}
A for-in loop variable can't be a ‘const’.
The analyzer produces this diagnostic when the loop variable declared in a for-in loop is declared to be a const. The variable can‘t be a const because the value can’t be computed at compile time.
The following code produces this diagnostic because the loop variable x is declared to be a const:
{% prettify dart tag=pre+code %} void f() { for ([!const!] x in [0, 1, 2]) { print(x); } } {% endprettify %}
If there's a type annotation, then remove the const modifier from the declaration.
If there's no type, then replace the const modifier with final, var, or a type annotation:
{% prettify dart tag=pre+code %} void f() { for (final x in [0, 1, 2]) { print(x); } } {% endprettify %}
A method tear-off on a receiver whose type is ‘dynamic’ can't have type arguments.
The analyzer produces this diagnostic when an instance method is being torn off from a receiver whose type is dynamic, and the tear-off includes type arguments. Because the analyzer can‘t know how many type parameters the method has, or whether it has any type parameters, there’s no way it can validate that the type arguments are correct. As a result, the type arguments aren't allowed.
The following code produces this diagnostic because the type of p is dynamic and the tear-off of m has type arguments:
{% prettify dart tag=pre+code %} void f(dynamic list) { [!list.fold!]; } {% endprettify %}
If you can use a more specific type than dynamic, then change the type of the receiver:
{% prettify dart tag=pre+code %} void f(List list) { list.fold; } {% endprettify %}
If you can't use a more specific type, then remove the type arguments:
{% prettify dart tag=pre+code %} void f(dynamic list) { list.cast; } {% endprettify %}
The class ‘{0}’ can't extend ‘Struct’ or ‘Union’ because ‘{0}’ is generic.
The analyzer produces this diagnostic when a subclass of either Struct or Union has a type parameter.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class S defines the type parameter T:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class [!S!] extends Struct { external Pointer notEmpty; } {% endprettify %}
Remove the type parameters from the class:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class S extends Struct { external Pointer notEmpty; } {% endprettify %}
The return type of getter ‘{0}’ is ‘{1}’ which isn't a subtype of the type ‘{2}’ of its setter ‘{3}’.
The analyzer produces this diagnostic when the return type of a getter isn't a subtype of the type of the parameter of a setter with the same name.
The subtype relationship is a requirement whether the getter and setter are in the same class or whether one of them is in a superclass of the other.
The following code produces this diagnostic because the return type of the getter x is num, the parameter type of the setter x is int, and num isn't a subtype of int:
{% prettify dart tag=pre+code %} class C { num get [!x!] => 0;
set x(int y) {} } {% endprettify %}
If the type of the getter is correct, then change the type of the setter:
{% prettify dart tag=pre+code %} class C { num get x => 0;
set x(num y) {} } {% endprettify %}
If the type of the setter is correct, then change the type of the getter:
{% prettify dart tag=pre+code %} class C { int get x => 0;
set x(int y) {} } {% endprettify %}
Functions marked ‘async*’ must have a return type that is a supertype of ‘Stream’ for some type ‘T’.
The analyzer produces this diagnostic when the body of a function has the async* modifier even though the return type of the function isn't either Stream or a supertype of Stream.
The following code produces this diagnostic because the body of the function f has the ‘async*’ modifier even though the return type int isn't a supertype of Stream:
{% prettify dart tag=pre+code %} [!int!] f() async* {} {% endprettify %}
If the function should be asynchronous, then change the return type to be either Stream or a supertype of Stream:
{% prettify dart tag=pre+code %} Stream f() async* {} {% endprettify %}
If the function should be synchronous, then remove the async* modifier:
{% prettify dart tag=pre+code %} int f() => 0; {% endprettify %}
Functions marked ‘async’ must have a return type assignable to ‘Future’.
The analyzer produces this diagnostic when the body of a function has the async modifier even though the return type of the function isn't assignable to Future.
The following code produces this diagnostic because the body of the function f has the async modifier even though the return type isn't assignable to Future:
{% prettify dart tag=pre+code %} [!int!] f() async { return 0; } {% endprettify %}
If the function should be asynchronous, then change the return type to be assignable to Future:
{% prettify dart tag=pre+code %} Future f() async { return 0; } {% endprettify %}
If the function should be synchronous, then remove the async modifier:
{% prettify dart tag=pre+code %} int f() => 0; {% endprettify %}
Functions marked ‘sync*’ must have a return type that is a supertype of ‘Iterable’ for some type ‘T’.
The analyzer produces this diagnostic when the body of a function has the sync* modifier even though the return type of the function isn't either Iterable or a supertype of Iterable.
The following code produces this diagnostic because the body of the function f has the ‘sync*’ modifier even though the return type int isn't a supertype of Iterable:
{% prettify dart tag=pre+code %} [!int!] f() sync* {} {% endprettify %}
If the function should return an iterable, then change the return type to be either Iterable or a supertype of Iterable:
{% prettify dart tag=pre+code %} Iterable f() sync* {} {% endprettify %}
If the function should return a single value, then remove the sync* modifier:
{% prettify dart tag=pre+code %} int f() => 0; {% endprettify %}
Classes and mixins can only implement other classes and mixins.
The analyzer produces this diagnostic when a name used in the implements clause of a class or mixin declaration is defined to be something other than a class or mixin.
The following code produces this diagnostic because x is a variable rather than a class or mixin:
{% prettify dart tag=pre+code %} var x; class C implements [!x!] {} {% endprettify %}
If the name is the name of an existing class or mixin that‘s already being imported, then add a prefix to the import so that the local definition of the name doesn’t shadow the imported name.
If the name is the name of an existing class or mixin that isn't being imported, then add an import, with a prefix, for the library in which it’s declared.
Otherwise, either replace the name in the implements clause with the name of an existing class or mixin, or remove the name from the implements clause.
‘{0}’ can only be implemented once.
The analyzer produces this diagnostic when a single class is specified more than once in an implements clause.
The following code produces this diagnostic because A is in the list twice:
{% prettify dart tag=pre+code %} class A {} class B implements A, [!A!] {} {% endprettify %}
Remove all except one occurrence of the class name:
{% prettify dart tag=pre+code %} class A {} class B implements A {} {% endprettify %}
‘{0}’ can't be used in both the ‘extends’ and ‘implements’ clauses.
The analyzer produces this diagnostic when one class is listed in both the extends and implements clauses of another class.
The following code produces this diagnostic because the class A is used in both the extends and implements clauses for the class B:
{% prettify dart tag=pre+code %} class A {}
class B extends A implements [!A!] {} {% endprettify %}
If you want to inherit the implementation from the class, then remove the class from the implements clause:
{% prettify dart tag=pre+code %} class A {}
class B extends A {} {% endprettify %}
If you don't want to inherit the implementation from the class, then remove the extends clause:
{% prettify dart tag=pre+code %} class A {}
class B implements A {} {% endprettify %}
The instance member ‘{0}’ can't be accessed in an initializer.
The analyzer produces this diagnostic when it finds a reference to an instance member in a constructor's initializer list.
The following code produces this diagnostic because defaultX is an instance member:
{% prettify dart tag=pre+code %} class C { int x;
C() : x = [!defaultX!];
int get defaultX => 0; } {% endprettify %}
If the member can be made static, then do so:
{% prettify dart tag=pre+code %} class C { int x;
C() : x = defaultX;
static int get defaultX => 0; } {% endprettify %}
If not, then replace the reference in the initializer with a different expression that doesn't use an instance member:
{% prettify dart tag=pre+code %} class C { int x;
C() : x = 0;
int get defaultX => 0; } {% endprettify %}
The imported library defines a top-level function named ‘loadLibrary’ that is hidden by deferring this library.
The analyzer produces this diagnostic when a library that declares a function named loadLibrary is imported using a deferred import. A deferred import introduces an implicit function named loadLibrary. This function is used to load the contents of the deferred library, and the implicit function hides the explicit declaration in the deferred library.
For more information, see the language tour's coverage of deferred loading.
Given a file (a.dart) that defines a function named loadLibrary:
{% prettify dart tag=pre+code %} void loadLibrary(Library library) {}
class Library {} {% endprettify %}
The following code produces this diagnostic because the implicit declaration of a.loadLibrary is hiding the explicit declaration of loadLibrary in a.dart:
{% prettify dart tag=pre+code %} [!import ‘a.dart’ deferred as a;!]
void f() { a.Library(); } {% endprettify %}
If the imported library isn't required to be deferred, then remove the keyword deferred:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a;
void f() { a.Library(); } {% endprettify %}
If the imported library is required to be deferred and you need to reference the imported function, then rename the function in the imported library:
{% prettify dart tag=pre+code %} void populateLibrary(Library library) {}
class Library {} {% endprettify %}
If the imported library is required to be deferred and you don't need to reference the imported function, then add a hide clause:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a hide loadLibrary;
void f() { a.Library(); } {% endprettify %}
If type arguments shouldn't be required for the class, then mark the class with the [optionalTypeArgs][meta-optionalTypeArgs] annotation (from package:meta):
The library ‘{0}’ is internal and can't be imported.
The analyzer produces this diagnostic when it finds an import whose dart: URI references an internal library.
The following code produces this diagnostic because _interceptors is an internal library:
{% prettify dart tag=pre+code %} import [!‘dart:_interceptors’!]; {% endprettify %}
Remove the import directive.
The library ‘{0}’ is legacy, and shouldn't be imported into a null safe library.
The analyzer produces this diagnostic when a library that is null safe imports a library that isn't null safe.
Given a file named a.dart that contains the following:
{% prettify dart tag=pre+code %} // @dart = 2.9
class A {} {% endprettify %}
The following code produces this diagnostic because a library that null safe is importing a library that isn't null safe:
{% prettify dart tag=pre+code %} import [!‘a.dart’!];
A? f() => null; {% endprettify %}
If you can migrate the imported library to be null safe, then migrate it and update or remove the migrated library's language version.
If you can't migrate the imported library, then the importing library needs to have a language version that is before 2.12, when null safety was enabled by default.
The imported library ‘{0}’ can't have a part-of directive.
The analyzer produces this diagnostic when a part file is imported into a library.
Given a part file named part.dart containing the following:
{% prettify dart tag=pre+code %} part of lib;
class C{} {% endprettify %}
The following code produces this diagnostic because imported files can't have a part-of directive:
{% prettify dart tag=pre+code %} library lib;
import [!‘part.dart’!];
C c = C(); {% endprettify %}
Import the library that contains the part file rather than the part file itself.
Superinterfaces don't have a valid override for ‘{0}’: {1}.
The analyzer produces this diagnostic when a class inherits two or more conflicting signatures for a member and doesn't provide an implementation that satisfies all the inherited signatures.
The following code produces this diagnostic because C is inheriting the declaration of m from A, and that implementation isn‘t consistent with the signature of m that’s inherited from B:
{% prettify dart tag=pre+code %} class A { void m({int a}) {} }
class B { void m({int b}) {} }
class [!C!] extends A implements B { } {% endprettify %}
Add an implementation of the method that satisfies all the inherited signatures:
{% prettify dart tag=pre+code %} class A { void m({int a}) {} }
class B { void m({int b}) {} }
class C extends A implements B { void m({int a, int b}) {} } {% endprettify %}
Parts must have exactly the same language version override as the library.
The analyzer produces this diagnostic when a part file has a language version override comment that specifies a different language version than the one being used for the library to which the part belongs.
Given a part file named part.dart that contains the following:
{% prettify dart tag=pre+code %} // @dart = 2.6 part of ‘test.dart’; {% endprettify %}
The following code produces this diagnostic because the parts of a library must have the same language version as the defining compilation unit:
{% prettify dart tag=pre+code %} // @dart = 2.5 part [!‘part.dart’!]; {% endprettify %}
Remove the language version override from the part file, so that it implicitly uses the same version as the defining compilation unit:
{% prettify dart tag=pre+code %} part of ‘test.dart’; {% endprettify %}
If necessary, either adjust the language version override in the defining compilation unit to be appropriate for the code in the part, or migrate the code in the part file to be consistent with the new language version.
‘{0}’ isn't a field in the enclosing class.
The analyzer produces this diagnostic when a constructor initializes a field that isn‘t declared in the class containing the constructor. Constructors can’t initialize fields that aren't declared and fields that are inherited from superclasses.
The following code produces this diagnostic because the initializer is initializing x, but x isn't a field in the class:
{% prettify dart tag=pre+code %} class C { int y;
C() : [!x = 0!]; } {% endprettify %}
If a different field should be initialized, then change the name to the name of the field:
{% prettify dart tag=pre+code %} class C { int y;
C() : y = 0; } {% endprettify %}
If the field must be declared, then add a declaration:
{% prettify dart tag=pre+code %} class C { int x; int y;
C() : x = 0; } {% endprettify %}
‘{0}’ is a static field in the enclosing class. Fields initialized in a constructor can't be static.
The analyzer produces this diagnostic when a static field is initialized in a constructor using either an initializing formal parameter or an assignment in the initializer list.
The following code produces this diagnostic because the static field a is being initialized by the initializing formal parameter this.a:
{% prettify dart tag=pre+code %} class C { static int? a; C([!this.a!]); } {% endprettify %}
If the field should be an instance field, then remove the keyword static:
{% prettify dart tag=pre+code %} class C { int? a; C(this.a); } {% endprettify %}
If you intended to initialize an instance field and typed the wrong name, then correct the name of the field being initialized:
{% prettify dart tag=pre+code %} class C { static int? a; int? b; C(this.b); } {% endprettify %}
If you really want to initialize the static field, then move the initialization into the constructor body:
{% prettify dart tag=pre+code %} class C { static int? a; C(int? c) { a = c; } } {% endprettify %}
‘{0}’ isn't a field in the enclosing class.
The analyzer produces this diagnostic when an initializing formal parameter is found in a constructor in a class that doesn‘t declare the field being initialized. Constructors can’t initialize fields that aren't declared and fields that are inherited from superclasses.
The following code produces this diagnostic because the field x isn't defined:
{% prettify dart tag=pre+code %} class C { int y;
C([!this.x!]); } {% endprettify %}
If the field name was wrong, then change it to the name of an existing field:
{% prettify dart tag=pre+code %} class C { int y;
C(this.y); } {% endprettify %}
If the field name is correct but hasn't yet been defined, then declare the field:
{% prettify dart tag=pre+code %} class C { int x; int y;
C(this.x); } {% endprettify %}
If the parameter is needed but shouldn't initialize a field, then convert it to a normal parameter and use it:
{% prettify dart tag=pre+code %} class C { int y;
C(int x) : y = x * 2; } {% endprettify %}
If the parameter isn't needed, then remove it:
{% prettify dart tag=pre+code %} class C { int y;
C(); } {% endprettify %}
The static {1} ‘{0}’ can't be accessed through an instance.
The analyzer produces this diagnostic when an access operator is used to access a static member through an instance of the class.
The following code produces this diagnostic because zero is a static field, but it’s being accessed as if it were an instance field:
{% prettify dart tag=pre+code %} void f(C c) { c.[!zero!]; }
class C { static int zero = 0; } {% endprettify %}
Use the class to access the static member:
{% prettify dart tag=pre+code %} void f(C c) { C.zero; }
class C { static int zero = 0; } {% endprettify %}
Instance members can't be accessed from a factory constructor.
The analyzer produces this diagnostic when a factory constructor contains an unqualified reference to an instance member. In a generative constructor, the instance of the class is created and initialized before the body of the constructor is executed, so the instance can be bound to this and accessed just like it would be in an instance method. But, in a factory constructor, the instance isn‘t created before executing the body, so this can’t be used to reference it.
The following code produces this diagnostic because x isn't in scope in the factory constructor:
{% prettify dart tag=pre+code %} class C { int x; factory C() { return C.([!x!]); } C.(this.x); } {% endprettify %}
Rewrite the code so that it doesn't reference the instance member:
{% prettify dart tag=pre+code %} class C { int x; factory C() { return C.(0); } C.(this.x); } {% endprettify %}
Instance members can't be accessed from a static method.
The analyzer produces this diagnostic when a static method contains an unqualified reference to an instance member.
The following code produces this diagnostic because the instance field x is being referenced in a static method:
{% prettify dart tag=pre+code %} class C { int x;
static int m() { return [!x!]; } } {% endprettify %}
If the method must reference the instance member, then it can't be static, so remove the keyword:
{% prettify dart tag=pre+code %} class C { int x;
int m() { return x; } } {% endprettify %}
If the method can't be made an instance method, then add a parameter so that an instance of the class can be passed in:
{% prettify dart tag=pre+code %} class C { int x;
static int m(C c) { return c.x; } } {% endprettify %}
Abstract classes can't be instantiated.
The analyzer produces this diagnostic when it finds a constructor invocation and the constructor is declared in an abstract class. Even though you can't create an instance of an abstract class, abstract classes can declare constructors that can be invoked by subclasses.
The following code produces this diagnostic because C is an abstract class:
{% prettify dart tag=pre+code %} abstract class C {}
var c = new !C!; {% endprettify %}
If there's a concrete subclass of the abstract class that can be used, then create an instance of the concrete subclass.
Enums can't be instantiated.
The analyzer produces this diagnostic when an enum is instantiated. It's invalid to create an instance of an enum by invoking a constructor; only the instances named in the declaration of the enum can exist.
The following code produces this diagnostic because the enum E is being instantiated:
{% prettify dart tag=pre+code %} // @dart = 2.16 enum E {a}
var e = !E!; {% endprettify %}
If you intend to use an instance of the enum, then reference one of the constants defined in the enum:
{% prettify dart tag=pre+code %} // @dart = 2.16 enum E {a}
var e = E.a; {% endprettify %}
If you intend to use an instance of a class, then use the name of that class in place of the name of the enum.
Type aliases that expand to a type parameter can't be instantiated.
The analyzer produces this diagnostic when a constructor invocation is found where the type being instantiated is a type alias for one of the type parameters of the type alias. This isn’t allowed because the value of the type parameter is a type rather than a class.
The following code produces this diagnostic because it creates an instance of A, even though A is a type alias that is defined to be equivalent to a type parameter:
{% prettify dart tag=pre+code %} typedef A = T;
void f() { const [!A!](); } {% endprettify %}
Use either a class name or a type alias defined to be a class, rather than a type alias defined to be a type parameter:
{% prettify dart tag=pre+code %} typedef A = C;
void f() { const A(); }
class C { const C(); } {% endprettify %}
The integer literal is being used as a double, but can't be represented as a 64-bit double without overflow or loss of precision: ‘{0}’.
The analyzer produces this diagnostic when an integer literal is being implicitly converted to a double, but can't be represented as a 64-bit double without overflow or loss of precision. Integer literals are implicitly converted to a double if the context requires the type double.
The following code produces this diagnostic because the integer value 9223372036854775807 can't be represented exactly as a double:
{% prettify dart tag=pre+code %} double x = [!9223372036854775807!]; {% endprettify %}
If you need to use the exact value, then use the class BigInt to represent the value:
{% prettify dart tag=pre+code %} var x = BigInt.parse(‘9223372036854775807’); {% endprettify %}
If you need to use a double, then change the value to one that can be represented exactly:
{% prettify dart tag=pre+code %} double x = 9223372036854775808; {% endprettify %}
The integer literal {0} can't be represented in 64 bits.
The analyzer produces this diagnostic when an integer literal has a value that is too large (positive) or too small (negative) to be represented in a 64-bit word.
The following code produces this diagnostic because the value can't be represented in 64 bits:
{% prettify dart tag=pre+code %} var x = [!9223372036854775810!]; {% endprettify %}
If you need to represent the current value, then wrap it in an instance of the class BigInt:
{% prettify dart tag=pre+code %} var x = BigInt.parse(‘9223372036854775810’); {% endprettify %}
Annotation must be either a const variable reference or const constructor invocation.
The analyzer produces this diagnostic when an annotation is found that is using something that is neither a variable marked as const or the invocation of a const constructor.
Getters can't be used as annotations.
The following code produces this diagnostic because the variable v isn't a const variable:
{% prettify dart tag=pre+code %} var v = 0;
[!@v!] void f() { } {% endprettify %}
The following code produces this diagnostic because f isn't a variable:
{% prettify dart tag=pre+code %} [!@f!] void f() { } {% endprettify %}
The following code produces this diagnostic because f isn't a constructor:
{% prettify dart tag=pre+code %} [!@f()!] void f() { } {% endprettify %}
The following code produces this diagnostic because g is a getter:
{% prettify dart tag=pre+code %} [!@g!] int get g => 0; {% endprettify %}
If the annotation is referencing a variable that isn‘t a const constructor, add the keyword const to the variable’s declaration:
{% prettify dart tag=pre+code %} const v = 0;
@v void f() { } {% endprettify %}
If the annotation isn't referencing a variable, then remove it:
{% prettify dart tag=pre+code %} int v = 0;
void f() { } {% endprettify %}
Constant values from a deferred library can't be used in annotations.
The analyzer produces this diagnostic when a constant defined in a library that is imported as a deferred library is referenced in the argument list of an annotation. Annotations are evaluated at compile time, and values from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
The following code produces this diagnostic because the constant pi is being referenced in the argument list of an annotation, even though the library that defines it is being imported as a deferred library:
{% prettify dart tag=pre+code %} import ‘dart:math’ deferred as math;
class C { const C(double d); }
@C([!math.pi!]) void f () {} {% endprettify %}
If you need to reference the imported constant, then remove the deferred keyword:
{% prettify dart tag=pre+code %} import ‘dart:math’ as math;
class C { const C(double d); }
@C(math.pi) void f () {} {% endprettify %}
If the import is required to be deferred and there's another constant that is appropriate, then use that constant in place of the constant from the deferred library.
Constant values from a deferred library can't be used as annotations.
The analyzer produces this diagnostic when a constant from a library that is imported using a deferred import is used as an annotation. Annotations are evaluated at compile time, and constants from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
The following code produces this diagnostic because the constant pi is being used as an annotation when the library dart:math is imported as deferred:
{% prettify dart tag=pre+code %} import ‘dart:math’ deferred as math;
@[!math.pi!] void f() {} {% endprettify %}
If you need to reference the constant as an annotation, then remove the keyword deferred from the import:
{% prettify dart tag=pre+code %} import ‘dart:math’ as math;
@math.pi void f() {} {% endprettify %}
If you can use a different constant as an annotation, then replace the annotation with a different constant:
{% prettify dart tag=pre+code %} @deprecated void f() {} {% endprettify %}
The annotation ‘{0}’ can only be used on {1}.
The analyzer produces this diagnostic when an annotation is applied to a kind of declaration that it doesn't support.
The following code produces this diagnostic because the optionalTypeArgs annotation isn't defined to be valid for top-level variables:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@[!optionalTypeArgs!] int x = 0; {% endprettify %}
Remove the annotation from the declaration.
A value of type ‘{0}’ can't be assigned to a variable of type ‘{1}’.
The analyzer produces this diagnostic when the static type of an expression that is assigned to a variable isn't assignable to the type of the variable.
The following code produces this diagnostic because the type of the initializer (int) isn't assignable to the type of the variable (String):
{% prettify dart tag=pre+code %} int i = 0; String s = [!i!]; {% endprettify %}
If the value being assigned is always assignable at runtime, even though the static types don't reflect that, then add an explicit cast.
Otherwise, change the value being assigned so that it has the expected type. In the previous example, this might look like:
{% prettify dart tag=pre+code %} int i = 0; String s = i.toString(); {% endprettify %}
If you can’t change the value, then change the type of the variable to be compatible with the type of the value being assigned:
{% prettify dart tag=pre+code %} int i = 0; int s = i; {% endprettify %}
Publishable packages can't have ‘{0}’ dependencies.
The analyzer produces this diagnostic when a package under either dependencies or dev_dependencies isn't a pub, git, or path based dependency.
See Package dependencies for more information about the kind of dependencies that are supported.
The following code produces this diagnostic because the dependency on the package transmogrify isn't a pub, git, or path based dependency:
name: example dependencies: transmogrify: hosted: name: transmogrify url: http://your-package-server.com version: ^1.4.0
If you want to publish your package to pub.dev, then change the dependencies to ones that are supported by pub.
If you don‘t want to publish your package to pub.dev, then add a publish_to: none entry to mark the package as one that isn’t intended to be published:
name: example publish_to: none dependencies: transmogrify: hosted: name: transmogrify url: http://your-package-server.com version: ^1.4.0
The method ‘Pointer.fromFunction’ can't have an exceptional return value (the second argument) when the return type of the function is either ‘void’, ‘Handle’ or ‘Pointer’.
The analyzer produces this diagnostic when an invocation of the method Pointer.fromFunction has a second argument (the exceptional return value) and the type to be returned from the invocation is either void, Handle or Pointer.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because a second argument is provided when the return type of f is void:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef T = Void Function(Int8);
void f(int i) {}
void g() { Pointer.fromFunction(f, [!42!]); } {% endprettify %}
Remove the exception value:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef T = Void Function(Int8);
void f(int i) {}
void g() { Pointer.fromFunction(f); } {% endprettify %}
The member ‘{0}’ can‘t be exported as a part of a package’s public API.
The analyzer produces this diagnostic when a public library exports a declaration that is marked with the [internal][meta-internal] annotation.
Given a file named a.dart in the src directory that contains:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@internal class One {} {% endprettify %}
The following code, when found in a public library produces this diagnostic because the export directive is exporting a name that is only intended to be used internally:
{% prettify dart tag=pre+code %} [!export ‘src/a.dart’;!] {% endprettify %}
If the export is needed, then add a hide clause to hide the internal names:
{% prettify dart tag=pre+code %} export ‘src/a.dart’ hide One; {% endprettify %}
If the export isn't needed, then remove it.
The member ‘{0}’ can‘t be exported as a part of a package’s public API, but is indirectly exported as part of the signature of ‘{1}’.
The analyzer produces this diagnostic when a public library exports a top-level function with a return type or at least one parameter type that is marked with the [internal][meta-internal] annotation.
Given a file named a.dart in the src directory that contains the following:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@internal typedef IntFunction = int Function();
int f(IntFunction g) => g(); {% endprettify %}
The following code produces this diagnostic because the function f has a parameter of type IntFunction, and IntFunction is only intended to be used internally:
{% prettify dart tag=pre+code %} [!export ‘src/a.dart’ show f;!] {% endprettify %}
If the function must be public, then make all the types in the function's signature public types.
If the function doesn't need to be exported, then stop exporting it, either by removing it from the show clause, adding it to the hide clause, or by removing the export.
Extension overrides must have exactly one argument: the value of ‘this’ in the extension method.
The analyzer produces this diagnostic when an extension override doesn't have exactly one argument. The argument is the expression used to compute the value of this within the extension method, so there must be one argument.
The following code produces this diagnostic because there are no arguments:
{% prettify dart tag=pre+code %} extension E on String { String join(String other) => ‘$this $other’; }
void f() { E[!()!].join(‘b’); } {% endprettify %}
And, the following code produces this diagnostic because there's more than one argument:
{% prettify dart tag=pre+code %} extension E on String { String join(String other) => ‘$this $other’; }
void f() { E[!(‘a’, ‘b’)!].join(‘c’); } {% endprettify %}
Provide one argument for the extension override:
{% prettify dart tag=pre+code %} extension E on String { String join(String other) => ‘$this $other’; }
void f() { E(‘a’).join(‘b’); } {% endprettify %}
Factory method ‘{0}’ must have a return type.
The analyzer produces this diagnostic when a method that is annotated with the [factory][meta-factory] annotation has a return type of void.
The following code produces this diagnostic because the method createC is annotated with the [factory][meta-factory] annotation but doesn't return any value:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class Factory { @factory void !createC! {} }
class C {} {% endprettify %}
Change the return type to something other than void:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class Factory { @factory C createC() => C(); }
class C {} {% endprettify %}
Factory method ‘{0}’ doesn't return a newly allocated object.
The analyzer produces this diagnostic when a method that is annotated with the [factory][meta-factory] annotation doesn't return a newly allocated object.
The following code produces this diagnostic because the method createC returns the value of a field rather than a newly created instance of C:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class Factory { C c = C();
@factory C !createC! => c; }
class C {} {% endprettify %}
Change the method to return a newly created instance of the return type:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class Factory { @factory C createC() => C(); }
class C {} {% endprettify %}
The name of a factory constructor must be the same as the name of the immediately enclosing class.
The analyzer produces this diagnostic when the name of a factory constructor isn't the same as the name of the surrounding class.
The following code produces this diagnostic because the name of the factory constructor (A) isn't the same as the surrounding class (C):
{% prettify dart tag=pre+code %} class A {}
class C { factory !A! => throw 0; } {% endprettify %}
If the factory returns an instance of the surrounding class, then rename the factory:
{% prettify dart tag=pre+code %} class A {}
class C { factory C() => throw 0; } {% endprettify %}
If the factory returns an instance of a different class, then move the factory to that class:
{% prettify dart tag=pre+code %} class A { factory A() => throw 0; }
class C {} {% endprettify %}
If the factory returns an instance of a different class, but you can‘t modify that class or don’t want to move the factory, then convert it to be a static method:
{% prettify dart tag=pre+code %} class A {}
class C { static A a() => throw 0; } {% endprettify %}
Fields in struct classes can't have the type ‘{0}’. They can only be declared as ‘int’, ‘double’, ‘Array’, ‘Pointer’, or subtype of ‘Struct’ or ‘Union’.
The analyzer produces this diagnostic when a field in a subclass of Struct has a type other than int, double, Array, Pointer, or subtype of Struct or Union.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field str has the type String, which isn't one of the allowed types for fields in a subclass of Struct:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { external [!String!] s;
@Int32() external int i; } {% endprettify %}
Use one of the allowed types for the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’; import ‘package:ffi/ffi.dart’;
class C extends Struct { external Pointer s;
@Int32() external int i; } {% endprettify %}
‘{1}.{0}’ (‘{2}’) isn‘t a valid concrete implementation of ‘{3}.{0}’ (’{4}').
The analyzer produces this diagnostic when all of the following are true:
The concrete implementation can be invalid because of incompatibilities in either the return type, the types of parameters, or the type variables.
The following code produces this diagnostic because the method A.add has a parameter of type int, and the overriding method B.add has a corresponding parameter of type num:
{% prettify dart tag=pre+code %} class A { int add(int a) => a; } class [!B!] extends A { int add(num a); } {% endprettify %}
This is a problem because in an invocation of B.add like the following:
{% prettify dart tag=pre+code %} void f(B b) { b.add(3.4); } {% endprettify %}
B.add is expecting to be able to take, for example, a double, but when the method A.add is executed (because it‘s the only concrete implementation of add), a runtime exception will be thrown because a double can’t be assigned to a parameter of type int.
If the method in the subclass can conform to the implementation in the superclass, then change the declaration in the subclass (or remove it if it's the same):
{% prettify dart tag=pre+code %} class A { int add(int a) => a; } class B extends A { int add(int a); } {% endprettify %}
If the method in the superclass can be generalized to be a valid implementation of the method in the subclass, then change the superclass method:
{% prettify dart tag=pre+code %} class A { int add(num a) => a.floor(); } class B extends A { int add(num a); } {% endprettify %}
If neither the method in the superclass nor the method in the subclass can be changed, then provide a concrete implementation of the method in the subclass:
{% prettify dart tag=pre+code %} class A { int add(int a) => a; } class B extends A { int add(num a) => a.floor(); } {% endprettify %}
Inline function types can't be used for parameters in a generic function type.
The analyzer produces this diagnostic when a generic function type has a function-valued parameter that is written using the older inline function type syntax.
The following code produces this diagnostic because the parameter f, in the generic function type used to define F, uses the inline function type syntax:
{% prettify dart tag=pre+code %} typedef F = int Function(int f[!(!]String s)); {% endprettify %}
Use the generic function syntax for the parameter's type:
{% prettify dart tag=pre+code %} typedef F = int Function(int Function(String)); {% endprettify %}
Only public elements in a package's private API can be annotated as being internal.
The analyzer produces this diagnostic when a declaration is annotated with the [internal][meta-internal] annotation and that declaration is either in a public library or has a private name.
The following code, when in a public library, produces this diagnostic because the [internal][meta-internal] annotation can't be applied to declarations in a public library:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
[!@internal!] class C {} {% endprettify %}
The following code, whether in a public or internal library, produces this diagnostic because the [internal][meta-internal] annotation can't be applied to declarations with private names:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
[!@internal!] class _C {}
void f(_C c) {} {% endprettify %}
If the declaration has a private name, then remove the annotation:
{% prettify dart tag=pre+code %} class _C {}
void f(_C c) {} {% endprettify %}
If the declaration has a public name and is intended to be internal to the package, then move the annotated declaration into an internal library (in other words, a library inside the src directory).
Otherwise, remove the use of the annotation:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
The Dart language version override comment can't be followed by any non-whitespace characters.
The Dart language version override comment must be specified with a version number, like ‘2.0’, after the ‘=’ character.
The Dart language version override comment must be specified with an ‘=’ character.
The Dart language version override comment must be specified with exactly two slashes.
The Dart language version override comment must be specified with the word ‘dart’ in all lower case.
The Dart language version override number can't be prefixed with a letter.
The Dart language version override number must begin with ‘@dart’.
The language version override can't specify a version greater than the latest known language version: {0}.{1}.
The language version override must be specified before any declaration or directive.
The analyzer produces this diagnostic when a comment that appears to be an attempt to specify a language version override doesn't conform to the requirements for such a comment. For more information, see Per-library language version selection.
The following code produces this diagnostic because the word dart must be lowercase in such a comment and because there's no equal sign between the word dart and the version number:
{% prettify dart tag=pre+code %} [!// @Dart 2.9!] {% endprettify %}
If the comment is intended to be a language version override, then change the comment to follow the correct format:
{% prettify dart tag=pre+code %} // @dart = 2.9 {% endprettify %}
Only const constructors can have the @literal annotation.
The analyzer produces this diagnostic when the [literal][[meta-literal]] annotation is applied to anything other than a const constructor.
The following code produces this diagnostic because the constructor isn't a const constructor:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { [!@literal!] C(); } {% endprettify %}
The following code produces this diagnostic because x isn't a constructor:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
[!@literal!] var x; {% endprettify %}
If the annotation is on a constructor and the constructor should always be invoked with const, when possible, then mark the constructor with the const keyword:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { @literal const C(); } {% endprettify %}
If the constructor can't be marked as const, then remove the annotation.
If the annotation is on anything other than a constructor, then remove the annotation:
{% prettify dart tag=pre+code %} var x; {% endprettify %}
The modifier ‘{0}’ can't be applied to the body of a constructor.
The analyzer produces this diagnostic when the body of a constructor is prefixed by one of the following modifiers: async, async*, or sync*. Constructor bodies must be synchronous.
The following code produces this diagnostic because the body of the constructor for C is marked as being async:
{% prettify dart tag=pre+code %} class C { C() [!async!] {} } {% endprettify %}
If the constructor can be synchronous, then remove the modifier:
{% prettify dart tag=pre+code %} class C { C(); } {% endprettify %}
If the constructor can't be synchronous, then use a static method to create the instance instead:
{% prettify dart tag=pre+code %} class C { C(); static Future c() async { return C(); } } {% endprettify %}
Setters can't use ‘async’, ‘async*’, or ‘sync*’.
The analyzer produces this diagnostic when the body of a setter is prefixed by one of the following modifiers: async, async*, or sync*. Setter bodies must be synchronous.
The following code produces this diagnostic because the body of the setter x is marked as being async:
{% prettify dart tag=pre+code %} class C { set x(int i) [!async!] {} } {% endprettify %}
If the setter can be synchronous, then remove the modifier:
{% prettify dart tag=pre+code %} class C { set x(int i) {} } {% endprettify %}
If the setter can't be synchronous, then use a method to set the value instead:
{% prettify dart tag=pre+code %} class C { void x(int i) async {} } {% endprettify %}
The receiver can‘t be null because of short-circuiting, so the null-aware operator ‘{0}’ can’t be used.
The receiver can't be null, so the null-aware operator ‘{0}’ is unnecessary.
The analyzer produces this diagnostic when a null-aware operator (?., ?.., ?[, ?..[, or ...?) is used on a receiver that's known to be non-nullable.
The following code produces this diagnostic because s can't be null:
{% prettify dart tag=pre+code %} int? getLength(String s) { return s[!?.!]length; } {% endprettify %}
The following code produces this diagnostic because a can't be null:
{% prettify dart tag=pre+code %} var a = []; var b = [[!...?!]a]; {% endprettify %}
The following code produces this diagnostic because s?.length can't return null:
{% prettify dart tag=pre+code %} void f(String? s) { s?.length[!?.!]isEven; } {% endprettify %}
The reason s?.length can‘t return null is because the null-aware operator following s short-circuits the evaluation of both length and isEven if s is null. In other words, if s is null, then neither length nor isEven will be invoked, and if s is non-null, then length can’t return a null value. Either way, isEven can‘t be invoked on a null value, so the null-aware operator isn’t necessary. See Understanding null safety for more details.
The following code produces this diagnostic because s can't be null.
{% prettify dart tag=pre+code %} void f(Object? o) { var s = o as String; s[!?.!]length; } {% endprettify %}
The reason s can't be null, despite the fact that o can be null, is because of the cast to String, which is a non-nullable type. If o ever has the value null, the cast will fail and the invocation of length will not happen.
Replace the null-aware operator with a non-null-aware equivalent; for example, change ?. to .:
{% prettify dart tag=pre+code %} int getLength(String s) { return s.length; } {% endprettify %}
(Note that the return type was also changed to be non-nullable, which might not be appropriate in some cases.)
‘{1}.{0}’ (‘{2}’) isn‘t a valid override of ‘{3}.{0}’ (’{4}').
The analyzer produces this diagnostic when a member of a class is found that overrides a member from a supertype and the override isn't valid. An override is valid if all of these are true:
The following code produces this diagnostic because the type of the parameter s (String) isn't assignable to the type of the parameter i (int):
{% prettify dart tag=pre+code %} class A { void m(int i) {} }
class B extends A { void [!m!](String s) {} } {% endprettify %}
If the invalid method is intended to override the method from the superclass, then change it to conform:
{% prettify dart tag=pre+code %} class A { void m(int i) {} }
class B extends A { void m(int i) {} } {% endprettify %}
If it isn't intended to override the method from the superclass, then rename it:
{% prettify dart tag=pre+code %} class A { void m(int i) {} }
class B extends A { void m2(String s) {} } {% endprettify %}
Invalid reference to ‘this’ expression.
The analyzer produces this diagnostic when this is used outside of an instance method or a generative constructor. The reserved word this is only defined in the context of an instance method or a generative constructor.
The following code produces this diagnostic because v is a top-level variable:
{% prettify dart tag=pre+code %} C f() => [!this!];
class C {} {% endprettify %}
Use a variable of the appropriate type in place of this, declaring it if necessary:
{% prettify dart tag=pre+code %} C f(C c) => c;
class C {} {% endprettify %}
A value of type ‘{0}’ can't be returned by the ‘onError’ handler because it must be assignable to ‘{1}’.
The return type ‘{0}’ isn't assignable to ‘{1}’, as required by ‘Future.catchError’.
The analyzer produces this diagnostic when an invocation of Future.catchError has an argument whose return type isn't compatible with the type returned by the instance of Future. At runtime, the method catchError attempts to return the value from the callback as the result of the future, which results in another exception being thrown.
The following code produces this diagnostic because future is declared to return an int while callback is declared to return a String, and String isn't a subtype of int:
{% prettify dart tag=pre+code %} void f(Future future, String Function(dynamic, StackTrace) callback) { future.catchError([!callback!]); } {% endprettify %}
The following code produces this diagnostic because the closure being passed to catchError returns an int while future is declared to return a String:
{% prettify dart tag=pre+code %} void f(Future future) { future.catchError((error, stackTrace) => [!3!]); } {% endprettify %}
If the instance of Future is declared correctly, then change the callback to match:
{% prettify dart tag=pre+code %} void f(Future future, int Function(dynamic, StackTrace) callback) { future.catchError(callback); } {% endprettify %}
If the declaration of the instance of Future is wrong, then change it to match the callback:
{% prettify dart tag=pre+code %} void f(Future future, String Function(dynamic, StackTrace) callback) { future.catchError(callback); } {% endprettify %}
Constant list literals can't include a type parameter as a type argument, such as ‘{0}’.
Constant map literals can't include a type parameter as a type argument, such as ‘{0}’.
Constant set literals can't include a type parameter as a type argument, such as ‘{0}’.
The analyzer produces this diagnostic when a type parameter is used as a type argument in a list, map, or set literal that is prefixed by const. This isn‘t allowed because the value of the type parameter (the actual type that will be used at runtime) can’t be known at compile time.
The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant list:
{% prettify dart tag=pre+code %} List newList() => const <[!T!]>[]; {% endprettify %}
The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant map:
{% prettify dart tag=pre+code %} Map<String, T> newSet() => const <String, [!T!]>{}; {% endprettify %}
The following code produces this diagnostic because the type parameter T is being used as a type argument when creating a constant set:
{% prettify dart tag=pre+code %} Set newSet() => const <[!T!]>{}; {% endprettify %}
If the type that will be used for the type parameter can be known at compile time, then remove the type parameter:
{% prettify dart tag=pre+code %} List newList() => const []; {% endprettify %}
If the type that will be used for the type parameter can't be known until runtime, then remove the keyword const:
{% prettify dart tag=pre+code %} List newList() => []; {% endprettify %}
Invalid URI syntax: ‘{0}’.
The analyzer produces this diagnostic when a URI in a directive doesn't conform to the syntax of a valid URI.
The following code produces this diagnostic because '#' isn't a valid URI:
{% prettify dart tag=pre+code %} import [!‘#’!]; {% endprettify %}
Replace the invalid URI with a valid URI.
Can't have modifier ‘{0}’ in an extension.
The analyzer produces this diagnostic when a member declared inside an extension uses the keyword covariant in the declaration of a parameter. Extensions aren‘t classes and don’t have subclasses, so the keyword serves no purpose.
The following code produces this diagnostic because i is marked as being covariant:
{% prettify dart tag=pre+code %} extension E on String { void a([!covariant!] int i) {} } {% endprettify %}
Remove the covariant keyword:
{% prettify dart tag=pre+code %} extension E on String { void a(int i) {} } {% endprettify %}
The member ‘{0}’ can only be used within its package.
The analyzer produces this diagnostic when a reference to a declaration that is annotated with the [internal][meta-internal] annotation is found outside the package containing the declaration.
Given a package p that defines a library containing a declaration marked with the [internal][meta-internal] annotation:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@internal class C {} {% endprettify %}
The following code produces this diagnostic because it‘s referencing the class C, which isn’t intended to be used outside the package p:
{% prettify dart tag=pre+code %} import ‘package:p/src/p.dart’;
void f([!C!] c) {} {% endprettify %}
Remove the reference to the internal declaration.
An expression whose value is always ‘null’ can't be dereferenced.
The analyzer produces this diagnostic when an expression whose value will always be null is dereferenced.
The following code produces this diagnostic because x will always be null:
{% prettify dart tag=pre+code %} int f(Null x) { return [!x!].length; } {% endprettify %}
If the value is allowed to be something other than null, then change the type of the expression:
{% prettify dart tag=pre+code %} int f(String? x) { return x!.length; } {% endprettify %}
The member ‘{0}’ can only be used for overriding.
The analyzer produces this diagnostic when an instance member that is annotated with [visibleForOverriding][meta-visibleForOverriding] is referenced outside the library in which it's declared for any reason other than to override it.
Given a file named a.dart containing the following declaration:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class A { @visibleForOverriding void a() {} } {% endprettify %}
The following code produces this diagnostic because the method m is being invoked even though the only reason it's public is to allow it to be overridden:
{% prettify dart tag=pre+code %} import ‘a.dart’;
class B extends A { void b() { !a!; } } {% endprettify %}
Remove the invalid use of the member.
The member ‘{0}’ is annotated with ‘{1}’, but this annotation is only meaningful on declarations of public members.
The analyzer produces this diagnostic when either the visibleForTemplate or [visibleForTesting][meta-visibleForTesting] annotation is applied to a non-public declaration.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
[!@visibleForTesting!] void _someFunction() {}
void f() => _someFunction(); {% endprettify %}
If the declaration doesn't need to be used by test code, then remove the annotation:
{% prettify dart tag=pre+code %} void _someFunction() {}
void f() => _someFunction(); {% endprettify %}
If it does, then make it public:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@visibleForTesting void someFunction() {}
void f() => someFunction(); {% endprettify %}
The annotation ‘visibleForOverriding’ can only be applied to a public instance member that can be overridden.
The analyzer produces this diagnostic when anything other than a public instance member of a class is annotated with [visibleForOverriding][meta-visibleForOverriding]. Because only public instance members can be overridden outside the defining library, there's no value to annotating any other declarations.
The following code produces this diagnostic because the annotation is on a class, and classes can't be overridden:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
[!@visibleForOverriding!] class C {} {% endprettify %}
Remove the annotation:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
The extension ‘{0}’ doesn‘t define a ‘call’ method so the override can’t be used in an invocation.
The analyzer produces this diagnostic when an extension override is used to invoke a function but the extension doesn't declare a call method.
The following code produces this diagnostic because the extension E doesn't define a call method:
{% prettify dart tag=pre+code %} extension E on String {}
void f() { !E('')!; } {% endprettify %}
If the extension is intended to define a call method, then declare it:
{% prettify dart tag=pre+code %} extension E on String { int call() => 0; }
void f() { E('')(); } {% endprettify %}
If the extended type defines a call method, then remove the extension override.
If the call method isn‘t defined, then rewrite the code so that it doesn’t invoke the call method.
‘{0}’ isn't a function.
The analyzer produces this diagnostic when it finds a function invocation, but the name of the function being invoked is defined to be something other than a function.
The following code produces this diagnostic because Binary is the name of a function type, not a function:
{% prettify dart tag=pre+code %} typedef Binary = int Function(int, int);
int f() { return [!Binary!](1, 2); } {% endprettify %}
Replace the name with the name of a function.
The expression doesn‘t evaluate to a function, so it can’t be invoked.
The analyzer produces this diagnostic when a function invocation is found, but the name being referenced isn‘t the name of a function, or when the expression computing the function doesn’t compute a function.
The following code produces this diagnostic because x isn't a function:
{% prettify dart tag=pre+code %} int x = 0;
int f() => x;
var y = !x!; {% endprettify %}
The following code produces this diagnostic because f() doesn't return a function:
{% prettify dart tag=pre+code %} int x = 0;
int f() => x;
var y = !f()!; {% endprettify %}
If you need to invoke a function, then replace the code before the argument list with the name of a function or with an expression that computes a function:
{% prettify dart tag=pre+code %} int x = 0;
int f() => x;
var y = f(); {% endprettify %}
Can't reference label ‘{0}’ declared in an outer method.
The analyzer produces this diagnostic when a break or continue statement references a label that is declared in a method or function containing the function in which the break or continue statement appears. The break and continue statements can't be used to transfer control outside the function that contains them.
The following code produces this diagnostic because the label loop is declared outside the local function g:
{% prettify dart tag=pre+code %} void f() { loop: while (true) { void g() { break [!loop!]; }
g();
} } {% endprettify %}
Try rewriting the code so that it isn't necessary to transfer control outside the local function, possibly by inlining the local function:
{% prettify dart tag=pre+code %} void f() { loop: while (true) { break loop; } } {% endprettify %}
If that isn't possible, then try rewriting the local function so that a value returned by the function can be used to determine whether control is transferred:
{% prettify dart tag=pre+code %} void f() { loop: while (true) { bool g() { return true; }
if (g()) {
break loop;
}
} } {% endprettify %}
Can't reference an undefined label ‘{0}’.
The analyzer produces this diagnostic when it finds a reference to a label that isn't defined in the scope of the break or continue statement that is referencing it.
The following code produces this diagnostic because the label loop isn't defined anywhere:
{% prettify dart tag=pre+code %} void f() { for (int i = 0; i < 10; i++) { for (int j = 0; j < 10; j++) { break [!loop!]; } } } {% endprettify %}
If the label should be on the innermost enclosing do, for, switch, or while statement, then remove the label:
{% prettify dart tag=pre+code %} void f() { for (int i = 0; i < 10; i++) { for (int j = 0; j < 10; j++) { break; } } } {% endprettify %}
If the label should be on some other statement, then add the label:
{% prettify dart tag=pre+code %} void f() { loop: for (int i = 0; i < 10; i++) { for (int j = 0; j < 10; j++) { break loop; } } } {% endprettify %}
Can't have a late final field in a class with a generative const constructor.
The analyzer produces this diagnostic when a class that has at least one const constructor also has a field marked both late and final.
The following code produces this diagnostic because the class A has a const constructor and the final field f is marked as late:
{% prettify dart tag=pre+code %} class A { [!late!] final int f;
const A(); } {% endprettify %}
If the field doesn't need to be marked late, then remove the late modifier from the field:
{% prettify dart tag=pre+code %} class A { final int f = 0;
const A(); } {% endprettify %}
If the field must be marked late, then remove the const modifier from the constructors:
{% prettify dart tag=pre+code %} class A { late final int f;
A(); } {% endprettify %}
The late final local variable is already assigned.
The analyzer produces this diagnostic when the analyzer can prove that a local variable marked as both late and final was already assigned a value at the point where another assignment occurs.
Because final variables can only be assigned once, subsequent assignments are guaranteed to fail, so they're flagged.
The following code produces this diagnostic because the final variable v is assigned a value in two places:
{% prettify dart tag=pre+code %} int f() { late final int v; v = 0; [!v!] += 1; return v; } {% endprettify %}
If you need to be able to reassign the variable, then remove the final keyword:
{% prettify dart tag=pre+code %} int f() { late int v; v = 0; v += 1; return v; } {% endprettify %}
If you don't need to reassign the variable, then remove all except the first of the assignments:
{% prettify dart tag=pre+code %} int f() { late final int v; v = 0; return v; } {% endprettify %}
FFI leaf call can't return a ‘Handle’.
The analyzer produces this diagnostic when the value of the isLeaf argument in an invocation of either Pointer.asFunction or DynamicLibrary.lookupFunction is true and the function that would be returned would have a return type of Handle.
The analyzer also produces this diagnostic when the value of the isLeaf argument in an FfiNative annotation is true and the type argument on the annotation is a function type whose return type is Handle.
In all of these cases, leaf calls are only supported for the types bool, int, float, double, and, as a return type void.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the function p returns a Handle, but the isLeaf argument is true:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
void f(Pointer<NativeFunction<Handle Function()>> p) { [!p.asFunction<Object Function()>(isLeaf: true)!]; } {% endprettify %}
If the function returns a handle, then remove the isLeaf argument:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
void f(Pointer<NativeFunction<Handle Function()>> p) { p.asFunction<Object Function()>(); } {% endprettify %}
If the function returns one of the supported types, then correct the type information:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
void f(Pointer<NativeFunction<Int32 Function()>> p) { p.asFunction<int Function()>(isLeaf: true); } {% endprettify %}
FFI leaf call can't take arguments of type ‘Handle’.
The analyzer produces this diagnostic when the value of the isLeaf argument in an invocation of either Pointer.asFunction or DynamicLibrary.lookupFunction is true and the function that would be returned would have a parameter of type Handle.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the function p has a parameter of type Handle, but the isLeaf argument is true:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
void f(Pointer<NativeFunction<Void Function(Handle)>> p) { [!p.asFunction<void Function(Object)>(isLeaf: true)!]; } {% endprettify %}
If the function has at least one parameter of type Handle, then remove the isLeaf argument:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
void f(Pointer<NativeFunction<Void Function(Handle)>> p) { p.asFunction<void Function(Object)>(); } {% endprettify %}
If none of the function's parameters are Handles, then correct the type information:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
void f(Pointer<NativeFunction<Void Function(Int8)>> p) { p.asFunction<void Function(int)>(isLeaf: true); } {% endprettify %}
The element type ‘{0}’ can't be assigned to the list type ‘{1}’.
The analyzer produces this diagnostic when the type of an element in a list literal isn't assignable to the element type of the list.
The following code produces this diagnostic because 2.5 is a double, and the list can hold only integers:
{% prettify dart tag=pre+code %} List x = [1, [!2.5!], 3]; {% endprettify %}
If you intended to add a different object to the list, then replace the element with an expression that computes the intended object:
{% prettify dart tag=pre+code %} List x = [1, 2, 3]; {% endprettify %}
If the object shouldn't be in the list, then remove the element:
{% prettify dart tag=pre+code %} List x = [1, 3]; {% endprettify %}
If the object being computed is correct, then widen the element type of the list to allow all of the different types of objects it needs to contain:
{% prettify dart tag=pre+code %} List x = [1, 2.5, 3]; {% endprettify %}
The type of the first positional parameter of the ‘main’ function must be a supertype of ‘List’.
The analyzer produces this diagnostic when the first positional parameter of a function named main isn't a supertype of List<String>.
The following code produces this diagnostic because List<int> isn't a supertype of List<String>:
{% prettify dart tag=pre+code %} void main([!List!] args) {} {% endprettify %}
If the function is an entry point, then change the type of the first positional parameter to be a supertype of List<String>:
{% prettify dart tag=pre+code %} void main(List args) {} {% endprettify %}
If the function isn't an entry point, then change the name of the function:
{% prettify dart tag=pre+code %} void f(List args) {} {% endprettify %}
The function ‘main’ can't have any required named parameters.
The analyzer produces this diagnostic when a function named main has one or more required named parameters.
The following code produces this diagnostic because the function named main has a required named parameter (x):
{% prettify dart tag=pre+code %} void [!main!]({required int x}) {} {% endprettify %}
If the function is an entry point, then remove the required keyword:
{% prettify dart tag=pre+code %} void main({int? x}) {} {% endprettify %}
If the function isn't an entry point, then change the name of the function:
{% prettify dart tag=pre+code %} void f({required int x}) {} {% endprettify %}
The function ‘main’ can't have more than two required positional parameters.
The analyzer produces this diagnostic when a function named main has more than two required positional parameters.
The following code produces this diagnostic because the function main has three required positional parameters:
{% prettify dart tag=pre+code %} void [!main!](List args, int x, int y) {} {% endprettify %}
If the function is an entry point and the extra parameters aren't used, then remove them:
{% prettify dart tag=pre+code %} void main(List args, int x) {} {% endprettify %}
If the function is an entry point, but the extra parameters used are for when the function isn't being used as an entry point, then make the extra parameters optional:
{% prettify dart tag=pre+code %} void main(List args, int x, [int y = 0]) {} {% endprettify %}
If the function isn't an entry point, then change the name of the function:
{% prettify dart tag=pre+code %} void f(List args, int x, int y) {} {% endprettify %}
The declaration named ‘main’ must be a function.
The analyzer produces this diagnostic when a library contains a declaration of the name main that isn't the declaration of a top-level function.
The following code produces this diagnostic because the name main is being used to declare a top-level variable:
{% prettify dart tag=pre+code %} var [!main!] = 3; {% endprettify %}
Use a different name for the declaration:
{% prettify dart tag=pre+code %} var mainIndex = 3; {% endprettify %}
Map entries can only be used in a map literal.
The analyzer produces this diagnostic when a map entry (a key/value pair) is found in a set literal.
The following code produces this diagnostic because the literal has a map entry even though it's a set literal:
{% prettify dart tag=pre+code %} const collection = {[!‘a’ : ‘b’!]}; {% endprettify %}
If you intended for the collection to be a map, then change the code so that it is a map. In the previous example, you could do this by adding another type argument:
{% prettify dart tag=pre+code %} const collection = <String, String>{‘a’ : ‘b’}; {% endprettify %}
In other cases, you might need to change the explicit type from Set to Map.
If you intended for the collection to be a set, then remove the map entry, possibly by replacing the colon with a comma if both values should be included in the set:
{% prettify dart tag=pre+code %} const collection = {‘a’, ‘b’}; {% endprettify %}
The element type ‘{0}’ can't be assigned to the map key type ‘{1}’.
The analyzer produces this diagnostic when a key of a key-value pair in a map literal has a type that isn't assignable to the key type of the map.
The following code produces this diagnostic because 2 is an int, but the keys of the map are required to be Strings:
{% prettify dart tag=pre+code %} var m = <String, String>{[!2!] : ‘a’}; {% endprettify %}
If the type of the map is correct, then change the key to have the correct type:
{% prettify dart tag=pre+code %} var m = <String, String>{‘2’ : ‘a’}; {% endprettify %}
If the type of the key is correct, then change the key type of the map:
{% prettify dart tag=pre+code %} var m = <int, String>{2 : ‘a’}; {% endprettify %}
The element type ‘{0}’ can't be assigned to the map value type ‘{1}’.
The analyzer produces this diagnostic when a value of a key-value pair in a map literal has a type that isn't assignable to the value type of the map.
The following code produces this diagnostic because 2 is an int, but/ the values of the map are required to be Strings:
{% prettify dart tag=pre+code %} var m = <String, String>{‘a’ : [!2!]}; {% endprettify %}
If the type of the map is correct, then change the value to have the correct type:
{% prettify dart tag=pre+code %} var m = <String, String>{‘a’ : ‘2’}; {% endprettify %}
If the type of the value is correct, then change the value type of the map:
{% prettify dart tag=pre+code %} var m = <String, int>{‘a’ : 2}; {% endprettify %}
The annotation doesn't match the declared type of the field.
The analyzer produces this diagnostic when the annotation on a field in a subclass of Struct or Union doesn't match the Dart type of the field.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the annotation Double doesn't match the Dart type int:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { [!@Double()!] external int x; } {% endprettify %}
If the type of the field is correct, then change the annotation to match:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int32() external int x; } {% endprettify %}
If the annotation is correct, then change the type of the field to match:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Double() external double x; } {% endprettify %}
Fields in a struct class must either have the type ‘Pointer’ or an annotation indicating the native type.
The analyzer produces this diagnostic when a field in a subclass of Struct or Union whose type requires an annotation doesn't have one. The Dart types int, double, and Array are used to represent multiple C types, and the annotation specifies which of the compatible C types the field represents.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field x doesn't have an annotation indicating the underlying width of the integer value:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { external [!int!] x; } {% endprettify %}
Add an appropriate annotation to the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int64() external int x; } {% endprettify %}
Required library ‘{0}’ is missing.
The analyzer produces this diagnostic when either the Dart or Flutter SDK isn’t installed correctly, and, as a result, one of the dart: libraries can't be found.
Reinstall the Dart or Flutter SDK.
The parameter ‘{0}’ can't have a value of ‘null’ because of its type, but the implicit default value is ‘null’.
With null safety, use the ‘required’ keyword, not the ‘@required’ annotation.
The analyzer produces this diagnostic when an optional parameter, whether positional or named, has a potentially non-nullable type and doesn‘t specify a default value. Optional parameters that have no explicit default value have an implicit default value of null. If the type of the parameter doesn’t allow the parameter to have a value of null, then the implicit default value isn't valid.
The following code produces this diagnostic because x can't be null, and no non-null default value is specified:
{% prettify dart tag=pre+code %} void f([int [!x!]]) {} {% endprettify %}
As does this:
{% prettify dart tag=pre+code %} void g({int [!x!]}) {} {% endprettify %}
If you want to use null to indicate that no value was provided, then you need to make the type nullable:
{% prettify dart tag=pre+code %} void f([int? x]) {} void g({int? x}) {} {% endprettify %}
If the parameter can't be null, then either provide a default value:
{% prettify dart tag=pre+code %} void f([int x = 1]) {} void g({int x = 2}) {} {% endprettify %}
or make the parameter a required parameter:
{% prettify dart tag=pre+code %} void f(int x) {} void g({required int x}) {} {% endprettify %}
Missing case clause for ‘{0}’.
The analyzer produces this diagnostic when a switch statement for an enum doesn't include an option for one of the values in the enum.
Note that null is always a possible value for an enum and therefore also must be handled.
The following code produces this diagnostic because the enum constant e2 isn't handled:
{% prettify dart tag=pre+code %} enum E { e1, e2 }
void f(E e) { [!switch (e)!] { case E.e1: break; } } {% endprettify %}
If there's special handling for the missing values, then add a case clause for each of the missing values:
{% prettify dart tag=pre+code %} enum E { e1, e2 }
void f(E e) { switch (e) { case E.e1: break; case E.e2: break; } } {% endprettify %}
If the missing values should be handled the same way, then add a default clause:
{% prettify dart tag=pre+code %} enum E { e1, e2 }
void f(E e) { switch (e) { case E.e1: break; default: break; } } {% endprettify %}
The method ‘Pointer.fromFunction’ must have an exceptional return value (the second argument) when the return type of the function is neither ‘void’, ‘Handle’, nor ‘Pointer’.
The analyzer produces this diagnostic when an invocation of the method Pointer.fromFunction doesn't have a second argument (the exceptional return value) when the type to be returned from the invocation is neither void, Handle, nor Pointer.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the type returned by f is expected to be an 8-bit integer but the call to fromFunction doesn't include an exceptional return argument:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
int f(int i) => i * 2;
void g() { Pointer.[!fromFunction!]<Int8 Function(Int8)>(f); } {% endprettify %}
Add an exceptional return type:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
int f(int i) => i * 2;
void g() { Pointer.fromFunction<Int8 Function(Int8)>(f, 0); } {% endprettify %}
Fields in struct classes must have an explicitly declared type of ‘int’, ‘double’ or ‘Pointer’.
The analyzer produces this diagnostic when a field in a subclass of Struct or Union doesn't have a type annotation. Every field must have an explicit type, and the type must either be int, double, Pointer, or a subclass of either Struct or Union.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field str doesn't have a type annotation:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { external var [!str!];
@Int32() external int i; } {% endprettify %}
Explicitly specify the type of the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’; import ‘package:ffi/ffi.dart’;
class C extends Struct { external Pointer str;
@Int32() external int i; } {% endprettify %}
The ‘name’ field is required but missing.
The analyzer produces this diagnostic when there's no top-level name key. The name key provides the name of the package, which is required.
The following code produces this diagnostic because the package doesn't have a name:
dependencies: meta: ^1.0.2
Add the top-level key name with a value that's the name of the package:
name: example dependencies: meta: ^1.0.2
The named parameter ‘{0}’ is required, but there's no corresponding argument.
The analyzer produces this diagnostic when an invocation of a function is missing a required named parameter.
The following code produces this diagnostic because the invocation of f doesn't include a value for the required named parameter end:
{% prettify dart tag=pre+code %} void f(int start, {required int end}) {} void g() { !f!; } {% endprettify %}
Add a named argument corresponding to the missing required parameter:
{% prettify dart tag=pre+code %} void f(int start, {required int end}) {} void g() { f(3, end: 5); } {% endprettify %}
The parameter ‘{0}’ is required.
The parameter ‘{0}’ is required. {1}.
The analyzer produces this diagnostic when a method or function with a named parameter that is annotated as being required is invoked without providing a value for the parameter.
The following code produces this diagnostic because the named parameter x is required:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
void f({@required int x}) {}
void g() { !f!; } {% endprettify %}
Provide the required value:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
void f({@required int x}) {}
void g() { f(x: 2); } {% endprettify %}
This function has a return type of ‘{0}’, but doesn't end with a return statement.
Any function or method that doesn't end with either an explicit return or a throw implicitly returns null. This is rarely the desired behavior. The analyzer produces this diagnostic when it finds an implicit return.
The following code produces this diagnostic because f doesn't end with a return:
{% prettify dart tag=pre+code %} int [!f!](int x) { if (x < 0) { return 0; } } {% endprettify %}
Add a return statement that makes the return value explicit, even if null is the appropriate value.
Fields of type ‘Array’ must have exactly one ‘Array’ annotation.
The analyzer produces this diagnostic when a field in a subclass of either Struct or Union has a type of Array but doesn't have a single Array annotation indicating the dimensions of the array.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field a0 doesn't have an Array annotation:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { external [!Array!] a0; } {% endprettify %}
Ensure that there's exactly one Array annotation on the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(8) external Array a0; } {% endprettify %}
The super-invoked member ‘{0}’ has the type ‘{1}’, and the concrete member in the class has the type ‘{2}’.
The analyzer produces this diagnostic when a mixin that invokes a method using super is used in a class where the concrete implementation of that method has a different signature than the signature defined for that method by the mixin‘s on type. The reason this is an error is because the invocation in the mixin might invoke the method in a way that’s incompatible with the method that will actually be executed.
The following code produces this diagnostic because the class C uses the mixin M, the mixin M invokes foo using super, and the abstract version of foo declared in I (the mixin‘s on type) doesn’t have the same signature as the concrete version of foo declared in A:
{% prettify dart tag=pre+code %} class I { void foo([int? p]) {} }
class A { void foo(int p) {} }
abstract class B extends A implements I { @override void foo([int? p]); }
mixin M on I { void bar() { super.foo(42); } }
abstract class C extends B with [!M!] {} {% endprettify %}
If the class doesn't need to use the mixin, then remove it from the with clause:
{% prettify dart tag=pre+code %} class I { void foo([int? p]) {} }
class A { void foo(int? p) {} }
abstract class B extends A implements I { @override void foo([int? p]); }
mixin M on I { void bar() { super.foo(42); } }
abstract class C extends B {} {% endprettify %}
If the class needs to use the mixin, then ensure that there's a concrete implementation of the method that conforms to the signature expected by the mixin:
{% prettify dart tag=pre+code %} class I { void foo([int? p]) {} }
class A { void foo(int? p) {} }
abstract class B extends A implements I { @override void foo([int? p]) { super.foo(p); } }
mixin M on I { void bar() { super.foo(42); } }
abstract class C extends B with M {} {% endprettify %}
‘{0}’ can‘t be mixed onto ‘{1}’ because ‘{1}’ doesn’t implement ‘{2}’.
The analyzer produces this diagnostic when a mixin that has a superclass constraint is used in a mixin application with a superclass that doesn't implement the required constraint.
The following code produces this diagnostic because the mixin M requires that the class to which it‘s applied be a subclass of A, but Object isn’t a subclass of A:
{% prettify dart tag=pre+code %} class A {}
mixin M on A {}
class X = Object with [!M!]; {% endprettify %}
If you need to use the mixin, then change the superclass to be either the same as or a subclass of the superclass constraint:
{% prettify dart tag=pre+code %} class A {}
mixin M on A {}
class X = A with M; {% endprettify %}
The class doesn't have a concrete implementation of the super-invoked member ‘{0}’.
The analyzer produces this diagnostic when a mixin application contains an invocation of a member from its superclass, and there‘s no concrete member of that name in the mixin application’s superclass.
The following code produces this diagnostic because the mixin M contains the invocation super.m(), and the class A, which is the superclass of the mixin application A+M, doesn't define a concrete implementation of m:
{% prettify dart tag=pre+code %} abstract class A { void m(); }
mixin M on A { void bar() { super.m(); } }
abstract class B extends A with [!M!] {} {% endprettify %}
If you intended to apply the mixin M to a different class, one that has a concrete implementation of m, then change the superclass of B to that class:
{% prettify dart tag=pre+code %} abstract class A { void m(); }
mixin M on A { void bar() { super.m(); } }
class C implements A { void m() {} }
abstract class B extends C with M {} {% endprettify %}
If you need to make B a subclass of A, then add a concrete implementation of m in A:
{% prettify dart tag=pre+code %} abstract class A { void m() {} }
mixin M on A { void bar() { super.m(); } }
abstract class B extends A with M {} {% endprettify %}
The class ‘{0}’ can't be used as a mixin because it declares a constructor.
The analyzer produces this diagnostic when a class is used as a mixin and the mixed-in class defines a constructor.
The following code produces this diagnostic because the class A, which defines a constructor, is being used as a mixin:
{% prettify dart tag=pre+code %} class A { A(); }
class B with [!A!] {} {% endprettify %}
If it's possible to convert the class to a mixin, then do so:
{% prettify dart tag=pre+code %} mixin A { }
class B with A {} {% endprettify %}
If the class can‘t be a mixin and it’s possible to remove the constructor, then do so:
{% prettify dart tag=pre+code %} class A { }
class B with A {} {% endprettify %}
If the class can‘t be a mixin and you can’t remove the constructor, then try extending or implementing the class rather than mixing it in:
{% prettify dart tag=pre+code %} class A { A(); }
class B extends A {} {% endprettify %}
The class ‘{0}’ can't be used as a mixin because it extends a class other than ‘Object’.
The analyzer produces this diagnostic when a class that extends a class other than Object is used as a mixin.
The following code produces this diagnostic because the class B, which extends A, is being used as a mixin by C:
{% prettify dart tag=pre+code %} class A {}
class B extends A {}
class C with [!B!] {} {% endprettify %}
If the class being used as a mixin can be changed to extend Object, then change it:
{% prettify dart tag=pre+code %} class A {}
class B {}
class C with B {} {% endprettify %}
If the class being used as a mixin can‘t be changed and the class that’s using it extends Object, then extend the class being used as a mixin:
{% prettify dart tag=pre+code %} class A {}
class B extends A {}
class C extends B {} {% endprettify %}
If the class doesn't extend Object or if you want to be able to mix in the behavior from B in other places, then create a real mixin:
{% prettify dart tag=pre+code %} class A {}
mixin M on A {}
class B extends A with M {}
class C extends A with M {} {% endprettify %}
Mixins can't be instantiated.
The analyzer produces this diagnostic when a mixin is instantiated.
The following code produces this diagnostic because the mixin M is being instantiated:
{% prettify dart tag=pre+code %} mixin M {}
var m = !M!; {% endprettify %}
If you intend to use an instance of a class, then use the name of that class in place of the name of the mixin.
Classes can only mix in mixins and classes.
The analyzer produces this diagnostic when a name in a with clause is defined to be something other than a mixin or a class.
The following code produces this diagnostic because F is defined to be a function type:
{% prettify dart tag=pre+code %} typedef F = int Function(String);
class C with [!F!] {} {% endprettify %}
Remove the invalid name from the list, possibly replacing it with the name of the intended mixin or class:
{% prettify dart tag=pre+code %} typedef F = int Function(String);
class C {} {% endprettify %}
The class ‘{0}’ shouldn't be used as a mixin constraint because it is sealed, and any class mixing in this mixin must have ‘{0}’ as a superclass.
The analyzer produces this diagnostic when the superclass constraint of a mixin is a class from a different package that was marked as [sealed][meta-sealed]. Classes that are sealed can't be extended, implemented, mixed in, or used as a superclass constraint.
If the package p defines a sealed class:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@sealed class C {} {% endprettify %}
Then, the following code, when in a package other than p, produces this diagnostic:
{% prettify dart tag=pre+code %} import ‘package:p/p.dart’;
[!mixin M on C {}!] {% endprettify %}
If the classes that use the mixin don't need to be subclasses of the sealed class, then consider adding a field and delegating to the wrapped instance of the sealed class.
Only classes and mixins can be used as superclass constraints.
The analyzer produces this diagnostic when a type following the on keyword in a mixin declaration is neither a class nor a mixin.
The following code produces this diagnostic because F is neither a class nor a mixin:
{% prettify dart tag=pre+code %} typedef F = void Function();
mixin M on [!F!] {} {% endprettify %}
If the type was intended to be a class but was mistyped, then replace the name.
Otherwise, remove the type from the on clause.
Constructors can have only one ‘this’ redirection, at most.
The analyzer produces this diagnostic when a constructor redirects to more than one other constructor in the same class (using this).
The following code produces this diagnostic because the unnamed constructor in C is redirecting to both this.a and this.b:
{% prettify dart tag=pre+code %} class C { C() : this.a(), [!this.b()!]; C.a(); C.b(); } {% endprettify %}
Remove all but one of the redirections:
{% prettify dart tag=pre+code %} class C { C() : this.a(); C.a(); C.b(); } {% endprettify %}
A constructor can have at most one ‘super’ initializer.
The analyzer produces this diagnostic when the initializer list of a constructor contains more than one invocation of a constructor from the superclass. The initializer list is required to have exactly one such call, which can either be explicit or implicit.
The following code produces this diagnostic because the initializer list for B’s constructor invokes both the constructor one and the constructor two from the superclass A:
{% prettify dart tag=pre+code %} class A { int? x; String? s; A.one(this.x); A.two(this.s); }
class B extends A { B() : super.one(0), [!super.two('')!]; } {% endprettify %}
If one of the super constructors will initialize the instance fully, then remove the other:
{% prettify dart tag=pre+code %} class A { int? x; String? s; A.one(this.x); A.two(this.s); }
class B extends A { B() : super.one(0); } {% endprettify %}
If the initialization achieved by one of the super constructors can be performed in the body of the constructor, then remove its super invocation and perform the initialization in the body:
{% prettify dart tag=pre+code %} class A { int? x; String? s; A.one(this.x); A.two(this.s); }
class B extends A { B() : super.one(0) { s = ''; } } {% endprettify %}
If the initialization can only be performed in a constructor in the superclass, then either add a new constructor or modify one of the existing constructors so there's a constructor that allows all the required initialization to occur in a single call:
{% prettify dart tag=pre+code %} class A { int? x; String? s; A.one(this.x); A.two(this.s); A.three(this.x, this.s); }
class B extends A { B() : super.three(0, ''); } {% endprettify %}
The type ‘{0}’ given to ‘{1}’ must be a valid ‘dart:ffi’ native function type.
The analyzer produces this diagnostic when an invocation of either Pointer.fromFunction or DynamicLibrary.lookupFunction has a type argument(whether explicit or inferred) that isn't a native function type.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the type T can be any subclass of Function but the type argument for fromFunction is required to be a native function type:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
int f(int i) => i * 2;
class C { void g() { Pointer.fromFunction<[!T!]>(f, 0); } } {% endprettify %}
Use a native function type as the type argument to the invocation:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
int f(int i) => i * 2;
class C { void g() { Pointer.fromFunction<Int32 Function(Int32)>(f, 0); } } {% endprettify %}
The type ‘{0}’ must be a subtype of ‘{1}’ for ‘{2}’.
The analyzer produces this diagnostic in two cases:
Pointer.fromFunction where the type argument (whether explicit or inferred) isn't a supertype of the type of the function passed as the first argument to the method.DynamicLibrary.lookupFunction where the first type argument isn't a supertype of the second type argument.For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the type of the function f (String Function(int)) isn't a subtype of the type argument T (Int8 Function(Int8)):
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef T = Int8 Function(Int8);
double f(double i) => i;
void g() { Pointer.fromFunction([!f!], 5.0); } {% endprettify %}
If the function is correct, then change the type argument to match:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef T = Float Function(Float);
double f(double i) => i;
void g() { Pointer.fromFunction(f, 5.0); } {% endprettify %}
If the type argument is correct, then change the function to match:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef T = Int8 Function(Int8);
int f(int i) => i;
void g() { Pointer.fromFunction(f, 5); } {% endprettify %}
This class (or a class that this class inherits from) is marked as ‘@immutable’, but one or more of its instance fields aren't final: {0}
The analyzer produces this diagnostic when an immutable class defines one or more instance fields that aren‘t final. A class is immutable if it’s marked as being immutable using the annotation [immutable][meta-immutable] or if it's a subclass of an immutable class.
The following code produces this diagnostic because the field x isn't final:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@immutable class [!C!] { int x;
C(this.x); } {% endprettify %}
If instances of the class should be immutable, then add the keyword final to all non-final field declarations:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@immutable class C { final int x;
C(this.x); } {% endprettify %}
If the instances of the class should be mutable, then remove the annotation, or choose a different superclass if the annotation is inherited:
{% prettify dart tag=pre+code %} class C { int x;
C(this.x); } {% endprettify %}
This method overrides a method annotated as ‘@mustCallSuper’ in ‘{0}’, but doesn't invoke the overridden method.
The analyzer produces this diagnostic when a method that overrides a method that is annotated as [mustCallSuper][meta-mustCallSuper] doesn't invoke the overridden method as required.
The following code produces this diagnostic because the method m in B doesn't invoke the overridden method m in A:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class A { @mustCallSuper m() {} }
class B extends A { @override !m! {} } {% endprettify %}
Add an invocation of the overridden method in the overriding method:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class A { @mustCallSuper m() {} }
class B extends A { @override m() { super.m(); } } {% endprettify %}
The value of the ‘name’ field is required to be a string.
The analyzer produces this diagnostic when the top-level name key has a value that isn't a string.
The following code produces this diagnostic because the value following the name key is a list:
name: - example
Replace the value with a string:
name: example
The class ‘{0}’ doesn't have an unnamed constructor.
The analyzer produces this diagnostic when an unnamed constructor is invoked on a class that defines named constructors but the class doesn’t have an unnamed constructor.
The following code produces this diagnostic because A doesn't define an unnamed constructor:
{% prettify dart tag=pre+code %} class A { A.a(); }
A f() => !A!; {% endprettify %}
If one of the named constructors does what you need, then use it:
{% prettify dart tag=pre+code %} class A { A.a(); }
A f() => A.a(); {% endprettify %}
If none of the named constructors does what you need, and you're able to add an unnamed constructor, then add the constructor:
{% prettify dart tag=pre+code %} class A { A(); A.a(); }
A f() => A(); {% endprettify %}
Missing concrete implementation of ‘{0}’.
Missing concrete implementations of ‘{0}’ and ‘{1}’.
Missing concrete implementations of ‘{0}’, ‘{1}’, ‘{2}’, ‘{3}’, and {4} more.
Missing concrete implementations of ‘{0}’, ‘{1}’, ‘{2}’, and ‘{3}’.
Missing concrete implementations of ‘{0}’, ‘{1}’, and ‘{2}’.
The analyzer produces this diagnostic when a concrete class inherits one or more abstract members, and doesn't provide or inherit an implementation for at least one of those abstract members.
The following code produces this diagnostic because the class B doesn't have a concrete implementation of m:
{% prettify dart tag=pre+code %} abstract class A { void m(); }
class [!B!] extends A {} {% endprettify %}
If the subclass can provide a concrete implementation for some or all of the abstract inherited members, then add the concrete implementations:
{% prettify dart tag=pre+code %} abstract class A { void m(); }
class B extends A { void m() {} } {% endprettify %}
If there is a mixin that provides an implementation of the inherited methods, then apply the mixin to the subclass:
{% prettify dart tag=pre+code %} abstract class A { void m(); }
class B extends A with M {}
mixin M { void m() {} } {% endprettify %}
If the subclass can't provide a concrete implementation for all of the abstract inherited members, then mark the subclass as being abstract:
{% prettify dart tag=pre+code %} abstract class A { void m(); }
abstract class B extends A {} {% endprettify %}
Conditions must have a static type of ‘bool’.
The analyzer produces this diagnostic when a condition, such as an if or while loop, doesn't have the static type bool.
The following code produces this diagnostic because x has the static type int:
{% prettify dart tag=pre+code %} void f(int x) { if ([!x!]) { // ... } } {% endprettify %}
Change the condition so that it produces a Boolean value:
{% prettify dart tag=pre+code %} void f(int x) { if (x == 0) { // ... } } {% endprettify %}
The expression in an assert must be of type ‘bool’.
The analyzer produces this diagnostic when the first expression in an assert has a type other than bool.
The following code produces this diagnostic because the type of p is int, but a bool is required:
{% prettify dart tag=pre+code %} void f(int p) { assert([!p!]); } {% endprettify %}
Change the expression so that it has the type bool:
{% prettify dart tag=pre+code %} void f(int p) { assert(p > 0); } {% endprettify %}
A negation operand must have a static type of ‘bool’.
The analyzer produces this diagnostic when the operand of the unary negation operator (!) doesn't have the type bool.
The following code produces this diagnostic because x is an int when it must be a bool:
{% prettify dart tag=pre+code %} int x = 0; bool y = ![!x!]; {% endprettify %}
Replace the operand with an expression that has the type bool:
{% prettify dart tag=pre+code %} int x = 0; bool y = !(x > 0); {% endprettify %}
The operands of the operator ‘{0}’ must be assignable to ‘bool’.
The analyzer produces this diagnostic when one of the operands of either the && or || operator doesn't have the type bool.
The following code produces this diagnostic because a isn't a Boolean value:
{% prettify dart tag=pre+code %} int a = 3; bool b = [!a!] || a > 1; {% endprettify %}
Change the operand to a Boolean value:
{% prettify dart tag=pre+code %} int a = 3; bool b = a == 0 || a > 1; {% endprettify %}
Annotation creation can only call a const constructor.
The analyzer produces this diagnostic when an annotation is the invocation of an existing constructor even though the invoked constructor isn't a const constructor.
The following code produces this diagnostic because the constructor for C isn't a const constructor:
{% prettify dart tag=pre+code %} [!@C()!] void f() { }
class C { C(); } {% endprettify %}
If it's valid for the class to have a const constructor, then create a const constructor that can be used for the annotation:
{% prettify dart tag=pre+code %} @C() void f() { }
class C { const C(); } {% endprettify %}
If it isn't valid for the class to have a const constructor, then either remove the annotation or use a different class for the annotation.
Case expressions must be constant.
The analyzer produces this diagnostic when the expression in a case clause isn't a constant expression.
The following code produces this diagnostic because j isn't a constant:
{% prettify dart tag=pre+code %} void f(int i, int j) { switch (i) { case [!j!]: // ... break; } } {% endprettify %}
Either make the expression a constant expression, or rewrite the switch statement as a sequence of if statements:
{% prettify dart tag=pre+code %} void f(int i, int j) { if (i == j) { // ... } } {% endprettify %}
Constant values from a deferred library can't be used as a case expression.
The analyzer produces this diagnostic when the expression in a case clause references a constant from a library that is imported using a deferred import. In order for switch statements to be compiled efficiently, the constants referenced in case clauses need to be available at compile time, and constants from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
Given a file (a.dart) that defines the constant zero:
{% prettify dart tag=pre+code %} const zero = 0; {% endprettify %}
The following code produces this diagnostic because the library a.dart is imported using a deferred import, and the constant a.zero, declared in the imported library, is used in a case clause:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a;
void f(int x) { switch (x) { case [!a.zero!]: // ... break; } } {% endprettify %}
If you need to reference the constant from the imported library, then remove the deferred keyword:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a;
void f(int x) { switch (x) { case a.zero: // ... break; } } {% endprettify %}
If you need to reference the constant from the imported library and also need the imported library to be deferred, then rewrite the switch statement as a sequence of if statements:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a;
void f(int x) { if (x == a.zero) { // ... } } {% endprettify %}
If you don't need to reference the constant, then replace the case expression:
{% prettify dart tag=pre+code %} void f(int x) { switch (x) { case 0: // ... break; } } {% endprettify %}
The default value of an optional parameter must be constant.
The analyzer produces this diagnostic when an optional parameter, either named or positional, has a default value that isn't a compile-time constant.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} var defaultValue = 3;
void f([int value = [!defaultValue!]]) {} {% endprettify %}
If the default value can be converted to be a constant, then convert it:
{% prettify dart tag=pre+code %} const defaultValue = 3;
void f([int value = defaultValue]) {} {% endprettify %}
If the default value needs to change over time, then apply the default value inside the function:
{% prettify dart tag=pre+code %} var defaultValue = 3;
void f([int value]) { value ??= defaultValue; } {% endprettify %}
Constant values from a deferred library can't be used as a default parameter value.
The analyzer produces this diagnostic when the default value of an optional parameter uses a constant from a library imported using a deferred import. Default values need to be available at compile time, and constants from deferred libraries aren't available at compile time.
For more information, see the language tour's coverage of deferred loading.
Given a file (a.dart) that defines the constant zero:
{% prettify dart tag=pre+code %} const zero = 0; {% endprettify %}
The following code produces this diagnostic because zero is declared in a library imported using a deferred import:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a;
void f({int x = [!a.zero!]}) {} {% endprettify %}
If you need to reference the constant from the imported library, then remove the deferred keyword:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a;
void f({int x = a.zero}) {} {% endprettify %}
If you don't need to reference the constant, then replace the default value:
{% prettify dart tag=pre+code %} void f({int x = 0}) {} {% endprettify %}
The values in a const list literal must be constants.
The analyzer produces this diagnostic when an element in a constant list literal isn‘t a constant value. The list literal can be constant either explicitly (because it’s prefixed by the const keyword) or implicitly (because it appears in a constant context).
The following code produces this diagnostic because x isn't a constant, even though it appears in an implicitly constant list literal:
{% prettify dart tag=pre+code %} var x = 2; var y = const [0, 1, [!x!]]; {% endprettify %}
If the list needs to be a constant list, then convert the element to be a constant. In the example above, you might add the const keyword to the declaration of x:
{% prettify dart tag=pre+code %} const x = 2; var y = const [0, 1, x]; {% endprettify %}
If the expression can‘t be made a constant, then the list can’t be a constant either, so you must change the code so that the list isn't a constant. In the example above this means removing the const keyword before the list literal:
{% prettify dart tag=pre+code %} var x = 2; var y = [0, 1, x]; {% endprettify %}
The elements in a const map literal must be constant.
The analyzer produces this diagnostic when an if element or a spread element in a constant map isn't a constant element.
The following code produces this diagnostic because it's attempting to spread a non-constant map:
{% prettify dart tag=pre+code %} var notConst = <int, int>{}; var map = const <int, int>{...[!notConst!]}; {% endprettify %}
Similarly, the following code produces this diagnostic because the condition in the if element isn't a constant expression:
{% prettify dart tag=pre+code %} bool notConst = true; var map = const <int, int>{if ([!notConst!]) 1 : 2}; {% endprettify %}
If the map needs to be a constant map, then make the elements constants. In the spread example, you might do that by making the collection being spread a constant:
{% prettify dart tag=pre+code %} const notConst = <int, int>{}; var map = const <int, int>{...notConst}; {% endprettify %}
If the map doesn't need to be a constant map, then remove the const keyword:
{% prettify dart tag=pre+code %} bool notConst = true; var map = <int, int>{if (notConst) 1 : 2}; {% endprettify %}
The keys in a const map literal must be constant.
The analyzer produces this diagnostic when a key in a constant map literal isn't a constant value.
The following code produces this diagnostic because a isn't a constant:
{% prettify dart tag=pre+code %} var a = ‘a’; var m = const {[!a!]: 0}; {% endprettify %}
If the map needs to be a constant map, then make the key a constant:
{% prettify dart tag=pre+code %} const a = ‘a’; var m = const {a: 0}; {% endprettify %}
If the map doesn't need to be a constant map, then remove the const keyword:
{% prettify dart tag=pre+code %} var a = ‘a’; var m = {a: 0}; {% endprettify %}
The values in a const map literal must be constant.
The analyzer produces this diagnostic when a value in a constant map literal isn't a constant value.
The following code produces this diagnostic because a isn't a constant:
{% prettify dart tag=pre+code %} var a = ‘a’; var m = const {0: [!a!]}; {% endprettify %}
If the map needs to be a constant map, then make the key a constant:
{% prettify dart tag=pre+code %} const a = ‘a’; var m = const {0: a}; {% endprettify %}
If the map doesn't need to be a constant map, then remove the const keyword:
{% prettify dart tag=pre+code %} var a = ‘a’; var m = {0: a}; {% endprettify %}
The values in a const set literal must be constants.
The analyzer produces this diagnostic when a constant set literal contains an element that isn't a compile-time constant.
The following code produces this diagnostic because i isn't a constant:
{% prettify dart tag=pre+code %} var i = 0;
var s = const {[!i!]}; {% endprettify %}
If the element can be changed to be a constant, then change it:
{% prettify dart tag=pre+code %} const i = 0;
var s = const {i}; {% endprettify %}
If the element can't be a constant, then remove the keyword const:
{% prettify dart tag=pre+code %} var i = 0;
var s = {i}; {% endprettify %}
The type arguments to ‘{0}’ must be known at compile time, so they can't be type parameters.
The analyzer produces this diagnostic when the type arguments to a method are required to be known at compile time, but a type parameter, whose value can't be known at compile time, is used as a type argument.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the type argument to Pointer.asFunction must be known at compile time, but the type parameter R, which isn't known at compile time, is being used as the type argument:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef T = int Function(int);
class C { void m(Pointer<NativeFunction> p) { p.asFunction<[!R!]>(); } } {% endprettify %}
Remove any uses of type parameters:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C { void m(Pointer<NativeFunction<Int64 Function(Int64)>> p) { p.asFunction<int Function(int)>(); } } {% endprettify %}
This instance creation must be ‘const’, because the {0} constructor is marked as ‘@literal’.
The analyzer produces this diagnostic when a constructor that has the [literal][meta-literal] annotation is invoked without using the const keyword, but all of the arguments to the constructor are constants. The annotation indicates that the constructor should be used to create a constant value whenever possible.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { @literal const C(); }
C f() => [!C()!]; {% endprettify %}
Add the keyword const before the constructor invocation:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { @literal const C(); }
void f() => const C(); {% endprettify %}
The generative constructor ‘{0}’ is expected, but a factory was found.
The analyzer produces this diagnostic when the initializer list of a constructor invokes a constructor from the superclass, and the invoked constructor is a factory constructor. Only a generative constructor can be invoked in the initializer list.
The following code produces this diagnostic because the invocation of the constructor super.one() is invoking a factory constructor:
{% prettify dart tag=pre+code %} class A { factory A.one() = B; A.two(); }
class B extends A { B() : [!super.one()!]; } {% endprettify %}
Change the super invocation to invoke a generative constructor:
{% prettify dart tag=pre+code %} class A { factory A.one() = B; A.two(); }
class B extends A { B() : super.two(); } {% endprettify %}
If the generative constructor is the unnamed constructor, and if there are no arguments being passed to it, then you can remove the super invocation.
The unnamed constructor of superclass ‘{0}’ (called by the default constructor of ‘{1}’) must be a generative constructor, but factory found.
The analyzer produces this diagnostic when a class has an implicit generative constructor and the superclass has an explicit unnamed factory constructor. The implicit constructor in the subclass implicitly invokes the unnamed constructor in the superclass, but generative constructors can only invoke another generative constructor, not a factory constructor.
The following code produces this diagnostic because the implicit constructor in B invokes the unnamed constructor in A, but the constructor in A is a factory constructor, when a generative constructor is required:
{% prettify dart tag=pre+code %} class A { factory A() => throw 0; A.named(); }
class [!B!] extends A {} {% endprettify %}
If the unnamed constructor in the superclass can be a generative constructor, then change it to be a generative constructor:
{% prettify dart tag=pre+code %} class A { A(); A.named(); }
class B extends A { } {% endprettify %}
If the unnamed constructor can't be a generative constructor and there are other generative constructors in the superclass, then explicitly invoke one of them:
{% prettify dart tag=pre+code %} class A { factory A() => throw 0; A.named(); }
class B extends A { B() : super.named(); } {% endprettify %}
If there are no generative constructors that can be used and none can be added, then implement the superclass rather than extending it:
{% prettify dart tag=pre+code %} class A { factory A() => throw 0; A.named(); }
class B implements A {} {% endprettify %}
Can‘t invoke ‘asFunction’ because the function signature ‘{0}’ for the pointer isn’t a valid C function signature.
The analyzer produces this diagnostic when the method asFunction is invoked on a pointer to a native function, but the signature of the native function isn't a valid C function signature.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because function signature associated with the pointer p (FNative) isn't a valid C function signature:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef FNative = int Function(int); typedef F = int Function(int);
class C { void f(Pointer<NativeFunction> p) { p.asFunction<[!F!]>(); } } {% endprettify %}
Make the NativeFunction signature a valid C signature:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
typedef FNative = Int8 Function(Int8); typedef F = int Function(int);
class C { void f(Pointer<NativeFunction> p) { p.asFunction(); } } {% endprettify %}
Array dimensions must be positive numbers.
The analyzer produces this diagnostic when a dimension given in an Array annotation is less than or equal to zero (0).
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because an array dimension of -1 was provided:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class MyStruct extends Struct { @Array([!-8!]) external Array a0; } {% endprettify %}
Change the dimension to be a positive integer:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class MyStruct extends Struct { @Array(8) external Array a0; } {% endprettify %}
The type ‘{1}’ isn't a valid type argument for ‘{0}’. The type argument must be a native integer, ‘Float’, ‘Double’, ‘Pointer’, or subtype of ‘Struct’, ‘Union’, or ‘AbiSpecificInteger’.
The analyzer produces this diagnostic when the type argument for the class Array isn't one of the valid types: either a native integer, Float, Double, Pointer, or subtype of Struct, Union, or AbiSpecificInteger.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the type argument to Array is Void, and Void isn't one of the valid types:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(8) external Array<[!Void!]> a0; } {% endprettify %}
Change the type argument to one of the valid types:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(8) external Array a0; } {% endprettify %}
Factory bodies can't use ‘async’, ‘async*’, or ‘sync*’.
The analyzer produces this diagnostic when the body of a factory constructor is marked with async, async*, or sync*. All constructors, including factory constructors, are required to return an instance of the class in which they're declared, not a Future, Stream, or Iterator.
The following code produces this diagnostic because the body of the factory constructor is marked with async:
{% prettify dart tag=pre+code %} class C { factory C() [!async!] { return C.(); } C.(); } {% endprettify %}
If the member must be declared as a factory constructor, then remove the keyword appearing before the body:
{% prettify dart tag=pre+code %} class C { factory C() { return C.(); } C.(); } {% endprettify %}
If the member must return something other than an instance of the enclosing class, then make the member a static method:
{% prettify dart tag=pre+code %} class C { static Future m() async { return C.(); } C.(); } {% endprettify %}
The name ‘{0}’ isn‘t a type so it can’t be used as a type argument.
The analyzer produces this diagnostic when an identifier that isn't a type is used as a type argument.
The following code produces this diagnostic because x is a variable, not a type:
{% prettify dart tag=pre+code %} var x = 0; List<[!x!]> xList = []; {% endprettify %}
Change the type argument to be a type:
{% prettify dart tag=pre+code %} var x = 0; List xList = []; {% endprettify %}
The name ‘{0}’ isn‘t a type and can’t be used in an on-catch clause.
The analyzer produces this diagnostic when the identifier following the on in a catch clause is defined to be something other than a type.
The following code produces this diagnostic because f is a function, not a type:
{% prettify dart tag=pre+code %} void f() { try { // ... } on [!f!] { // ... } } {% endprettify %}
Change the name to the type of object that should be caught:
{% prettify dart tag=pre+code %} void f() { try { // ... } on FormatException { // ... } } {% endprettify %}
The return type of the operator []= must be ‘void’.
The analyzer produces this diagnostic when a declaration of the operator []= has a return type other than void.
The following code produces this diagnostic because the declaration of the operator []= has a return type of int:
{% prettify dart tag=pre+code %} class C { [!int!] operator []=(int index, int value) => 0; } {% endprettify %}
Change the return type to void:
{% prettify dart tag=pre+code %} class C { void operator []=(int index, int value) => 0; } {% endprettify %}
The return type of the setter must be ‘void’ or absent.
The analyzer produces this diagnostic when a setter is defined with a return type other than void.
The following code produces this diagnostic because the setter p has a return type of int:
{% prettify dart tag=pre+code %} class C { [!int!] set p(int i) => 0; } {% endprettify %}
Change the return type to void or omit the return type:
{% prettify dart tag=pre+code %} class C { set p(int i) => 0; } {% endprettify %}
The non-nullable local variable ‘{0}’ must be assigned before it can be used.
The analyzer produces this diagnostic when a local variable is referenced and has all these characteristics:
late.The following code produces this diagnostic because x can't have a value of null, but is referenced before a value was assigned to it:
{% prettify dart tag=pre+code %} String f() { int x; return [!x!].toString(); } {% endprettify %}
The following code produces this diagnostic because the assignment to x might not be executed, so it might have a value of null:
{% prettify dart tag=pre+code %} int g(bool b) { int x; if (b) { x = 1; } return [!x!] * 2; } {% endprettify %}
The following code produces this diagnostic because the analyzer can‘t prove, based on definite assignment analysis, that x won’t be referenced without having a value assigned to it:
{% prettify dart tag=pre+code %} int h(bool b) { int x; if (b) { x = 1; } if (b) { return [!x!] * 2; } return 0; } {% endprettify %}
If null is a valid value, then make the variable nullable:
{% prettify dart tag=pre+code %} String f() { int? x; return x!.toString(); } {% endprettify %}
If null isn’t a valid value, and there's a reasonable default value, then add an initializer:
{% prettify dart tag=pre+code %} int g(bool b) { int x = 2; if (b) { x = 1; } return x * 2; } {% endprettify %}
Otherwise, ensure that a value was assigned on every possible code path before the value is accessed:
{% prettify dart tag=pre+code %} int g(bool b) { int x; if (b) { x = 1; } else { x = 2; } return x * 2; } {% endprettify %}
You can also mark the variable as late, which removes the diagnostic, but if the variable isn‘t assigned a value before it’s accessed, then it results in an exception being thrown at runtime. This approach should only be used if you‘re sure that the variable will always be assigned, even though the analyzer can’t prove it based on definite assignment analysis.
{% prettify dart tag=pre+code %} int h(bool b) { late int x; if (b) { x = 1; } if (b) { return x * 2; } return 0; } {% endprettify %}
{0} isn't a type.
The analyzer produces this diagnostic when a name is used as a type but declared to be something other than a type.
The following code produces this diagnostic because f is a function:
{% prettify dart tag=pre+code %} f() {} g([!f!] v) {} {% endprettify %}
Replace the name with the name of a type.
‘{0}’ isn't a binary operator.
The analyzer produces this diagnostic when an operator that can only be used as a unary operator is used as a binary operator.
The following code produces this diagnostic because the operator ~ can only be used as a unary operator:
{% prettify dart tag=pre+code %} var a = 5 [!~!] 3; {% endprettify %}
Replace the operator with the correct binary operator:
{% prettify dart tag=pre+code %} var a = 5 - 3; {% endprettify %}
{0} positional argument(s) expected, but {1} found.
The analyzer produces this diagnostic when a method or function invocation has fewer positional arguments than the number of required positional parameters.
The following code produces this diagnostic because f declares two required parameters, but only one argument is provided:
{% prettify dart tag=pre+code %} void f(int a, int b) {} void g() { f[!(0)!]; } {% endprettify %}
Add arguments corresponding to the remaining parameters:
{% prettify dart tag=pre+code %} void f(int a, int b) {} void g() { f(0, 1); } {% endprettify %}
Non-nullable instance field ‘{0}’ must be initialized.
The analyzer produces this diagnostic when a field is declared and has all these characteristics:
lateThe following code produces this diagnostic because x is implicitly initialized to null when it isn't allowed to be null:
{% prettify dart tag=pre+code %} class C { int [!x!]; } {% endprettify %}
Similarly, the following code produces this diagnostic because x is implicitly initialized to null, when it isn‘t allowed to be null, by one of the constructors, even though it’s initialized by other constructors:
{% prettify dart tag=pre+code %} class C { int x;
C(this.x);
[!C!].n(); } {% endprettify %}
If there's a reasonable default value for the field that’s the same for all instances, then add an initializer expression:
{% prettify dart tag=pre+code %} class C { int x = 0; } {% endprettify %}
If the value of the field should be provided when an instance is created, then add a constructor that sets the value of the field or update an existing constructor:
{% prettify dart tag=pre+code %} class C { int x;
C(this.x); } {% endprettify %}
You can also mark the field as late, which removes the diagnostic, but if the field isn‘t assigned a value before it’s accessed, then it results in an exception being thrown at runtime. This approach should only be used if you‘re sure that the field will always be assigned before it’s referenced.
{% prettify dart tag=pre+code %} class C { late int x; } {% endprettify %}
The non-nullable variable ‘{0}’ must be initialized.
The analyzer produces this diagnostic when a static field or top-level variable has a type that‘s non-nullable and doesn’t have an initializer. Fields and variables that don‘t have an initializer are normally initialized to null, but the type of the field or variable doesn’t allow it to be set to null, so an explicit initializer must be provided.
The following code produces this diagnostic because the field f can't be initialized to null:
{% prettify dart tag=pre+code %} class C { static int [!f!]; } {% endprettify %}
Similarly, the following code produces this diagnostic because the top-level variable v can't be initialized to null:
{% prettify dart tag=pre+code %} int [!v!]; {% endprettify %}
If the field or variable can't be initialized to null, then add an initializer that sets it to a non-null value:
{% prettify dart tag=pre+code %} class C { static int f = 0; } {% endprettify %}
If the field or variable should be initialized to null, then change the type to be nullable:
{% prettify dart tag=pre+code %} int? v; {% endprettify %}
If the field or variable can‘t be initialized in the declaration but will always be initialized before it’s referenced, then mark it as being late:
{% prettify dart tag=pre+code %} class C { static late int f; } {% endprettify %}
Spread elements in list or set literals must implement ‘Iterable’.
The analyzer produces this diagnostic when the static type of the expression of a spread element that appears in either a list literal or a set literal doesn't implement the type Iterable.
The following code produces this diagnostic:
{% prettify dart tag=pre+code %} var m = <String, int>{‘a’: 0, ‘b’: 1}; var s = {...[!m!]}; {% endprettify %}
The most common fix is to replace the expression with one that produces an iterable object:
{% prettify dart tag=pre+code %} var m = <String, int>{‘a’: 0, ‘b’: 1}; var s = {...m.keys}; {% endprettify %}
Spread elements in map literals must implement ‘Map’.
The analyzer produces this diagnostic when the static type of the expression of a spread element that appears in a map literal doesn't implement the type Map.
The following code produces this diagnostic because l isn't a Map:
{% prettify dart tag=pre+code %} var l = [‘a’, ‘b’]; var m = <int, String>{...[!l!]}; {% endprettify %}
The most common fix is to replace the expression with one that produces a map:
{% prettify dart tag=pre+code %} var l = [‘a’, ‘b’]; var m = <int, String>{...l.asMap()}; {% endprettify %}
Annotation creation must have arguments.
The analyzer produces this diagnostic when an annotation consists of a single identifier, but that identifier is the name of a class rather than a variable. To create an instance of the class, the identifier must be followed by an argument list.
The following code produces this diagnostic because C is a class, and a class can't be used as an annotation without invoking a const constructor from the class:
{% prettify dart tag=pre+code %} class C { const C(); }
[!@C!] var x; {% endprettify %}
Add the missing argument list:
{% prettify dart tag=pre+code %} class C { const C(); }
@C() var x; {% endprettify %}
Can't infer missing types in ‘{0}’ from overridden methods: {1}.
The analyzer produces this diagnostic when there is a method declaration for which one or more types needs to be inferred, and those types can't be inferred because none of the overridden methods has a function type that is a supertype of all the other overridden methods, as specified by override inference.
The following code produces this diagnostic because the method m declared in the class C is missing both the return type and the type of the parameter a, and neither of the missing types can be inferred for it:
{% prettify dart tag=pre+code %} abstract class A { A m(String a); }
abstract class B { B m(int a); }
abstract class C implements A, B { !m!; } {% endprettify %}
In this example, override inference can't be performed because the overridden methods are incompatible in these ways:
String and int) is a supertype of the other.If possible, add types to the method in the subclass that are consistent with the types from all the overridden methods:
{% prettify dart tag=pre+code %} abstract class A { A m(String a); }
abstract class B { B m(int a); }
abstract class C implements A, B { C m(Object a); } {% endprettify %}
The class ‘{0}’ can't extend ‘{1}’ because ‘{1}’ only has factory constructors (no generative constructors), and ‘{0}’ has at least one generative constructor.
The analyzer produces this diagnostic when a class that has at least one generative constructor (whether explicit or implicit) has a superclass that doesn't have any generative constructors. Every generative constructor, except the one defined in Object, invokes, either explicitly or implicitly, one of the generative constructors from its superclass.
The following code produces this diagnostic because the class B has an implicit generative constructor that can‘t invoke a generative constructor from A because A doesn’t have any generative constructors:
{% prettify dart tag=pre+code %} class A { factory A.none() => throw ''; }
class B extends [!A!] {} {% endprettify %}
If the superclass should have a generative constructor, then add one:
{% prettify dart tag=pre+code %} class A { A(); factory A.none() => throw ''; }
class B extends A {} {% endprettify %}
If the subclass shouldn't have a generative constructor, then remove it by adding a factory constructor:
{% prettify dart tag=pre+code %} class A { factory A.none() => throw ''; }
class B extends A { factory B.none() => throw ''; } {% endprettify %}
If the subclass must have a generative constructor but the superclass can't have one, then implement the superclass instead:
{% prettify dart tag=pre+code %} class A { factory A.none() => throw ''; }
class B implements A {} {% endprettify %}
A potentially nullable type can‘t be used in an ‘on’ clause because it isn’t valid to throw a nullable expression.
The analyzer produces this diagnostic when the type following on in a catch clause is a nullable type. It isn‘t valid to specify a nullable type because it isn’t possible to catch null (because it's a runtime error to throw null).
The following code produces this diagnostic because the exception type is specified to allow null when null can't be thrown:
{% prettify dart tag=pre+code %} void f() { try { // ... } on [!FormatException?!] { } } {% endprettify %}
Remove the question mark from the type:
{% prettify dart tag=pre+code %} void f() { try { // ... } on FormatException { } } {% endprettify %}
A class can't extend a nullable type.
The analyzer produces this diagnostic when a class declaration uses an extends clause to specify a superclass, and the superclass is followed by a ?.
It isn‘t valid to specify a nullable superclass because doing so would have no meaning; it wouldn’t change either the interface or implementation being inherited by the class containing the extends clause.
Note, however, that it is valid to use a nullable type as a type argument to the superclass, such as class A extends B<C?> {}.
The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in an extends clause:
{% prettify dart tag=pre+code %} class A {} class B extends [!A?!] {} {% endprettify %}
Remove the question mark from the type:
{% prettify dart tag=pre+code %} class A {} class B extends A {} {% endprettify %}
A class or mixin can't implement a nullable type.
The analyzer produces this diagnostic when a class or mixin declaration has an implements clause, and an interface is followed by a ?.
It isn‘t valid to specify a nullable interface because doing so would have no meaning; it wouldn’t change the interface being inherited by the class containing the implements clause.
Note, however, that it is valid to use a nullable type as a type argument to the interface, such as class A implements B<C?> {}.
The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in an implements clause:
{% prettify dart tag=pre+code %} class A {} class B implements [!A?!] {} {% endprettify %}
Remove the question mark from the type:
{% prettify dart tag=pre+code %} class A {} class B implements A {} {% endprettify %}
A mixin can't have a nullable type as a superclass constraint.
The analyzer produces this diagnostic when a mixin declaration uses an on clause to specify a superclass constraint, and the class that's specified is followed by a ?.
It isn‘t valid to specify a nullable superclass constraint because doing so would have no meaning; it wouldn’t change the interface being depended on by the mixin containing the on clause.
Note, however, that it is valid to use a nullable type as a type argument to the superclass constraint, such as mixin A on B<C?> {}.
The following code produces this diagnostic because A? is a nullable type and nullable types can't be used in an on clause:
{% prettify dart tag=pre+code %} class C {} mixin M on [!C?!] {} {% endprettify %}
Remove the question mark from the type:
{% prettify dart tag=pre+code %} class C {} mixin M on C {} {% endprettify %}
A class or mixin can't mix in a nullable type.
The analyzer produces this diagnostic when a class or mixin declaration has a with clause, and a mixin is followed by a ?.
It isn‘t valid to specify a nullable mixin because doing so would have no meaning; it wouldn’t change either the interface or implementation being inherited by the class containing the with clause.
Note, however, that it is valid to use a nullable type as a type argument to the mixin, such as class A with B<C?> {}.
The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in a with clause:
{% prettify dart tag=pre+code %} mixin M {} class C with [!M?!] {} {% endprettify %}
Remove the question mark from the type:
{% prettify dart tag=pre+code %} mixin M {} class C with M {} {% endprettify %}
‘{0}’ shouldn't be called with a null argument for the non-nullable type argument ‘{1}’.
The analyzer produces this diagnostic when null is passed to either the constructor Future.value or the method Completer.complete when the type argument used to create the instance was non-nullable. Even though the type system can't express this restriction, passing in a null results in a runtime exception.
The following code produces this diagnostic because null is being passed to the constructor Future.value even though the type argument is the non-nullable type String:
{% prettify dart tag=pre+code %} Future f() { return Future.value([!null!]); } {% endprettify %}
Pass in a non-null value:
{% prettify dart tag=pre+code %} Future f() { return Future.value(''); } {% endprettify %}
This null-check will always throw an exception because the expression will always evaluate to ‘null’.
The analyzer produces this diagnostic when the null check operator (!) is used on an expression whose value can only be null. In such a case the operator always throws an exception, which likely isn't the intended behavior.
The following code produces this diagnostic because the function g will always return null, which means that the null check in f will always throw:
{% prettify dart tag=pre+code %} void f() { [!g()!!]; }
Null g() => null; {% endprettify %}
If you intend to always throw an exception, then replace the null check with an explicit throw expression to make the intent more clear:
{% prettify dart tag=pre+code %} void f() { g(); throw TypeError(); }
Null g() => null; {% endprettify %}
The type ‘{0}’ can be included in the superclass constraints only once.
The analyzer produces this diagnostic when the same type is listed in the superclass constraints of a mixin multiple times.
The following code produces this diagnostic because A is included twice in the superclass constraints for M:
{% prettify dart tag=pre+code %} mixin M on A, [!A!] { }
class A {} class B {} {% endprettify %}
If a different type should be included in the superclass constraints, then replace one of the occurrences with the other type:
{% prettify dart tag=pre+code %} mixin M on A, B { }
class A {} class B {} {% endprettify %}
If no other type was intended, then remove the repeated type name:
{% prettify dart tag=pre+code %} mixin M on A { }
class A {} class B {} {% endprettify %}
Optional parameters aren't allowed when defining an operator.
The analyzer produces this diagnostic when one or more of the parameters in an operator declaration are optional.
The following code produces this diagnostic because the parameter other is an optional parameter:
{% prettify dart tag=pre+code %} class C { C operator +([[!C? other!]]) => this; } {% endprettify %}
Make all of the parameters be required parameters:
{% prettify dart tag=pre+code %} class C { C operator +(C other) => this; } {% endprettify %}
The field doesn't override an inherited getter or setter.
The getter doesn't override an inherited getter.
The method doesn't override an inherited method.
The setter doesn't override an inherited setter.
The analyzer produces this diagnostic when a class member is annotated with the @override annotation, but the member isn’t declared in any of the supertypes of the class.
The following code produces this diagnostic because m isn't declared in any of the supertypes of C:
{% prettify dart tag=pre+code %} class C { @override String !m! => ''; } {% endprettify %}
If the member is intended to override a member with a different name, then update the member to have the same name:
{% prettify dart tag=pre+code %} class C { @override String toString() => ''; } {% endprettify %}
If the member is intended to override a member that was removed from the superclass, then consider removing the member from the subclass.
If the member can't be removed, then remove the annotation.
Structs must have at most one ‘Packed’ annotation.
The analyzer produces this diagnostic when a subclass of Struct has more than one Packed annotation.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class C, which is a subclass of Struct, has two Packed annotations:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed(1) [!@Packed(1)!] class C extends Struct { external Pointer notEmpty; } {% endprettify %}
Remove all but one of the annotations:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed(1) class C extends Struct { external Pointer notEmpty; } {% endprettify %}
Only packing to 1, 2, 4, 8, and 16 bytes is supported.
The analyzer produces this diagnostic when the argument to the Packed annotation isn't one of the allowed values: 1, 2, 4, 8, or 16.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the argument to the Packed annotation (3) isn't one of the allowed values:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed([!3!]) class C extends Struct { external Pointer notEmpty; } {% endprettify %}
Change the alignment to be one of the allowed values:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed(4) class C extends Struct { external Pointer notEmpty; } {% endprettify %}
Nesting the non-packed or less tightly packed struct ‘{0}’ in a packed struct ‘{1}’ isn't supported.
The analyzer produces this diagnostic when a subclass of Struct that is annotated as being Packed declares a field whose type is also a subclass of Struct and the field's type is either not packed or is packed less tightly.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class Outer, which is a subclass of Struct and is packed on 1-byte boundaries, declared a field whose type (Inner) is packed on 8-byte boundaries:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed(8) class Inner extends Struct { external Pointer notEmpty; }
@Packed(1) class Outer extends Struct { external Pointer notEmpty;
external [!Inner!] nestedLooselyPacked; } {% endprettify %}
If the inner struct should be packed more tightly, then change the argument to the inner struct's Packed annotation:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed(1) class Inner extends Struct { external Pointer notEmpty; }
@Packed(1) class Outer extends Struct { external Pointer notEmpty;
external Inner nestedLooselyPacked; } {% endprettify %}
If the outer struct should be packed less tightly, then change the argument to the outer struct's Packed annotation:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
@Packed(8) class Inner extends Struct { external Pointer notEmpty; }
@Packed(8) class Outer extends Struct { external Pointer notEmpty;
external Inner nestedLooselyPacked; } {% endprettify %}
If the inner struct doesn't have an annotation and should be packed, then add an annotation.
If the inner struct doesn‘t have an annotation and the outer struct shouldn’t be packed, then remove its annotation.
Expected this library to be part of ‘{0}’, not ‘{1}’.
The analyzer produces this diagnostic when a library attempts to include a file as a part of itself when the other file is a part of a different library.
Given a file named part.dart containing
{% prettify dart tag=pre+code %} part of ‘library.dart’; {% endprettify %}
The following code, in any file other than library.dart, produces this diagnostic because it attempts to include part.dart as a part of itself when part.dart is a part of a different library:
{% prettify dart tag=pre+code %} part [!‘package:a/part.dart’!]; {% endprettify %}
If the library should be using a different file as a part, then change the URI in the part directive to be the URI of the other file.
If the part file should be a part of this library, then update the URI (or library name) in the part-of directive to be the URI (or name) of the correct library.
The included part ‘{0}’ must have a part-of directive.
The analyzer produces this diagnostic when a part directive is found and the referenced file doesn't have a part-of directive.
Given a file (a.dart) containing:
{% prettify dart tag=pre+code %} class A {} {% endprettify %}
The following code produces this diagnostic because a.dart doesn't contain a part-of directive:
{% prettify dart tag=pre+code %} part [!‘a.dart’!]; {% endprettify %}
If the referenced file is intended to be a part of another library, then add a part-of directive to the file:
{% prettify dart tag=pre+code %} part of ‘test.dart’;
class A {} {% endprettify %}
If the referenced file is intended to be a library, then replace the part directive with an import directive:
{% prettify dart tag=pre+code %} import ‘a.dart’; {% endprettify %}
The library is unnamed. A URI is expected, not a library name ‘{0}’, in the part-of directive.
The analyzer produces this diagnostic when a library that doesn‘t have a library directive (and hence has no name) contains a part directive and the part of directive in the part file uses a name to specify the library that it’s a part of.
Given a part file named part_file.dart containing the following code:
{% prettify dart tag=pre+code %} part of lib; {% endprettify %}
The following code produces this diagnostic because the library including the part file doesn‘t have a name even though the part file uses a name to specify which library it’s a part of:
{% prettify dart tag=pre+code %} part [!‘part_file.dart’!]; {% endprettify %}
Change the part of directive in the part file to specify its library by URI:
{% prettify dart tag=pre+code %} part of ‘test.dart’; {% endprettify %}
The path ‘{0}’ doesn't exist.
The analyzer produces this diagnostic when a dependency has a path key referencing a directory that doesn't exist.
Assuming that the directory doesNotExist doesn‘t exist, the following code produces this diagnostic because it’s listed as the path of a package:
name: example dependencies: local_package: path: doesNotExist
If the path is correct, then create a directory at that path.
If the path isn't correct, then change the path to match the path to the root of the package.
The path ‘{0}’ isn't a POSIX-style path.
The analyzer produces this diagnostic when a dependency has a path key whose value is a string, but isn't a POSIX-style path.
The following code produces this diagnostic because the path following the path key is a Windows path:
name: example dependencies: local_package: path: E:\local_package
Convert the path to a POSIX path.
The directory ‘{0}’ doesn't contain a pubspec.
The analyzer produces this diagnostic when a dependency has a path key that references a directory that doesn't contain a pubspec.yaml file.
Assuming that the directory local_package doesn‘t contain a file named pubspec.yaml, the following code produces this diagnostic because it’s listed as the path of a package:
name: example dependencies: local_package: path: local_package
If the path is intended to be the root of a package, then add a pubspec.yaml file in the directory:
name: local_package
If the path is wrong, then replace it with the correct path.
The name ‘{0}’ is already used as an import prefix and can't be used to name a top-level element.
The analyzer produces this diagnostic when a name is used as both an import prefix and the name of a top-level declaration in the same library.
The following code produces this diagnostic because f is used as both an import prefix and the name of a function:
{% prettify dart tag=pre+code %} import ‘dart:math’ as f;
int !f! => f.min(0, 1); {% endprettify %}
If you want to use the name for the import prefix, then rename the top-level declaration:
{% prettify dart tag=pre+code %} import ‘dart:math’ as f;
int g() => f.min(0, 1); {% endprettify %}
If you want to use the name for the top-level declaration, then rename the import prefix:
{% prettify dart tag=pre+code %} import ‘dart:math’ as math;
int f() => math.min(0, 1); {% endprettify %}
The name ‘{0}’ refers to an import prefix, so it must be followed by ‘.’.
The analyzer produces this diagnostic when an import prefix is used by itself, without accessing any of the names declared in the libraries associated with the prefix. Prefixes aren‘t variables, and therefore can’t be used as a value.
The following code produces this diagnostic because the prefix math is being used as if it were a variable:
{% prettify dart tag=pre+code %} import ‘dart:math’ as math;
void f() { print([!math!]); } {% endprettify %}
If the code is incomplete, then reference something in one of the libraries associated with the prefix:
{% prettify dart tag=pre+code %} import ‘dart:math’ as math;
void f() { print(math.pi); } {% endprettify %}
If the name is wrong, then correct the name.
The prefix ‘{0}’ can‘t be used here because it’s shadowed by a local declaration.
The analyzer produces this diagnostic when an import prefix is used in a context where it isn't visible because it was shadowed by a local declaration.
The following code produces this diagnostic because the prefix a is being used to access the class Future, but isn‘t visible because it’s shadowed by the parameter a:
{% prettify dart tag=pre+code %} import ‘dart:async’ as a;
a.Future? f(int a) { [!a!].Future? x; return x; } {% endprettify %}
Rename either the prefix:
{% prettify dart tag=pre+code %} import ‘dart:async’ as p;
p.Future? f(int a) { p.Future? x; return x; } {% endprettify %}
Or rename the local variable:
{% prettify dart tag=pre+code %} import ‘dart:async’ as a;
a.Future? f(int p) { a.Future? x; return x; } {% endprettify %}
The private name ‘{0}’, defined by ‘{1}’, conflicts with the same name defined by ‘{2}’.
The analyzer produces this diagnostic when two mixins that define the same private member are used together in a single class in a library other than the one that defines the mixins.
Given a file named a.dart containing the following code:
{% prettify dart tag=pre+code %} class A { void _foo() {} }
class B { void _foo() {} } {% endprettify %}
The following code produces this diagnostic because the classes A and B both define the method _foo:
{% prettify dart tag=pre+code %} import ‘a.dart’;
class C extends Object with A, [!B!] {} {% endprettify %}
If you don't need both of the mixins, then remove one of them from the with clause:
{% prettify dart tag=pre+code %} import ‘a.dart’;
class C extends Object with A, [!B!] {} {% endprettify %}
If you need both of the mixins, then rename the conflicting member in one of the two mixins.
Named parameters can't start with an underscore.
The analyzer produces this diagnostic when the name of a named parameter starts with an underscore.
The following code produces this diagnostic because the named parameter _x starts with an underscore:
{% prettify dart tag=pre+code %} class C { void m({int [!_x!] = 0}) {} } {% endprettify %}
Rename the parameter so that it doesn't start with an underscore:
{% prettify dart tag=pre+code %} class C { void m({int x = 0}) {} } {% endprettify %}
The setter ‘{0}’ is private and can't be accessed outside the library that declares it.
The analyzer produces this diagnostic when a private setter is used in a library where it isn't visible.
Given a file named a.dart that contains the following:
{% prettify dart tag=pre+code %} class A { static int _f = 0; } {% endprettify %}
The following code produces this diagnostic because it references the private setter _f even though the setter isn't visible:
{% prettify dart tag=pre+code %} import ‘a.dart’;
void f() { A.[!_f!] = 0; } {% endprettify %}
If you're able to make the setter public, then do so:
{% prettify dart tag=pre+code %} class A { static int f = 0; } {% endprettify %}
If you aren't able to make the setter public, then find a different way to implement the code.
The final variable ‘{0}’ can‘t be read because it’s potentially unassigned at this point.
The analyzer produces this diagnostic when a final local variable that isn‘t initialized at the declaration site is read at a point where the compiler can’t prove that the variable is always initialized before it's referenced.
The following code produces this diagnostic because the final local variable x is read (on line 3) when it‘s possible that it hasn’t yet been initialized:
{% prettify dart tag=pre+code %} int f() { final int x; return [!x!]; } {% endprettify %}
Ensure that the variable has been initialized before it's read:
{% prettify dart tag=pre+code %} int f(bool b) { final int x; if (b) { x = 0; } else { x = 1; } return x; } {% endprettify %}
The compile-time constant expression depends on itself.
The analyzer produces this diagnostic when the value of a compile-time constant is defined in terms of itself, either directly or indirectly, creating an infinite loop.
The following code produces this diagnostic twice because both of the constants are defined in terms of the other:
{% prettify dart tag=pre+code %} const [!secondsPerHour!] = minutesPerHour * 60; const [!minutesPerHour!] = secondsPerHour / 60; {% endprettify %}
Break the cycle by finding an alternative way of defining at least one of the constants:
{% prettify dart tag=pre+code %} const secondsPerHour = minutesPerHour * 60; const minutesPerHour = 60; {% endprettify %}
Constructors can't redirect to themselves either directly or indirectly.
The analyzer produces this diagnostic when a constructor redirects to itself, either directly or indirectly, creating an infinite loop.
The following code produces this diagnostic because the generative constructors C.a and C.b each redirect to the other:
{% prettify dart tag=pre+code %} class C { C.a() : [!this.b()!]; C.b() : [!this.a()!]; } {% endprettify %}
The following code produces this diagnostic because the factory constructors A and B each redirect to the other:
{% prettify dart tag=pre+code %} abstract class A { factory A() = [!B!]; } class B implements A { factory B() = [!A!]; B.named(); } {% endprettify %}
In the case of generative constructors, break the cycle by finding defining at least one of the constructors to not redirect to another constructor:
{% prettify dart tag=pre+code %} class C { C.a() : this.b(); C.b(); } {% endprettify %}
In the case of factory constructors, break the cycle by defining at least one of the factory constructors to do one of the following:
{% prettify dart tag=pre+code %} abstract class A { factory A() = B; } class B implements A { factory B() = B.named; B.named(); } {% endprettify %}
{% prettify dart tag=pre+code %} abstract class A { factory A() = B; } class B implements A { factory B() { return B.named(); }
B.named(); } {% endprettify %}
{% prettify dart tag=pre+code %} abstract class A { factory A() = B; } class B implements A { B(); B.named(); } {% endprettify %}
‘{0}’ can't be a superinterface of itself: {1}.
‘{0}’ can't extend itself.
‘{0}’ can't implement itself.
‘{0}’ can't use itself as a mixin.
‘{0}’ can't use itself as a superclass constraint.
The analyzer produces this diagnostic when there's a circularity in the type hierarchy. This happens when a type, either directly or indirectly, is declared to be a subtype of itself.
The following code produces this diagnostic because the class A is declared to be a subtype of B, and B is a subtype of A:
{% prettify dart tag=pre+code %} class [!A!] extends B {} class B implements A {} {% endprettify %}
Change the type hierarchy so that there's no circularity.
The constructor ‘{0}’ couldn't be found in ‘{1}’.
The analyzer produces this diagnostic when a generative constructor redirects to a constructor that isn't defined.
The following code produces this diagnostic because the constructor C.a redirects to the constructor C.b, but C.b isn't defined:
{% prettify dart tag=pre+code %} class C { C.a() : [!this.b()!]; } {% endprettify %}
If the missing constructor must be called, then define it:
{% prettify dart tag=pre+code %} class C { C.a() : this.b(); C.b(); } {% endprettify %}
If the missing constructor doesn't need to be called, then remove the redirect:
{% prettify dart tag=pre+code %} class C { C.a(); } {% endprettify %}
Generative constructors can't redirect to a factory constructor.
The analyzer produces this diagnostic when a generative constructor redirects to a factory constructor.
The following code produces this diagnostic because the generative constructor C.a redirects to the factory constructor C.b:
{% prettify dart tag=pre+code %} class C { C.a() : [!this.b()!]; factory C.b() => C.a(); } {% endprettify %}
If the generative constructor doesn't need to redirect to another constructor, then remove the redirect.
{% prettify dart tag=pre+code %} class C { C.a(); factory C.b() => C.a(); } {% endprettify %}
If the generative constructor must redirect to another constructor, then make the other constructor be a generative (non-factory) constructor:
{% prettify dart tag=pre+code %} class C { C.a() : this.b(); C.b(); } {% endprettify %}
The redirecting constructor ‘{0}’ can't redirect to a constructor of the abstract class ‘{1}’.
The analyzer produces this diagnostic when a constructor redirects to a constructor in an abstract class.
The following code produces this diagnostic because the factory constructor in A redirects to a constructor in B, but B is an abstract class:
{% prettify dart tag=pre+code %} class A { factory A() = [!B!]; }
abstract class B implements A {} {% endprettify %}
If the code redirects to the correct constructor, then change the class so that it isn't abstract:
{% prettify dart tag=pre+code %} class A { factory A() = B; }
class B implements A {} {% endprettify %}
Otherwise, change the factory constructor so that it either redirects to a constructor in a concrete class, or has a concrete implementation.
The redirected constructor ‘{0}’ has incompatible parameters with ‘{1}’.
The analyzer produces this diagnostic when a factory constructor attempts to redirect to another constructor, but the two have incompatible parameters. The parameters are compatible if all of the parameters of the redirecting constructor can be passed to the other constructor and if the other constructor doesn‘t require any parameters that aren’t declared by the redirecting constructor.
The following code produces this diagnostic because the constructor for A doesn't declare a parameter that the constructor for B requires:
{% prettify dart tag=pre+code %} abstract class A { factory A() = [!B!]; }
class B implements A { B(int x); B.zero(); } {% endprettify %}
The following code produces this diagnostic because the constructor for A declares a named parameter (y) that the constructor for B doesn't allow:
{% prettify dart tag=pre+code %} abstract class A { factory A(int x, {int y}) = [!B!]; }
class B implements A { B(int x); } {% endprettify %}
If there's a different constructor that is compatible with the redirecting constructor, then redirect to that constructor:
{% prettify dart tag=pre+code %} abstract class A { factory A() = B.zero; }
class B implements A { B(int x); B.zero(); } {% endprettify %}
Otherwise, update the redirecting constructor to be compatible:
{% prettify dart tag=pre+code %} abstract class A { factory A(int x) = B; }
class B implements A { B(int x); } {% endprettify %}
The return type ‘{0}’ of the redirected constructor isn't a subtype of ‘{1}’.
The analyzer produces this diagnostic when a factory constructor redirects to a constructor whose return type isn't a subtype of the type that the factory constructor is declared to produce.
The following code produces this diagnostic because A isn‘t a subclass of C, which means that the value returned by the constructor A() couldn’t be returned from the constructor C():
{% prettify dart tag=pre+code %} class A {}
class B implements C {}
class C { factory C() = [!A!]; } {% endprettify %}
If the factory constructor is redirecting to a constructor in the wrong class, then update the factory constructor to redirect to the correct constructor:
{% prettify dart tag=pre+code %} class A {}
class B implements C {}
class C { factory C() = B; } {% endprettify %}
If the class defining the constructor being redirected to is the class that should be returned, then make it a subtype of the factory's return type:
{% prettify dart tag=pre+code %} class A implements C {}
class B implements C {}
class C { factory C() = A; } {% endprettify %}
The constructor ‘{0}’ couldn't be found in ‘{1}’.
The analyzer produces this diagnostic when a constructor redirects to a constructor that doesn't exist.
The following code produces this diagnostic because the factory constructor in A redirects to a constructor in B that doesn't exist:
{% prettify dart tag=pre+code %} class A { factory A() = [!B.name!]; }
class B implements A { B(); } {% endprettify %}
If the constructor being redirected to is correct, then define the constructor:
{% prettify dart tag=pre+code %} class A { factory A() = B.name; }
class B implements A { B(); B.name(); } {% endprettify %}
If a different constructor should be invoked, then update the redirect:
{% prettify dart tag=pre+code %} class A { factory A() = B; }
class B implements A { B(); } {% endprettify %}
The name ‘{0}’ isn‘t a type and can’t be used in a redirected constructor.
One way to implement a factory constructor is to redirect to another constructor by referencing the name of the constructor. The analyzer produces this diagnostic when the redirect is to something other than a constructor.
The following code produces this diagnostic because f is a function:
{% prettify dart tag=pre+code %} C f() => throw 0;
class C { factory C() = [!f!]; } {% endprettify %}
If the constructor isn't defined, then either define it or replace it with a constructor that is defined.
If the constructor is defined but the class that defines it isn't visible, then you probably need to add an import.
If you‘re trying to return the value returned by a function, then rewrite the constructor to return the value from the constructor’s body:
{% prettify dart tag=pre+code %} C f() => throw 0;
class C { factory C() => f(); } {% endprettify %}
A constant redirecting constructor can't redirect to a non-constant constructor.
The analyzer produces this diagnostic when a constructor marked as const redirects to a constructor that isn't marked as const.
The following code produces this diagnostic because the constructor C.a is marked as const but redirects to the constructor C.b, which isn't:
{% prettify dart tag=pre+code %} class C { const C.a() : this.!b!; C.b(); } {% endprettify %}
If the non-constant constructor can be marked as const, then mark it as const:
{% prettify dart tag=pre+code %} class C { const C.a() : this.b(); const C.b(); } {% endprettify %}
If the non-constant constructor can't be marked as const, then either remove the redirect or remove const from the redirecting constructor:
{% prettify dart tag=pre+code %} class C { C.a() : this.b(); C.b(); } {% endprettify %}
A redirecting constructor can't redirect to a type alias that expands to a type parameter.
The analyzer produces this diagnostic when a redirecting factory constructor redirects to a type alias, and the type alias expands to one of the type parameters of the type alias. This isn’t allowed because the value of the type parameter is a type rather than a class.
The following code produces this diagnostic because the redirect to B<A> is to a type alias whose value is T, even though it looks like the value should be A:
{% prettify dart tag=pre+code %} class A implements C {}
typedef B = T;
abstract class C { factory C() = [!B!]; } {% endprettify %}
Use either a class name or a type alias that is defined to be a class rather than a type alias defined to be a type parameter:
{% prettify dart tag=pre+code %} class A implements C {}
abstract class C { factory C() = A; } {% endprettify %}
Local variable ‘{0}’ can't be referenced before it is declared.
The analyzer produces this diagnostic when a variable is referenced before it’s declared. In Dart, variables are visible everywhere in the block in which they are declared, but can only be referenced after they are declared.
The analyzer also produces a context message that indicates where the declaration is located.
The following code produces this diagnostic because i is used before it is declared:
{% prettify dart tag=pre+code %} void f() { print([!i!]); int i = 5; } {% endprettify %}
If you intended to reference the local variable, move the declaration before the first reference:
{% prettify dart tag=pre+code %} void f() { int i = 5; print(i); } {% endprettify %}
If you intended to reference a name from an outer scope, such as a parameter, instance field or top-level variable, then rename the local declaration so that it doesn't hide the outer variable.
{% prettify dart tag=pre+code %} void f(int i) { print(i); int x = 5; print(x); } {% endprettify %}
A rethrow must be inside of a catch clause.
The analyzer produces this diagnostic when a rethrow statement is outside a catch clause. The rethrow statement is used to throw a caught exception again, but there's no caught exception outside of a catch clause.
The following code produces this diagnostic because therethrow statement is outside of a catch clause:
{% prettify dart tag=pre+code %} void f() { [!rethrow!]; } {% endprettify %}
If you're trying to rethrow an exception, then wrap the rethrow statement in a catch clause:
{% prettify dart tag=pre+code %} void f() { try { // ... } catch (exception) { rethrow; } } {% endprettify %}
If you're trying to throw a new exception, then replace the rethrow statement with a throw expression:
{% prettify dart tag=pre+code %} void f() { throw UnsupportedError(‘Not yet implemented’); } {% endprettify %}
Constructors can't return values.
The analyzer produces this diagnostic when a generative constructor contains a return statement that specifies a value to be returned. Generative constructors always return the object that was created, and therefore can't return a different object.
The following code produces this diagnostic because the return statement has an expression:
{% prettify dart tag=pre+code %} class C { C() { return [!this!]; } } {% endprettify %}
If the constructor should create a new instance, then remove either the return statement or the expression:
{% prettify dart tag=pre+code %} class C { C(); } {% endprettify %}
If the constructor shouldn't create a new instance, then convert it to be a factory constructor:
{% prettify dart tag=pre+code %} class C { factory C() { return _instance; }
static C instance = C.();
C._(); } {% endprettify %}
Can't return a value from a generator function that uses the ‘async*’ or ‘sync*’ modifier.
The analyzer produces this diagnostic when a generator function (one whose body is marked with either async* or sync*) uses either a return statement to return a value or implicitly returns a value because of using =>. In any of these cases, they should use yield instead of return.
The following code produces this diagnostic because the method f is a generator and is using return to return a value:
{% prettify dart tag=pre+code %} Iterable f() sync* { [!return 3!]; } {% endprettify %}
The following code produces this diagnostic because the function f is a generator and is implicitly returning a value:
{% prettify dart tag=pre+code %} Stream f() async* [!=>!] 3; {% endprettify %}
If the function is using => for the body of the function, then convert it to a block function body, and use yield to return a value:
{% prettify dart tag=pre+code %} Stream f() async* { yield 3; } {% endprettify %}
If the method is intended to be a generator, then use yield to return a value:
{% prettify dart tag=pre+code %} Iterable f() sync* { yield 3; } {% endprettify %}
If the method isn‘t intended to be a generator, then remove the modifier from the body (or use async if you’re returning a future):
{% prettify dart tag=pre+code %} int f() { return 3; } {% endprettify %}
‘{0}’ is annotated with ‘doNotStore’ and shouldn't be returned unless ‘{1}’ is also annotated.
The analyzer produces this diagnostic when a value that is annotated with the [doNotStore][meta-doNotStore] annotation is returned from a method, getter, or function that doesn't have the same annotation.
The following code produces this diagnostic because the result of invoking f shouldn‘t be stored, but the function g isn’t annotated to preserve that semantic:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@doNotStore int f() => 0;
int g() => [!f()!]; {% endprettify %}
If the value that shouldn't be stored is the correct value to return, then mark the function with the [doNotStore][meta-doNotStore] annotation:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@doNotStore int f() => 0;
@doNotStore int g() => f(); {% endprettify %}
Otherwise, return a different value from the function:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@doNotStore int f() => 0;
int g() => 0; {% endprettify %}
A value of type ‘{0}’ can't be returned from the constructor ‘{2}’ because it has a return type of ‘{1}’.
A value of type ‘{0}’ can't be returned from the function ‘{2}’ because it has a return type of ‘{1}’.
A value of type ‘{0}’ can't be returned from the method ‘{2}’ because it has a return type of ‘{1}’.
The analyzer produces this diagnostic when a method or function returns a value whose type isn't assignable to the declared return type.
The following code produces this diagnostic because f has a return type of String but is returning an int:
{% prettify dart tag=pre+code %} String f() => [!3!]; {% endprettify %}
If the return type is correct, then replace the value being returned with a value of the correct type, possibly by converting the existing value:
{% prettify dart tag=pre+code %} String f() => 3.toString(); {% endprettify %}
If the value is correct, then change the return type to match:
{% prettify dart tag=pre+code %} int f() => 3; {% endprettify %}
The return type ‘{0}’ isn‘t a ‘{1}’, as required by the closure’s context.
The analyzer produces this diagnostic when the static type of a returned expression isn't assignable to the return type that the closure is required to have.
The following code produces this diagnostic because f is defined to be a function that returns a String, but the closure assigned to it returns an int:
{% prettify dart tag=pre+code %} String Function(String) f = (s) => [!3!]; {% endprettify %}
If the return type is correct, then replace the returned value with a value of the correct type, possibly by converting the existing value:
{% prettify dart tag=pre+code %} String Function(String) f = (s) => 3.toString(); {% endprettify %}
The return value is missing after ‘return’.
The analyzer produces this diagnostic when it finds a return statement without an expression in a function that declares a return type.
The following code produces this diagnostic because the function f is expected to return an int, but no value is being returned:
{% prettify dart tag=pre+code %} int f() { [!return!]; } {% endprettify %}
Add an expression that computes the value to be returned:
{% prettify dart tag=pre+code %} int f() { return 0; } {% endprettify %}
The class ‘{0}’ wasn't exported from ‘dart:core’ until version 2.1, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when either the class Future or Stream is referenced in a library that doesn‘t import dart:async in code that has an SDK constraint whose lower bound is less than 2.1.0. In earlier versions, these classes weren’t defined in dart:core, so the import was necessary.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.1.0:
environment: sdk: '>=2.0.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} void f([!Future!] f) {} {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the classes to be referenced:
environment: sdk: '>=2.1.0 <2.4.0'
If you need to support older versions of the SDK, then import the dart:async library.
{% prettify dart tag=pre+code %} import ‘dart:async’;
void f(Future f) {} {% endprettify %}
The use of an as expression in a constant expression wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when an as expression inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using an as expression in a constant context wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:
environment: sdk: '>=2.1.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} const num n = 3; const int i = [!n as int!]; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the expression to be used:
environment: sdk: '>=2.3.2 <2.4.0'
If you need to support older versions of the SDK, then either rewrite the code to not use an as expression, or change the code so that the as expression isn't in a constant context:
{% prettify dart tag=pre+code %} num x = 3; int y = x as int; {% endprettify %}
The use of the operator ‘{0}’ for ‘bool’ operands in a constant context wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when any use of the &, |, or ^ operators on the class bool inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using these operators in a constant context wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:
environment: sdk: '>=2.1.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} const bool a = true; const bool b = false; const bool c = a [!&!] b; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operators to be used:
environment: sdk: '>=2.3.2 <2.4.0'
If you need to support older versions of the SDK, then either rewrite the code to not use these operators, or change the code so that the expression isn't in a constant context:
{% prettify dart tag=pre+code %} const bool a = true; const bool b = false; bool c = a & b; {% endprettify %}
Tearing off a constructor requires the ‘constructor-tearoffs’ language feature.
The analyzer produces this diagnostic when a constructor tear-off is found in code that has an SDK constraint whose lower bound is less than 2.15. Constructor tear-offs weren‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.15:
environment: sdk: '>=2.9.0 <2.15.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} var setConstructor = [!Set.identity!]; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operator to be used:
environment: sdk: '>=2.15.0 <2.16.0'
If you need to support older versions of the SDK, then rewrite the code to not use constructor tear-offs:
{% prettify dart tag=pre+code %} var setConstructor = () => Set.identity(); {% endprettify %}
Using the operator ‘==’ for non-primitive types wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when the operator == is used on a non-primitive type inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using this operator in a constant context wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:
environment: sdk: '>=2.1.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} class C {} const C a = null; const C b = null; const bool same = a [!==!] b; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operator to be used:
environment: sdk: '>=2.3.2 <2.4.0'
If you need to support older versions of the SDK, then either rewrite the code to not use the == operator, or change the code so that the expression isn't in a constant context:
{% prettify dart tag=pre+code %} class C {} const C a = null; const C b = null; bool same = a == b; {% endprettify %}
Extension methods weren't supported until version 2.6.0, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when an extension declaration or an extension override is found in code that has an SDK constraint whose lower bound is less than 2.6.0. Using extensions wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.6.0:
environment: sdk: '>=2.4.0 <2.7.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} [!extension!] E on String { void sayHello() { print(‘Hello $this’); } } {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:
environment: sdk: '>=2.6.0 <2.7.0'
If you need to support older versions of the SDK, then rewrite the code to not make use of extensions. The most common way to do this is to rewrite the members of the extension as top-level functions (or methods) that take the value that would have been bound to this as a parameter:
{% prettify dart tag=pre+code %} void sayHello(String s) { print(‘Hello $s’); } {% endprettify %}
The operator ‘>>>’ wasn't supported until version 2.14.0, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when the operator >>> is used in code that has an SDK constraint whose lower bound is less than 2.14.0. This operator wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.14.0:
environment: sdk: '>=2.0.0 <2.15.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} int x = 3 [!>>>!] 4; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the operator to be used:
environment: sdk: '>=2.14.0 <2.15.0'
If you need to support older versions of the SDK, then rewrite the code to not use the >>> operator:
{% prettify dart tag=pre+code %} int x = logicalShiftRight(3, 4);
int logicalShiftRight(int leftOperand, int rightOperand) { int divisor = 1 << rightOperand; if (divisor == 0) { return 0; } return leftOperand ~/ divisor; } {% endprettify %}
The use of an is expression in a constant context wasn't supported until version 2.3.2, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when an is expression inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.3.2. Using an is expression in a constant context wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.2:
environment: sdk: '>=2.1.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} const Object x = 4; const y = [!x is int!] ? 0 : 1; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the expression to be used:
environment: sdk: '>=2.3.2 <2.4.0'
If you need to support older versions of the SDK, then either rewrite the code to not use the is operator, or, if that isn‘t possible, change the code so that the is expression isn’t in a constant context:
{% prettify dart tag=pre+code %} const Object x = 4; var y = x is int ? 0 : 1; {% endprettify %}
The type ‘Never’ wasn't supported until version 2.12.0, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when a reference to the class Never is found in code that has an SDK constraint whose lower bound is less than 2.12.0. This class wasn‘t defined in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.12.0:
environment: sdk: '>=2.5.0 <2.6.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} [!Never!] n; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the type to be used:
environment: sdk: '>=2.12.0 <2.13.0'
If you need to support older versions of the SDK, then rewrite the code to not reference this class:
{% prettify dart tag=pre+code %} dynamic x; {% endprettify %}
Set literals weren't supported until version 2.2, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when a set literal is found in code that has an SDK constraint whose lower bound is less than 2.2.0. Set literals weren‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.2.0:
environment: sdk: '>=2.1.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} var s = [!{}!]; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:
environment: sdk: '>=2.2.0 <2.4.0'
If you do need to support older versions of the SDK, then replace the set literal with code that creates the set without the use of a literal:
{% prettify dart tag=pre+code %} var s = new Set(); {% endprettify %}
The for, if, and spread elements weren't supported until version 2.3.0, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when a for, if, or spread element is found in code that has an SDK constraint whose lower bound is less than 2.3.0. Using a for, if, or spread element wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.3.0:
environment: sdk: '>=2.2.0 <2.4.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} var digits = [[!for (int i = 0; i < 10; i++) i!]]; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:
environment: sdk: '>=2.3.0 <2.4.0'
If you need to support older versions of the SDK, then rewrite the code to not make use of those elements:
{% prettify dart tag=pre+code %} var digits = _initializeDigits();
List _initializeDigits() { var digits = []; for (int i = 0; i < 10; i++) { digits.add(i); } return digits; } {% endprettify %}
The if and spread elements weren't supported in constant expressions until version 2.5.0, but this code is required to be able to run on earlier versions.
The analyzer produces this diagnostic when an if or spread element inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.5.0. Using an if or spread element inside a constant context wasn‘t supported in earlier versions, so this code won’t be able to run against earlier versions of the SDK.
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.5.0:
environment: sdk: '>=2.4.0 <2.6.0'
In the package that has that pubspec, code like the following produces this diagnostic:
{% prettify dart tag=pre+code %} const a = [1, 2]; const b = [[!...a!]]; {% endprettify %}
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:
environment: sdk: '>=2.5.0 <2.6.0'
If you need to support older versions of the SDK, then rewrite the code to not make use of those elements:
{% prettify dart tag=pre+code %} const a = [1, 2]; const b = [1, 2]; {% endprettify %}
If that isn‘t possible, change the code so that the element isn’t in a constant context:
{% prettify dart tag=pre+code %} const a = [1, 2]; var b = [...a]; {% endprettify %}
The element type ‘{0}’ can't be assigned to the set type ‘{1}’.
The analyzer produces this diagnostic when an element in a set literal has a type that isn't assignable to the element type of the set.
The following code produces this diagnostic because the type of the string literal '0' is String, which isn't assignable to int, the element type of the set:
{% prettify dart tag=pre+code %} var s = {[!‘0’!]}; {% endprettify %}
If the element type of the set literal is wrong, then change the element type of the set:
{% prettify dart tag=pre+code %} var s = {‘0’}; {% endprettify %}
If the type of the element is wrong, then change the element:
{% prettify dart tag=pre+code %} var s = {‘0’.length}; {% endprettify %}
The prefix of a deferred import can't be used in other import directives.
The analyzer produces this diagnostic when a prefix in a deferred import is also used as a prefix in other imports (whether deferred or not). The prefix in a deferred import can't be shared with other imports because the prefix is used to load the imported library.
The following code produces this diagnostic because the prefix x is used as the prefix for a deferred import and is also used for one other import:
{% prettify dart tag=pre+code %} import ‘dart:math’ [!deferred!] as x; import ‘dart:convert’ as x;
var y = x.json.encode(x.min(0, 1)); {% endprettify %}
If you can use a different name for the deferred import, then do so:
{% prettify dart tag=pre+code %} import ‘dart:math’ deferred as math; import ‘dart:convert’ as x;
var y = x.json.encode(math.min(0, 1)); {% endprettify %}
If you can use a different name for the other imports, then do so:
{% prettify dart tag=pre+code %} import ‘dart:math’ deferred as x; import ‘dart:convert’ as convert;
var y = convert.json.encode(x.min(0, 1)); {% endprettify %}
‘Array’s must have an ‘Array’ annotation that matches the dimensions.
The analyzer produces this diagnostic when the number of dimensions specified in an Array annotation doesn't match the number of nested arrays specified by the type of a field.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the field a0 has a type with three nested arrays, but only two dimensions are given in the Array annotation:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { [!@Array(8, 8)!] external Array<Array<Array>> a0; } {% endprettify %}
If the type of the field is correct, then fix the annotation to have the required number of dimensions:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(8, 8, 4) external Array<Array<Array>> a0; } {% endprettify %}
If the type of the field is wrong, then fix the type of the field:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Array(8, 8) external Array<Array> a0; } {% endprettify %}
Instance member ‘{0}’ can't be accessed using static access.
The analyzer produces this diagnostic when a class name is used to access an instance field. Instance fields don't exist on a class; they exist only on an instance of the class.
The following code produces this diagnostic because x is an instance field:
{% prettify dart tag=pre+code %} class C { static int a;
int b; }
int f() => C.[!b!]; {% endprettify %}
If you intend to access a static field, then change the name of the field to an existing static field:
{% prettify dart tag=pre+code %} class C { static int a;
int b; }
int f() => C.a; {% endprettify %}
If you intend to access the instance field, then use an instance of the class to access the field:
{% prettify dart tag=pre+code %} class C { static int a;
int b; }
int f(C c) => c.b; {% endprettify %}
Classes and mixins can't implement deferred classes.
Classes can't extend deferred classes.
Classes can't mixin deferred classes.
The analyzer produces this diagnostic when a type (class or mixin) is a subtype of a class from a library being imported using a deferred import. The supertypes of a type must be compiled at the same time as the type, and classes from deferred libraries aren't compiled until the library is loaded.
For more information, see the language tour's coverage of deferred loading.
Given a file (a.dart) that defines the class A:
{% prettify dart tag=pre+code %} class A {} {% endprettify %}
The following code produces this diagnostic because the superclass of B is declared in a deferred library:
{% prettify dart tag=pre+code %} import ‘a.dart’ deferred as a;
class B extends [!a.A!] {} {% endprettify %}
If you need to create a subtype of a type from the deferred library, then remove the deferred keyword:
{% prettify dart tag=pre+code %} import ‘a.dart’ as a;
class B extends a.A {} {% endprettify %}
‘‘{0}’ can’t be used as a superclass constraint.
Classes and mixins can't implement ‘{0}’.
Classes can't extend ‘{0}’.
Classes can't mixin ‘{0}’.
The analyzer produces this diagnostic when one of the restricted classes is used in either an extends, implements, with, or on clause. The classes bool, double, FutureOr, int, Null, num, and String are all restricted in this way, to allow for more efficient implementations.
The following code produces this diagnostic because String is used in an extends clause:
{% prettify dart tag=pre+code %} class A extends [!String!] {} {% endprettify %}
The following code produces this diagnostic because String is used in an implements clause:
{% prettify dart tag=pre+code %} class B implements [!String!] {} {% endprettify %}
The following code produces this diagnostic because String is used in a with clause:
{% prettify dart tag=pre+code %} class C with [!String!] {} {% endprettify %}
The following code produces this diagnostic because String is used in an on clause:
{% prettify dart tag=pre+code %} mixin M on [!String!] {} {% endprettify %}
If a different type should be specified, then replace the type:
{% prettify dart tag=pre+code %} class A extends Object {} {% endprettify %}
If there isn't a different type that would be appropriate, then remove the type, and possibly the whole clause:
{% prettify dart tag=pre+code %} class B {} {% endprettify %}
The class ‘{0}’ can't extend ‘{1}’.
The class ‘{0}’ can't implement ‘{1}’.
The class ‘{0}’ can't mix in ‘{1}’.
The analyzer produces this diagnostic when a class extends any FFI class other than Struct or Union, or implements or mixes in any FFI class. Struct and Union are the only FFI classes that can be subtyped, and then only by extending them.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class C extends Double:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends [!Double!] {} {% endprettify %}
If the class should extend either Struct or Union, then change the declaration of the class:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class C extends Struct { @Int32() external int i; } {% endprettify %}
If the class shouldn't extend either Struct or Union, then remove any references to FFI classes:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
The class ‘{0}’ shouldn‘t be extended, mixed in, or implemented because it’s sealed.
The analyzer produces this diagnostic when a sealed class (one that either has the [sealed][meta-sealed] annotation or inherits or mixes in a sealed class) is referenced in either the extends, implements, or with clause of a class or mixin declaration if the declaration isn't in the same package as the sealed class.
Given a library in a package other than the package being analyzed that contains the following:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class A {}
@sealed class B {} {% endprettify %}
The following code produces this diagnostic because C, which isn't in the same package as B, is extending the sealed class B:
{% prettify dart tag=pre+code %} import ‘package:a/a.dart’;
[!class C extends B {}!] {% endprettify %}
If the class doesn‘t need to be a subtype of the sealed class, then change the declaration so that it isn’t:
{% prettify dart tag=pre+code %} import ‘package:a/a.dart’;
class B extends A {} {% endprettify %}
If the class needs to be a subtype of the sealed class, then either change the sealed class so that it's no longer sealed or move the subclass into the same package as the sealed class.
The class ‘{0}’ can't extend ‘{1}’ because ‘{1}’ is a subtype of ‘Struct’, ‘Union’, or ‘AbiSpecificInteger’.
The class ‘{0}’ can't implement ‘{1}’ because ‘{1}’ is a subtype of ‘Struct’, ‘Union’, or ‘AbiSpecificInteger’.
The class ‘{0}’ can't mix in ‘{1}’ because ‘{1}’ is a subtype of ‘Struct’, ‘Union’, or ‘AbiSpecificInteger’.
The analyzer produces this diagnostic when a class extends, implements, or mixes in a class that extends either Struct or Union. Classes can only extend either Struct or Union directly.
For more information about FFI, see [C interop using dart:ffi][].
The following code produces this diagnostic because the class C extends S, and S extends Struct:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class S extends Struct { external Pointer f; }
class C extends [!S!] { external Pointer g; } {% endprettify %}
If you're trying to define a struct or union that shares some fields declared by a different struct or union, then extend Struct or Union directly and copy the shared fields:
{% prettify dart tag=pre+code %} import ‘dart:ffi’;
class S extends Struct { external Pointer f; }
class C extends Struct { external Pointer f;
external Pointer g; } {% endprettify %}
A type alias that expands to a type parameter can't be implemented.
A type alias that expands to a type parameter can't be mixed in.
A type alias that expands to a type parameter can't be used as a superclass constraint.
A type alias that expands to a type parameter can't be used as a superclass.
The analyzer produces this diagnostic when a type alias that expands to a type parameter is used in an extends, implements, with, or on clause.
The following code produces this diagnostic because the type alias T, which expands to the type parameter S, is used in the extends clause of the class C:
{% prettify dart tag=pre+code %} typedef T = S;
class C extends [!T!] {} {% endprettify %}
Use the value of the type argument directly:
{% prettify dart tag=pre+code %} typedef T = S;
class C extends Object {} {% endprettify %}
(Previously known as invalid_super_invocation)
The superconstructor call must be last in an initializer list: ‘{0}’.
The analyzer produces this diagnostic when the initializer list of a constructor contains an invocation of a constructor in the superclass, but the invocation isn't the last item in the initializer list.
The following code produces this diagnostic because the invocation of the superclass' constructor isn't the last item in the initializer list:
{% prettify dart tag=pre+code %} class A { A(int x); }
class B extends A { B(int x) : !super!, assert(x >= 0); } {% endprettify %}
Move the invocation of the superclass' constructor to the end of the initializer list:
{% prettify dart tag=pre+code %} class A { A(int x); }
class B extends A { B(int x) : assert(x >= 0), super(x); } {% endprettify %}
The ‘super’ keyword can‘t be used in an extension because an extension doesn’t have a superclass.
The analyzer produces this diagnostic when a member declared inside an extension uses the super keyword . Extensions aren‘t classes and don’t have superclasses, so the super keyword serves no purpose.
The following code produces this diagnostic because super can't be used in an extension:
{% prettify dart tag=pre+code %} extension E on Object { String get displayString => [!super!].toString(); } {% endprettify %}
Remove the super keyword :
{% prettify dart tag=pre+code %} extension E on Object { String get displayString => toString(); } {% endprettify %}
Invalid context for ‘super’ invocation.
The analyzer produces this diagnostic when the keyword super is used outside of a instance method.
The following code produces this diagnostic because super is used in a top-level function:
{% prettify dart tag=pre+code %} void f() { [!super!].f(); } {% endprettify %}
Rewrite the code to not use super.
The redirecting constructor can't have a ‘super’ initializer.
The analyzer produces this diagnostic when a constructor that redirects to another constructor also attempts to invoke a constructor from the superclass. The superclass constructor will be invoked when the constructor that the redirecting constructor is redirected to is invoked.
The following code produces this diagnostic because the constructor C.a both redirects to C.b and invokes a constructor from the superclass:
{% prettify dart tag=pre+code %} class C { C.a() : this.b(), [!super()!]; C.b(); } {% endprettify %}
Remove the invocation of the super constructor:
{% prettify dart tag=pre+code %} class C { C.a() : this.b(); C.b(); } {% endprettify %}
The ‘case’ shouldn't complete normally.
The analyzer produces this diagnostic when the statements following a case label in a switch statement could fall through to the next case or default label.
The following code produces this diagnostic because the case label with a value of zero (0) falls through to the default statements:
{% prettify dart tag=pre+code %} void f(int a) { switch (a) { [!case!] 0: print(0); default: return; } } {% endprettify %}
Change the flow of control so that the case won't fall through. There are several ways that this can be done, including adding one of the following at the end of the current list of statements:
return statement,throw expression,break statement,continue, orNever.Type ‘{0}’ of the switch expression isn't assignable to the type ‘{1}’ of case expressions.
The analyzer produces this diagnostic when the type of the expression in a switch statement isn't assignable to the type of the expressions in the case clauses.
The following code produces this diagnostic because the type of s (String) isn't assignable to the type of 0 (int):
{% prettify dart tag=pre+code %} void f(String s) { switch ([!s!]) { case 0: break; } } {% endprettify %}
If the type of the case expressions is correct, then change the expression in the switch statement to have the correct type:
{% prettify dart tag=pre+code %} void f(String s) { switch (int.parse(s)) { case 0: break; } } {% endprettify %}
If the type of the switch expression is correct, then change the case expressions to have the correct type:
{% prettify dart tag=pre+code %} void f(String s) { switch (s) { case ‘0’: break; } } {% endprettify %}
A generative constructor of an abstract class can't be torn off.
The analyzer produces this diagnostic when a generative constructor from an abstract class is being torn off. This isn‘t allowed because it isn’t valid to create an instance of an abstract class, which means that there isn't any valid use for the torn off constructor.
The following code produces this diagnostic because the constructor C.new is being torn off and the class C is an abstract class:
{% prettify dart tag=pre+code %} abstract class C { C(); }
void f() { [!C.new!]; } {% endprettify %}
Tear off the constructor of a concrete class.
The Unicode code point ‘U+{0}’ changes the appearance of text from how it's interpreted by the compiler.
The analyzer produces this diagnostic when it encounters source that contains text direction Unicode code points. These code points cause source code in either a string literal or a comment to be interpreted and compiled differently than how it appears in editors, leading to possible security vulnerabilities.
The following code produces this diagnostic twice because there are hidden characters at the start and end of the label string:
{% prettify dart tag=pre+code %} var label = ‘[!I!]nteractive text[!’!]; {% endprettify %}
If the code points are intended to be included in the string literal, then escape them:
{% prettify dart tag=pre+code %} var label = ‘\u202AInteractive text\u202C’; {% endprettify %}
If the code points aren't intended to be included in the string literal, then remove them:
{% prettify dart tag=pre+code %} var label = ‘Interactive text’; {% endprettify %}
The Unicode code point ‘U+{0}’ changes the appearance of text from how it's interpreted by the compiler.
The analyzer produces this diagnostic when it encounters source that contains text direction Unicode code points. These code points cause source code in either a string literal or a comment to be interpreted and compiled differently than how it appears in editors, leading to possible security vulnerabilities.
The following code produces this diagnostic twice because there are hidden characters at the start and end of the label string:
{% prettify dart tag=pre+code %} var label = ‘[!I!]nteractive text[!’!]; {% endprettify %}
If the code points are intended to be included in the string literal, then escape them:
{% prettify dart tag=pre+code %} var label = ‘\u202AInteractive text\u202C’; {% endprettify %}
If the code points aren't intended to be included in the string literal, then remove them:
{% prettify dart tag=pre+code %} var label = ‘Interactive text’; {% endprettify %}
The type ‘{0}’ of the thrown expression must be assignable to ‘Object’.
The analyzer produces this diagnostic when the type of the expression in a throw expression isn‘t assignable to Object. It isn’t valid to throw null, so it isn't valid to use an expression that might evaluate to null.
The following code produces this diagnostic because s might be null:
{% prettify dart tag=pre+code %} void f(String? s) { throw [!s!]; } {% endprettify %}
Add an explicit null check to the expression:
{% prettify dart tag=pre+code %} void f(String? s) { throw s!; } {% endprettify %}
The type of ‘{0}’ can't be inferred because it depends on itself through the cycle: {1}.
The analyzer produces this diagnostic when a top-level variable has no type annotation and the variable's initializer refers to the variable, either directly or indirectly.
The following code produces this diagnostic because the variables x and y are defined in terms of each other, and neither has an explicit type, so the type of the other can't be inferred:
{% prettify dart tag=pre+code %} var x = y; var y = [!x!]; {% endprettify %}
If the two variables don't need to refer to each other, then break the cycle:
{% prettify dart tag=pre+code %} var x = 0; var y = x; {% endprettify %}
If the two variables need to refer to each other, then give at least one of them an explicit type:
{% prettify dart tag=pre+code %} int x = y; var y = x; {% endprettify %}
Note, however, that while this code doesn‘t produce any diagnostics, it will produce a stack overflow at runtime unless at least one of the variables is assigned a value that doesn’t depend on the other variables before any of the variables in the cycle are referenced.
Typedefs can't reference themselves directly or recursively via another typedef.
The analyzer produces this diagnostic when a typedef refers to itself, either directly or indirectly.
The following code produces this diagnostic because F depends on itself indirectly through G:
{% prettify dart tag=pre+code %} typedef [!F!] = void Function(G); typedef G = void Function(F); {% endprettify %}
Change one or more of the typedefs in the cycle so that none of them refer to themselves:
{% prettify dart tag=pre+code %} typedef F = void Function(G); typedef G = void Function(int); {% endprettify %}
The deferred type ‘{0}’ can't be used in a declaration, cast, or type test.
The analyzer produces this diagnostic when the type annotation is in a variable declaration, or the type used in a cast (as) or type test (is) is a type declared in a library that is imported using a deferred import. These types are required to be available at compile time, but aren't.
For more information, see the language tour's coverage of deferred loading.
The following code produces this diagnostic because the type of the parameter f is imported from a deferred library:
{% prettify dart tag=pre+code %} import ‘dart:io’ deferred as io;
void f([!io.File!] f) {} {% endprettify %}
If you need to reference the imported type, then remove the deferred keyword:
{% prettify dart tag=pre+code %} import ‘dart:io’ as io;
void f(io.File f) {} {% endprettify %}
If the import is required to be deferred and there's another type that is appropriate, then use that type in place of the type from the deferred library.
‘{0}’ doesn't conform to the bound ‘{2}’ of the type parameter ‘{1}’.
The analyzer produces this diagnostic when a type argument isn't the same as or a subclass of the bounds of the corresponding type parameter.
The following code produces this diagnostic because String isn't a subclass of num:
{% prettify dart tag=pre+code %} class A {}
var a = A<[!String!]>(); {% endprettify %}
Change the type argument to be a subclass of the bounds:
{% prettify dart tag=pre+code %} class A {}
var a = A(); {% endprettify %}
Tests for non-null should be done with ‘!= null’.
Tests for null should be done with ‘== null’.
The analyzer produces this diagnostic when there‘s a type check (using the as operator) where the type is Null. There’s only one value whose type is Null, so the code is both more readable and more performant when it tests for null explicitly.
The following code produces this diagnostic because the code is testing to see whether the value of s is null by using a type check:
{% prettify dart tag=pre+code %} void f(String? s) { if ([!s is Null!]) { return; } print(s); } {% endprettify %}
The following code produces this diagnostic because the code is testing to see whether the value of s is something other than null by using a type check:
{% prettify dart tag=pre+code %} void f(String? s) { if ([!s is! Null!]) { print(s); } } {% endprettify %}
Replace the type check with the equivalent comparison with null:
{% prettify dart tag=pre+code %} void f(String? s) { if (s == null) { return; } print(s); } {% endprettify %}
Static members can't reference type parameters of the class.
The analyzer produces this diagnostic when a static member references a type parameter that is declared for the class. Type parameters only have meaning for instances of the class.
The following code produces this diagnostic because the static method hasType has a reference to the type parameter T:
{% prettify dart tag=pre+code %} class C { static bool hasType(Object o) => o is [!T!]; } {% endprettify %}
If the member can be an instance member, then remove the keyword static:
{% prettify dart tag=pre+code %} class C { bool hasType(Object o) => o is T; } {% endprettify %}
If the member must be a static member, then make the member be generic:
{% prettify dart tag=pre+code %} class C { static bool hasType(Object o) => o is S; } {% endprettify %}
Note, however, that there isn’t a relationship between T and S, so this second option changes the semantics from what was likely to be intended.
‘{0}’ can't be a supertype of its upper bound.
The analyzer produces this diagnostic when the bound of a type parameter (the type following the extends keyword) is either directly or indirectly the type parameter itself. Stating that the type parameter must be the same as itself or a subtype of itself or a subtype of itself isn't helpful because it will always be the same as itself.
The following code produces this diagnostic because the bound of T is T:
{% prettify dart tag=pre+code %} class C<[!T!] extends T> {} {% endprettify %}
The following code produces this diagnostic because the bound of T1 is T2, and the bound of T2 is T1, effectively making the bound of T1 be T1:
{% prettify dart tag=pre+code %} class C<[!T1!] extends T2, T2 extends T1> {} {% endprettify %}
If the type parameter needs to be a subclass of some type, then replace the bound with the required type:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
If the type parameter can be any type, then remove the extends clause:
{% prettify dart tag=pre+code %} class C {} {% endprettify %}
The name ‘{0}’ isn‘t a type and can’t be used in an ‘is’ expression.
The analyzer produces this diagnostic when the right-hand side of an is or is! test isn't a type.
The following code produces this diagnostic because the right-hand side is a parameter, not a type:
{% prettify dart tag=pre+code %} typedef B = int Function(int);
void f(Object a, B b) { if (a is [!b!]) { return; } } {% endprettify %}
If you intended to use a type test, then replace the right-hand side with a type:
{% prettify dart tag=pre+code %} typedef B = int Function(int);
void f(Object a, B b) { if (a is B) { return; } } {% endprettify %}
If you intended to use a different kind of test, then change the test:
{% prettify dart tag=pre+code %} typedef B = int Function(int);
void f(Object a, B b) { if (a == b) { return; } } {% endprettify %}
The name ‘{0}’ isn‘t defined, so it can’t be used in an ‘is’ expression.
The analyzer produces this diagnostic when the name following the is in a type test expression isn't defined.
The following code produces this diagnostic because the name Srting isn't defined:
{% prettify dart tag=pre+code %} void f(Object o) { if (o is [!Srting!]) { // ... } } {% endprettify %}
Replace the name with the name of a type:
{% prettify dart tag=pre+code %} void f(Object o) { if (o is String) { // ... } } {% endprettify %}
A nullable expression can't be used as a condition.
A nullable expression can't be used as an iterator in a for-in loop.
A nullable expression can't be used in a spread.
A nullable expression can't be used in a yield-each statement.
The function can't be unconditionally invoked because it can be ‘null’.
The method ‘{0}’ can't be unconditionally invoked because the receiver can be ‘null’.
The operator ‘{0}’ can't be unconditionally invoked because the receiver can be ‘null’.
The property ‘{0}’ can't be unconditionally accessed because the receiver can be ‘null’.
The analyzer produces this diagnostic when an expression whose type is potentially non-nullable is dereferenced without first verifying that the value isn't null.
The following code produces this diagnostic because s can be null at the point where it's referenced:
{% prettify dart tag=pre+code %} void f(String? s) { if (s.[!length!] > 3) { // ... } } {% endprettify %}
If the value really can be null, then add a test to ensure that members are only accessed when the value isn't null:
{% prettify dart tag=pre+code %} void f(String? s) { if (s != null && s.length > 3) { // ... } } {% endprettify %}
If the expression is a variable and the value should never be null, then change the type of the variable to be non-nullable:
{% prettify dart tag=pre+code %} void f(String s) { if (s.length > 3) { // ... } } {% endprettify %}
If you believe that the value of the expression should never be null, but you can‘t change the type of the variable, and you’re willing to risk having an exception thrown at runtime if you‘re wrong, then you can assert that the value isn’t null:
{% prettify dart tag=pre+code %} void f(String? s) { if (s!.length > 3) { // ... } } {% endprettify %}
Undefined name ‘{0}’ used as an annotation.
The analyzer produces this diagnostic when a name that isn't defined is used as an annotation.
The following code produces this diagnostic because the name undefined isn't defined:
{% prettify dart tag=pre+code %} [!@undefined!] void f() {} {% endprettify %}
If the name is correct, but it isn’t declared yet, then declare the name as a constant value:
{% prettify dart tag=pre+code %} const undefined = ‘undefined’;
@undefined void f() {} {% endprettify %}
If the name is wrong, replace the name with the name of a valid constant:
{% prettify dart tag=pre+code %} @deprecated void f() {} {% endprettify %}
Otherwise, remove the annotation.
Undefined class ‘{0}’.
The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a class but either isn‘t defined or isn’t visible in the scope in which it's being referenced.
The following code produces this diagnostic because Piont isn't defined:
{% prettify dart tag=pre+code %} class Point {}
void f([!Piont!] p) {} {% endprettify %}
If the identifier isn't defined, then either define it or replace it with the name of a class that is defined. The example above can be corrected by fixing the spelling of the class:
{% prettify dart tag=pre+code %} class Point {}
void f(Point p) {} {% endprettify %}
If the class is defined but isn't visible, then you probably need to add an import.
The class ‘{0}’ doesn't have a constructor named ‘{1}’.
The class ‘{0}’ doesn't have an unnamed constructor.
The analyzer produces this diagnostic when a superclass constructor is invoked in the initializer list of a constructor, but the superclass doesn't define the constructor being invoked.
The following code produces this diagnostic because A doesn't have an unnamed constructor:
{% prettify dart tag=pre+code %} class A { A.n(); } class B extends A { B() : [!super()!]; } {% endprettify %}
The following code produces this diagnostic because A doesn't have a constructor named m:
{% prettify dart tag=pre+code %} class A { A.n(); } class B extends A { B() : [!super.m()!]; } {% endprettify %}
If the superclass defines a constructor that should be invoked, then change the constructor being invoked:
{% prettify dart tag=pre+code %} class A { A.n(); } class B extends A { B() : super.n(); } {% endprettify %}
If the superclass doesn't define an appropriate constructor, then define the constructor being invoked:
{% prettify dart tag=pre+code %} class A { A.m(); A.n(); } class B extends A { B() : super.m(); } {% endprettify %}
There's no constant named ‘{0}’ in ‘{1}’.
The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of an enum constant, and the name either isn‘t defined or isn’t visible in the scope in which it's being referenced.
The following code produces this diagnostic because E doesn't define a constant named c:
{% prettify dart tag=pre+code %} enum E {a, b}
var e = E.[!c!]; {% endprettify %}
If the constant should be defined, then add it to the declaration of the enum:
{% prettify dart tag=pre+code %} enum E {a, b, c}
var e = E.c; {% endprettify %}
If the constant shouldn't be defined, then change the name to the name of an existing constant:
{% prettify dart tag=pre+code %} enum E {a, b}
var e = E.b; {% endprettify %}
The getter ‘{0}’ isn't defined for the extension ‘{1}’.
The analyzer produces this diagnostic when an extension override is used to invoke a getter, but the getter isn‘t defined by the specified extension. The analyzer also produces this diagnostic when a static getter is referenced but isn’t defined by the specified extension.
The following code produces this diagnostic because the extension E doesn't declare an instance getter named b:
{% prettify dart tag=pre+code %} extension E on String { String get a => ‘a’; }
extension F on String { String get b => ‘b’; }
void f() { E(‘c’).[!b!]; } {% endprettify %}
The following code produces this diagnostic because the extension E doesn't declare a static getter named a:
{% prettify dart tag=pre+code %} extension E on String {}
var x = E.[!a!]; {% endprettify %}
If the name of the getter is incorrect, then change it to the name of an existing getter:
{% prettify dart tag=pre+code %} extension E on String { String get a => ‘a’; }
extension F on String { String get b => ‘b’; }
void f() { E(‘c’).a; } {% endprettify %}
If the name of the getter is correct but the name of the extension is wrong, then change the name of the extension to the correct name:
{% prettify dart tag=pre+code %} extension E on String { String get a => ‘a’; }
extension F on String { String get b => ‘b’; }
void f() { F(‘c’).b; } {% endprettify %}
If the name of the getter and extension are both correct, but the getter isn't defined, then define the getter:
{% prettify dart tag=pre+code %} extension E on String { String get a => ‘a’; String get b => ‘z’; }
extension F on String { String get b => ‘b’; }
void f() { E(‘c’).b; } {% endprettify %}
The method ‘{0}’ isn't defined for the extension ‘{1}’.
The analyzer produces this diagnostic when an extension override is used to invoke a method, but the method isn‘t defined by the specified extension. The analyzer also produces this diagnostic when a static method is referenced but isn’t defined by the specified extension.
The following code produces this diagnostic because the extension E doesn't declare an instance method named b:
{% prettify dart tag=pre+code %} extension E on String { String a() => ‘a’; }
extension F on String { String b() => ‘b’; }
void f() { E(‘c’).!b!; } {% endprettify %}
The following code produces this diagnostic because the extension E doesn't declare a static method named a:
{% prettify dart tag=pre+code %} extension E on String {}
var x = E.!a!; {% endprettify %}
If the name of the method is incorrect, then change it to the name of an existing method:
{% prettify dart tag=pre+code %} extension E on String { String a() => ‘a’; }
extension F on String { String b() => ‘b’; }
void f() { E(‘c’).a(); } {% endprettify %}
If the name of the method is correct, but the name of the extension is wrong, then change the name of the extension to the correct name:
{% prettify dart tag=pre+code %} extension E on String { String a() => ‘a’; }
extension F on String { String b() => ‘b’; }
void f() { F(‘c’).b(); } {% endprettify %}
If the name of the method and extension are both correct, but the method isn't defined, then define the method:
{% prettify dart tag=pre+code %} extension E on String { String a() => ‘a’; String b() => ‘z’; }
extension F on String { String b() => ‘b’; }
void f() { E(‘c’).b(); } {% endprettify %}
The operator ‘{0}’ isn't defined for the extension ‘{1}’.
The analyzer produces this diagnostic when an operator is invoked on a specific extension when that extension doesn't implement the operator.
The following code produces this diagnostic because the extension E doesn't define the operator *:
{% prettify dart tag=pre+code %} var x = E('') [!*!] 4;
extension E on String {} {% endprettify %}
If the extension is expected to implement the operator, then add an implementation of the operator to the extension:
{% prettify dart tag=pre+code %} var x = E('') * 4;
extension E on String { int operator *(int multiplier) => length * multiplier; } {% endprettify %}
If the operator is defined by a different extension, then change the name of the extension to the name of the one that defines the operator.
If the operator is defined on the argument of the extension override, then remove the extension override:
{% prettify dart tag=pre+code %} var x = '' * 4;
extension E on String {} {% endprettify %}
The setter ‘{0}’ isn't defined for the extension ‘{1}’.
The analyzer produces this diagnostic when an extension override is used to invoke a setter, but the setter isn‘t defined by the specified extension. The analyzer also produces this diagnostic when a static setter is referenced but isn’t defined by the specified extension.
The following code produces this diagnostic because the extension E doesn't declare an instance setter named b:
{% prettify dart tag=pre+code %} extension E on String { set a(String v) {} }
extension F on String { set b(String v) {} }
void f() { E(‘c’).[!b!] = ‘d’; } {% endprettify %}
The following code produces this diagnostic because the extension E doesn't declare a static setter named a:
{% prettify dart tag=pre+code %} extension E on String {}
void f() { E.[!a!] = 3; } {% endprettify %}
If the name of the setter is incorrect, then change it to the name of an existing setter:
{% prettify dart tag=pre+code %} extension E on String { set a(String v) {} }
extension F on String { set b(String v) {} }
void f() { E(‘c’).a = ‘d’; } {% endprettify %}
If the name of the setter is correct, but the name of the extension is wrong, then change the name of the extension to the correct name:
{% prettify dart tag=pre+code %} extension E on String { set a(String v) {} }
extension F on String { set b(String v) {} }
void f() { F(‘c’).b = ‘d’; } {% endprettify %}
If the name of the setter and extension are both correct, but the setter isn't defined, then define the setter:
{% prettify dart tag=pre+code %} extension E on String { set a(String v) {} set b(String v) {} }
extension F on String { set b(String v) {} }
void f() { E(‘c’).b = ‘d’; } {% endprettify %}
The function ‘{0}’ isn't defined.
The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a function but either isn‘t defined or isn’t visible in the scope in which it's being referenced.
The following code produces this diagnostic because the name emty isn't defined:
{% prettify dart tag=pre+code %} List empty() => [];
void main() { print(!emty!); } {% endprettify %}
If the identifier isn't defined, then either define it or replace it with the name of a function that is defined. The example above can be corrected by fixing the spelling of the function:
{% prettify dart tag=pre+code %} List empty() => [];
void main() { print(empty()); } {% endprettify %}
If the function is defined but isn't visible, then you probably need to add an import or re-arrange your code to make the function visible.
The getter ‘{0}’ isn't defined for the ‘{1}’ function type.
The getter ‘{0}’ isn't defined for the type ‘{1}’.
The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a getter but either isn‘t defined or isn’t visible in the scope in which it's being referenced.
The following code produces this diagnostic because String has no member named len:
{% prettify dart tag=pre+code %} int f(String s) => s.[!len!]; {% endprettify %}
If the identifier isn't defined, then either define it or replace it with the name of a getter that is defined. The example above can be corrected by fixing the spelling of the getter:
{% prettify dart tag=pre+code %} int f(String s) => s.length; {% endprettify %}
The library ‘{0}’ doesn't export a member with the hidden name ‘{1}’.
The analyzer produces this diagnostic when a hide combinator includes a name that isn't defined by the library being imported.
The following code produces this diagnostic because dart:math doesn't define the name String:
{% prettify dart tag=pre+code %} import ‘dart:math’ hide [!String!], max;
var x = min(0, 1); {% endprettify %}
If a different name should be hidden, then correct the name. Otherwise, remove the name from the list:
{% prettify dart tag=pre+code %} import ‘dart:math’ hide max;
var x = min(0, 1); {% endprettify %}
Undefined name ‘{0}’.
The analyzer produces this diagnostic when it encounters an identifier that either isn‘t defined or isn’t visible in the scope in which it's being referenced.
The following code produces this diagnostic because the name rihgt isn't defined:
{% prettify dart tag=pre+code %} int min(int left, int right) => left <= [!rihgt!] ? left : right; {% endprettify %}
If the identifier isn't defined, then either define it or replace it with an identifier that is defined. The example above can be corrected by fixing the spelling of the variable:
{% prettify dart tag=pre+code %} int min(int left, int right) => left <= right ? left : right; {% endprettify %}
If the identifier is defined but isn't visible, then you probably need to add an import or re-arrange your code to make the identifier visible.
Undefined name ‘await’ in function body not marked with ‘async’.
The analyzer produces this diagnostic when the name await is used in a method or function body without being declared, and the body isn't marked with the async keyword. The name await only introduces an await expression in an asynchronous function.
The following code produces this diagnostic because the name await is used in the body of f even though the body of f isn't marked with the async keyword:
{% prettify dart tag=pre+code %} void f(p) { [!await!] p; } {% endprettify %}
Add the keyword async to the function body:
{% prettify dart tag=pre+code %} void f(p) async { await p; } {% endprettify %}
The method ‘{0}’ isn't defined for the ‘{1}’ function type.
The method ‘{0}’ isn't defined for the type ‘{1}’.
The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a method but either isn‘t defined or isn’t visible in the scope in which it's being referenced.
The following code produces this diagnostic because the identifier removeMiddle isn't defined:
{% prettify dart tag=pre+code %} int f(List l) => l.!removeMiddle!; {% endprettify %}
If the identifier isn't defined, then either define it or replace it with the name of a method that is defined. The example above can be corrected by fixing the spelling of the method:
{% prettify dart tag=pre+code %} int f(List l) => l.removeLast(); {% endprettify %}
The named parameter ‘{0}’ isn't defined.
The analyzer produces this diagnostic when a method or function invocation has a named argument, but the method or function being invoked doesn't define a parameter with the same name.
The following code produces this diagnostic because m doesn't declare a named parameter named a:
{% prettify dart tag=pre+code %} class C { m({int b}) {} }
void f(C c) { c.m([!a!]: 1); } {% endprettify %}
If the argument name is mistyped, then replace it with the correct name. The example above can be fixed by changing a to b:
{% prettify dart tag=pre+code %} class C { m({int b}) {} }
void f(C c) { c.m(b: 1); } {% endprettify %}
If a subclass adds a parameter with the name in question, then cast the receiver to the subclass:
{% prettify dart tag=pre+code %} class C { m({int b}) {} }
class D extends C { m({int a, int b}) {} }
void f(C c) { (c as D).m(a: 1); } {% endprettify %}
If the parameter should be added to the function, then add it:
{% prettify dart tag=pre+code %} class C { m({int a, int b}) {} }
void f(C c) { c.m(a: 1); } {% endprettify %}
The operator ‘{0}’ isn't defined for the type ‘{1}’.
The analyzer produces this diagnostic when a user-definable operator is invoked on an object for which the operator isn't defined.
The following code produces this diagnostic because the class C doesn't define the operator +:
{% prettify dart tag=pre+code %} class C {}
C f(C c) => c [!+!] 2; {% endprettify %}
If the operator should be defined for the class, then define it:
{% prettify dart tag=pre+code %} class C { C operator +(int i) => this; }
C f(C c) => c + 2; {% endprettify %}
The name ‘{0}’ is being referenced through the prefix ‘{1}’, but it isn't defined in any of the libraries imported using that prefix.
The analyzer produces this diagnostic when a prefixed identifier is found where the prefix is valid, but the identifier isn't declared in any of the libraries imported using that prefix.
The following code produces this diagnostic because dart:core doesn't define anything named a:
{% prettify dart tag=pre+code %} import ‘dart:core’ as p;
void f() { p.[!a!]; } {% endprettify %}
If the library in which the name is declared isn't imported yet, add an import for the library.
If the name is wrong, then change it to one of the names that's declared in the imported libraries.
The parameter ‘{0}’ isn't defined by ‘{1}’.
The analyzer produces this diagnostic when an annotation of the form [UseResult][meta-UseResult].unless(parameterDefined: parameterName) specifies a parameter name that isn't defined by the annotated function.
The following code produces this diagnostic because the function f doesn't have a parameter named b:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@UseResult.unless(parameterDefined: [!‘b’!]) int f([int? a]) => a ?? 0; {% endprettify %}
Change the argument named parameterDefined to match the name of one of the parameters to the function:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
@UseResult.unless(parameterDefined: ‘a’) int f([int? a]) => a ?? 0; {% endprettify %}
The setter ‘{0}’ isn't defined for the ‘{1}’ function type.
The setter ‘{0}’ isn't defined for the type ‘{1}’.
The analyzer produces this diagnostic when it encounters an identifier that appears to be the name of a setter but either isn‘t defined or isn’t visible in the scope in which the identifier is being referenced.
The following code produces this diagnostic because there isn't a setter named z:
{% prettify dart tag=pre+code %} class C { int x = 0; void m(int y) { this.[!z!] = y; } } {% endprettify %}
If the identifier isn't defined, then either define it or replace it with the name of a setter that is defined. The example above can be corrected by fixing the spelling of the setter:
{% prettify dart tag=pre+code %} class C { int x = 0; void m(int y) { this.x = y; } } {% endprettify %}
The library ‘{0}’ doesn't export a member with the shown name ‘{1}’.
The analyzer produces this diagnostic when a show combinator includes a name that isn't defined by the library being imported.
The following code produces this diagnostic because dart:math doesn't define the name String:
{% prettify dart tag=pre+code %} import ‘dart:math’ show min, [!String!];
var x = min(0, 1); {% endprettify %}
If a different name should be shown, then correct the name. Otherwise, remove the name from the list:
{% prettify dart tag=pre+code %} import ‘dart:math’ show min;
var x = min(0, 1); {% endprettify %}
(Previously known as undefined_super_method)
The getter ‘{0}’ isn't defined in a superclass of ‘{1}’.
The method ‘{0}’ isn't defined in a superclass of ‘{1}’.
The operator ‘{0}’ isn't defined in a superclass of ‘{1}’.
The setter ‘{0}’ isn't defined in a superclass of ‘{1}’.
The analyzer produces this diagnostic when an inherited member (method, getter, setter, or operator) is referenced using super, but there’s no member with that name in the superclass chain.
The following code produces this diagnostic because Object doesn't define a method named n:
{% prettify dart tag=pre+code %} class C { void m() { super.!n!; } } {% endprettify %}
The following code produces this diagnostic because Object doesn't define a getter named g:
{% prettify dart tag=pre+code %} class C { void m() { super.[!g!]; } } {% endprettify %}
If the inherited member you intend to invoke has a different name, then make the name of the invoked member match the inherited member.
If the member you intend to invoke is defined in the same class, then remove the super..
If the member isn’t defined, then either add the member to one of the superclasses or remove the invocation.
Unnecessary cast.
The analyzer produces this diagnostic when the value being cast is already known to be of the type that it's being cast to.
The following code produces this diagnostic because n is already known to be an int as a result of the is test:
{% prettify dart tag=pre+code %} void f(num n) { if (n is int) { ([!n as int!]).isEven; } } {% endprettify %}
Remove the unnecessary cast:
{% prettify dart tag=pre+code %} void f(num n) { if (n is int) { n.isEven; } } {% endprettify %}
The dev dependency on {0} is unnecessary because there is also a normal dependency on that package.
The analyzer produces this diagnostic when there‘s an entry under dev_dependencies for a package that is also listed under dependencies. The packages under dependencies are available to all of the code in the package, so there’s no need to also list them under dev_dependencies.
The following code produces this diagnostic because the package meta is listed under both dependencies and dev_dependencies:
name: example dependencies: meta: ^1.0.2 dev_dependencies: meta: ^1.0.2
Remove the entry under dev_dependencies (and the dev_dependencies key if that's the only package listed there):
name: example dependencies: meta: ^1.0.2
The import of ‘{0}’ is unnecessary because all of the used elements are also provided by the import of ‘{1}’.
The analyzer produces this diagnostic when an import isn't needed because all of the names that are imported and referenced within the importing library are also visible through another import.
Given a file named a.dart that contains the following:
{% prettify dart tag=pre+code %} class A {} {% endprettify %}
And, given a file named b.dart that contains the following:
{% prettify dart tag=pre+code %} export ‘a.dart’;
class B {} {% endprettify %}
The following code produces this diagnostic because the class A, which is imported from a.dart, is also imported from b.dart. Removing the import of a.dart leaves the semantics unchanged:
{% prettify dart tag=pre+code %} import [!‘a.dart’!]; import ‘b.dart’;
void f(A a, B b) {} {% endprettify %}
If the import isn't needed, then remove it.
If some of the names imported by this import are intended to be used but aren‘t yet, and if those names aren’t imported by other imports, then add the missing references to those names.
The ‘!’ will have no effect because the receiver can't be null.
The analyzer produces this diagnostic when the operand of the ! operator can't be null.
The following code produces this diagnostic because x can't be null:
{% prettify dart tag=pre+code %} int f(int x) { return x[!!!]; } {% endprettify %}
Remove the null check operator (!):
{% prettify dart tag=pre+code %} int f(int x) { return x; } {% endprettify %}
Unnecessary ‘noSuchMethod’ declaration.
The analyzer produces this diagnostic when there‘s a declaration of noSuchMethod, the only thing the declaration does is invoke the overridden declaration, and the overridden declaration isn’t the declaration in Object.
Overriding the implementation of Object‘s noSuchMethod (no matter what the implementation does) signals to the analyzer that it shouldn’t flag any inherited abstract methods that aren‘t implemented in that class. This works even if the overriding implementation is inherited from a superclass, so there’s no value to declare it again in a subclass.
The following code produces this diagnostic because the declaration of noSuchMethod in A makes the declaration of noSuchMethod in B unnecessary:
{% prettify dart tag=pre+code %} class A { @override dynamic noSuchMethod(x) => super.noSuchMethod(x); } class B extends A { @override dynamic !noSuchMethod! { return super.noSuchMethod(y); } } {% endprettify %}
Remove the unnecessary declaration:
{% prettify dart tag=pre+code %} class A { @override dynamic noSuchMethod(x) => super.noSuchMethod(x); } class B extends A {} {% endprettify %}
The operand can't be null, so the condition is always false.
The operand can't be null, so the condition is always true.
The analyzer produces this diagnostic when it finds an equality comparison (either == or !=) with one operand of null and the other operand can't be null. Such comparisons are always either true or false, so they serve no purpose.
The following code produces this diagnostic because x can never be null, so the comparison always evaluates to true:
{% prettify dart tag=pre+code %} void f(int x) { if (x [!!= null!]) { print(x); } } {% endprettify %}
The following code produces this diagnostic because x can never be null, so the comparison always evaluates to false:
{% prettify dart tag=pre+code %} void f(int x) { if (x [!== null!]) { throw ArgumentError(“x can't be null”); } } {% endprettify %}
If the other operand should be able to be null, then change the type of the operand:
{% prettify dart tag=pre+code %} void f(int? x) { if (x != null) { print(x); } } {% endprettify %}
If the other operand really can't be null, then remove the condition:
{% prettify dart tag=pre+code %} void f(int x) { print(x); } {% endprettify %}
The ‘?’ is unnecessary because ‘{0}’ is nullable without it.
The analyzer produces this diagnostic when either the type dynamic or the type Null is followed by a question mark. Both of these types are inherently nullable so the question mark doesn't change the semantics.
The following code produces this diagnostic because the question mark following dynamic isn't necessary:
{% prettify dart tag=pre+code %} dynamic[!?!] x; {% endprettify %}
Remove the unneeded question mark:
{% prettify dart tag=pre+code %} dynamic x; {% endprettify %}
Unnecessary type check; the result is always ‘false’.
Unnecessary type check; the result is always ‘true’.
The analyzer produces this diagnostic when the value of a type check (using either is or is!) is known at compile time.
The following code produces this diagnostic because the test a is Object? is always true:
{% prettify dart tag=pre+code %} bool f(T a) => [!a is Object?!]; {% endprettify %}
If the type check doesn't check what you intended to check, then change the test:
{% prettify dart tag=pre+code %} bool f(T a) => a is Object; {% endprettify %}
If the type check does check what you intended to check, then replace the type check with its known value or completely remove it:
{% prettify dart tag=pre+code %} bool f(T a) => true; {% endprettify %}
Static members from supertypes must be qualified by the name of the defining type.
The analyzer produces this diagnostic when code in one class references a static member in a superclass without prefixing the member‘s name with the name of the superclass. Static members can only be referenced without a prefix in the class in which they’re declared.
The following code produces this diagnostic because the static field x is referenced in the getter g without prefixing it with the name of the defining class:
{% prettify dart tag=pre+code %} class A { static int x = 3; }
class B extends A { int get g => [!x!]; } {% endprettify %}
Prefix the name of the static member with the name of the declaring class:
{% prettify dart tag=pre+code %} class A { static int x = 3; }
class B extends A { int get g => A.x; } {% endprettify %}
Static members from the extended type or one of its superclasses must be qualified by the name of the defining type.
The analyzer produces this diagnostic when an undefined name is found, and the name is the same as a static member of the extended type or one of its superclasses.
The following code produces this diagnostic because m is a static member of the extended type C:
{% prettify dart tag=pre+code %} class C { static void m() {} }
extension E on C { void f() { !m!; } } {% endprettify %}
If you‘re trying to reference a static member that’s declared outside the extension, then add the name of the class or extension before the reference to the member:
{% prettify dart tag=pre+code %} class C { static void m() {} }
extension E on C { void f() { C.m(); } } {% endprettify %}
If you‘re referencing a member that isn’t declared yet, add a declaration:
{% prettify dart tag=pre+code %} class C { static void m() {} }
extension E on C { void f() { m(); }
void m() {} } {% endprettify %}
The exception variable ‘{0}’ isn't used, so the ‘catch’ clause can be removed.
The analyzer produces this diagnostic when a catch clause is found, and neither the exception parameter nor the optional stack trace parameter are used in the catch block.
The following code produces this diagnostic because e isn't referenced:
{% prettify dart tag=pre+code %} void f() { try { int.parse(‘;’); } on FormatException catch ([!e!]) { // ignored } } {% endprettify %}
Remove the unused catch clause:
{% prettify dart tag=pre+code %} void f() { try { int.parse(‘;’); } on FormatException { // ignored } } {% endprettify %}
The stack trace variable ‘{0}’ isn't used and can be removed.
The analyzer produces this diagnostic when the stack trace parameter in a catch clause isn't referenced within the body of the catch block.
The following code produces this diagnostic because stackTrace isn't referenced:
{% prettify dart tag=pre+code %} void f() { try { // ... } catch (exception, [!stackTrace!]) { // ... } } {% endprettify %}
If you need to reference the stack trace parameter, then add a reference to it. Otherwise, remove it:
{% prettify dart tag=pre+code %} void f() { try { // ... } catch (exception) { // ... } } {% endprettify %}
A value for optional parameter ‘{0}’ isn't ever given.
The declaration ‘{0}’ isn't referenced.
The analyzer produces this diagnostic when a private declaration isn't referenced in the library that contains the declaration. The following kinds of declarations are analyzed:
Assuming that no code in the library references _C, the following code produces this diagnostic:
{% prettify dart tag=pre+code %} class [!_C!] {} {% endprettify %}
Assuming that no code in the library passes a value for y in any invocation of _m, the following code produces this diagnostic:
{% prettify dart tag=pre+code %} class C { void _m(int x, [int [!y!]]) {}
void n() => _m(0); } {% endprettify %}
If the declaration isn't needed, then remove it:
{% prettify dart tag=pre+code %} class C { void _m(int x) {}
void n() => _m(0); } {% endprettify %}
If the declaration is intended to be used, then add the code to use it.
The value of the field ‘{0}’ isn't used.
The analyzer produces this diagnostic when a private field is declared but never read, even if it's written in one or more places.
The following code produces this diagnostic because the field _originalValue isn't read anywhere in the library:
{% prettify dart tag=pre+code %} class C { final String [!_originalValue!]; final String _currentValue;
C(this._originalValue) : _currentValue = _originalValue;
String get value => _currentValue; } {% endprettify %}
It might appear that the field _originalValue is being read in the initializer (_currentValue = _originalValue), but that is actually a reference to the parameter of the same name, not a reference to the field.
If the field isn't needed, then remove it.
If the field was intended to be used, then add the missing code.
Unused import: ‘{0}’.
The analyzer produces this diagnostic when an import isn't needed because none of the names that are imported are referenced within the importing library.
The following code produces this diagnostic because nothing defined in dart:async is referenced in the library:
{% prettify dart tag=pre+code %} import [!‘dart:async’!];
void main() {} {% endprettify %}
If the import isn't needed, then remove it.
If some of the imported names are intended to be used, then add the missing code.
The label ‘{0}’ isn't used.
The analyzer produces this diagnostic when a label that isn't used is found.
The following code produces this diagnostic because the label loop isn't referenced anywhere in the method:
{% prettify dart tag=pre+code %} void f(int limit) { [!loop:!] for (int i = 0; i < limit; i++) { print(i); } } {% endprettify %}
If the label isn't needed, then remove it:
{% prettify dart tag=pre+code %} void f(int limit) { for (int i = 0; i < limit; i++) { print(i); } } {% endprettify %}
If the label is needed, then use it:
{% prettify dart tag=pre+code %} void f(int limit) { loop: for (int i = 0; i < limit; i++) { print(i); break loop; } } {% endprettify %}
The value of the local variable ‘{0}’ isn't used.
The analyzer produces this diagnostic when a local variable is declared but never read, even if it's written in one or more places.
The following code produces this diagnostic because the value of count is never read:
{% prettify dart tag=pre+code %} void main() { int [!count!] = 0; } {% endprettify %}
If the variable isn't needed, then remove it.
If the variable was intended to be used, then add the missing code.
‘{0}’ should be used. {1}.
The value of ‘{0}’ should be used.
The analyzer produces this diagnostic when a function annotated with [useResult][meta-useResult] is invoked, and the value returned by that function isn't used. The value is considered to be used if a member of the value is invoked, if the value is passed to another function, or if the value is assigned to a variable or field.
The following code produces this diagnostic because the invocation of c.a() isn't used, even though the method a is annotated with [useResult][meta-useResult]:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { @useResult int a() => 0;
int b() => 0; }
void f(C c) { c.!a!; } {% endprettify %}
If you intended to invoke the annotated function, then use the value that was returned:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { @useResult int a() => 0;
int b() => 0; }
void f(C c) { print(c.a()); } {% endprettify %}
If you intended to invoke a different function, then correct the name of the function being invoked:
{% prettify dart tag=pre+code %} import ‘package:meta/meta.dart’;
class C { @useResult int a() => 0;
int b() => 0; }
void f(C c) { c.b(); } {% endprettify %}
The name {0} is shown, but isn’t used.
The analyzer produces this diagnostic when a show combinator includes a name that isn‘t used within the library. Because it isn’t referenced, the name can be removed.
The following code produces this diagnostic because the function max isn't used:
{% prettify dart tag=pre+code %} import ‘dart:math’ show min, [!max!];
var x = min(0, 1); {% endprettify %}
Either use the name or remove it:
{% prettify dart tag=pre+code %} import ‘dart:math’ show min;
var x = min(0, 1); {% endprettify %}
Target of URI doesn't exist: ‘{0}’.
The analyzer produces this diagnostic when an import, export, or part directive is found where the URI refers to a file that doesn't exist.
If the file lib.dart doesn't exist, the following code produces this diagnostic:
{% prettify dart tag=pre+code %} import [!‘lib.dart’!]; {% endprettify %}
If the URI was mistyped or invalid, then correct the URI.
If the URI is correct, then create the file.
Target of URI hasn't been generated: ‘{0}’.
The analyzer produces this diagnostic when an import, export, or part directive is found where the URI refers to a file that doesn‘t exist and the name of the file ends with a pattern that’s commonly produced by code generators, such as one of the following:
.g.dart.pb.dart.pbenum.dart.pbserver.dart.pbjson.dart.template.dartIf the file lib.g.dart doesn't exist, the following code produces this diagnostic:
{% prettify dart tag=pre+code %} import [!‘lib.g.dart’!]; {% endprettify %}
If the file is a generated file, then run the generator that generates the file.
If the file isn't a generated file, then check the spelling of the URI or create the file.
URIs can't use string interpolation.
The analyzer produces this diagnostic when the string literal in an import, export, or part directive contains an interpolation. The resolution of the URIs in directives must happen before the declarations are compiled, so expressions can’t be evaluated while determining the values of the URIs.
The following code produces this diagnostic because the string in the import directive contains an interpolation:
{% prettify dart tag=pre+code %} import [!‘dart:$m’!];
const m = ‘math’; {% endprettify %}
Remove the interpolation from the URI:
{% prettify dart tag=pre+code %} import ‘dart:math’;
var zero = min(0, 0); {% endprettify %}
Dart native extensions are deprecated and aren’t available in Dart 2.15.
The analyzer produces this diagnostic when a library is imported using the dart-ext scheme.
The following code produces this diagnostic because the native library x is being imported using a scheme of dart-ext:
{% prettify dart tag=pre+code %} import [!‘dart-ext:x’!]; {% endprettify %}
Rewrite the code to use dart:ffi as a way of invoking the contents of the native library.
This expression has a type of ‘void’ so its value can't be used.
The analyzer produces this diagnostic when it finds an expression whose type is void, and the expression is used in a place where a value is expected, such as before a member access or on the right-hand side of an assignment.
The following code produces this diagnostic because f doesn't produce an object on which toString can be invoked:
{% prettify dart tag=pre+code %} void f() {}
void g() { [!f()!].toString(); } {% endprettify %}
Either rewrite the code so that the expression has a value or rewrite the code so that it doesn't depend on the value.
A value of type ‘{0}’ can't be assigned to a const variable of type ‘{1}’.
The analyzer produces this diagnostic when the evaluation of a constant expression would result in a CastException.
The following code produces this diagnostic because the value of x is an int, which can‘t be assigned to y because an int isn’t a String:
{% prettify dart tag=pre+code %} const Object x = 0; const String y = [!x!]; {% endprettify %}
If the declaration of the constant is correct, then change the value being assigned to be of the correct type:
{% prettify dart tag=pre+code %} const Object x = 0; const String y = ‘$x’; {% endprettify %}
If the assigned value is correct, then change the declaration to have the correct type:
{% prettify dart tag=pre+code %} const Object x = 0; const int y = x; {% endprettify %}
Operator ‘-’ should declare 0 or 1 parameter, but {0} found.
Operator ‘{0}’ should declare exactly {1} parameters, but {2} found.
The analyzer produces this diagnostic when a declaration of an operator has the wrong number of parameters.
The following code produces this diagnostic because the operator + must have a single parameter corresponding to the right operand:
{% prettify dart tag=pre+code %} class C { int operator [!+!](a, b) => 0; } {% endprettify %}
Add or remove parameters to match the required number:
{% prettify dart tag=pre+code %} class C { int operator +(a) => 0; } {% endprettify %}
Setters must declare exactly one required positional parameter.
The analyzer produces this diagnostic when a setter is found that doesn't declare exactly one required positional parameter.
The following code produces this diagnostic because the setter s declares two required parameters:
{% prettify dart tag=pre+code %} class C { set [!s!](int x, int y) {} } {% endprettify %}
The following code produces this diagnostic because the setter s declares one optional parameter:
{% prettify dart tag=pre+code %} class C { set [!s!]([int x]) {} } {% endprettify %}
Change the declaration so that there's exactly one required positional parameter:
{% prettify dart tag=pre+code %} class C { set s(int x) {} } {% endprettify %}
The type ‘{0}’ is declared with {1} type parameters, but {2} type arguments were given.
The analyzer produces this diagnostic when a type that has type parameters is used and type arguments are provided, but the number of type arguments isn't the same as the number of type parameters.
The analyzer also produces this diagnostic when a constructor is invoked and the number of type arguments doesn't match the number of type parameters declared for the class.
The following code produces this diagnostic because C has one type parameter but two type arguments are provided when it is used as a type annotation:
{% prettify dart tag=pre+code %} class C {}
void f([!C<int, int>!] x) {} {% endprettify %}
The following code produces this diagnostic because C declares one type parameter, but two type arguments are provided when creating an instance:
{% prettify dart tag=pre+code %} class C {}
var c = !C<int, int>!; {% endprettify %}
Add or remove type arguments, as necessary, to match the number of type parameters defined for the type:
{% prettify dart tag=pre+code %} class C {}
void f(C x) {} {% endprettify %}
The constructor ‘{0}.{1}’ doesn't have type parameters.
The analyzer produces this diagnostic when type arguments are provided after the name of a named constructor. Constructors can't declare type parameters, so invocations can only provide the type arguments associated with the class, and those type arguments are required to follow the name of the class rather than the name of the constructor.
The following code produces this diagnostic because the type parameters (<String>) follow the name of the constructor rather than the name of the class:
{% prettify dart tag=pre+code %} class C { C.named(); } C f() => C.named!!; {% endprettify %}
If the type arguments are for the class' type parameters, then move the type arguments to follow the class name:
{% prettify dart tag=pre+code %} class C { C.named(); } C f() => C.named(); {% endprettify %}
If the type arguments aren‘t for the class’ type parameters, then remove them:
{% prettify dart tag=pre+code %} class C { C.named(); } C f() => C.named(); {% endprettify %}
The extension ‘{0}’ is declared with {1} type parameters, but {2} type arguments were given.
The analyzer produces this diagnostic when an extension that has type parameters is used and type arguments are provided, but the number of type arguments isn't the same as the number of type parameters.
The following code produces this diagnostic because the extension E is declared to have a single type parameter (T), but the extension override has two type arguments:
{% prettify dart tag=pre+code %} extension E on List { int get len => length; }
void f(List p) { E!<int, String>!.len; } {% endprettify %}
Change the type arguments so that there are the same number of type arguments as there are type parameters:
{% prettify dart tag=pre+code %} extension E on List { int get len => length; }
void f(List p) { E(p).len; } {% endprettify %}
The method ‘{0}’ is declared with {1} type parameters, but {2} type arguments are given.
The analyzer produces this diagnostic when a method or function is invoked with a different number of type arguments than the number of type parameters specified in its declaration. There must either be no type arguments or the number of arguments must match the number of parameters.
The following code produces this diagnostic because the invocation of the method m has two type arguments, but the declaration of m only has one type parameter:
{% prettify dart tag=pre+code %} class C { int m(A a) => 0; }
int f(C c) => c.m!<int, int>!; {% endprettify %}
If the type arguments are necessary, then make them match the number of type parameters by either adding or removing type arguments:
{% prettify dart tag=pre+code %} class C { int m(A a) => 0; }
int f(C c) => c.m(2); {% endprettify %}
If the type arguments aren't necessary, then remove them:
{% prettify dart tag=pre+code %} class C { int m(A a) => 0; }
int f(C c) => c.m(2); {% endprettify %}
Yield statements must be in a generator function (one marked with either ‘async*’ or ‘sync*’).
Yield-each statements must be in a generator function (one marked with either ‘async*’ or ‘sync*’).
The analyzer produces this diagnostic when a yield or yield* statement appears in a function whose body isn't marked with one of the async* or sync* modifiers.
The following code produces this diagnostic because yield is being used in a function whose body doesn't have a modifier:
{% prettify dart tag=pre+code %} Iterable get digits { yield* [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]; } {% endprettify %}
The following code produces this diagnostic because yield* is being used in a function whose body has the async modifier rather than the async* modifier:
{% prettify dart tag=pre+code %} Stream get digits async { yield* [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]; } {% endprettify %}
Add a modifier, or change the existing modifier to be either async* or sync*:
{% prettify dart tag=pre+code %} Iterable get digits sync* { yield* [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]; } {% endprettify %}
A yielded value of type ‘{0}’ must be assignable to ‘{1}’.
The type ‘{0}’ implied by the ‘yield*’ expression must be assignable to ‘{1}’.
The analyzer produces this diagnostic when the type of object produced by a yield or yield* expression doesn't match the type of objects that are to be returned from the Iterable or Stream types that are returned from a generator (a function or method marked with either sync* or async*).
The following code produces this diagnostic because the getter zero is declared to return an Iterable that returns integers, but the yield is returning a string from the iterable:
{% prettify dart tag=pre+code %} Iterable get zero sync* { yield [!‘0’!]; } {% endprettify %}
If the return type of the function is correct, then fix the expression following the keyword yield to return the correct type:
{% prettify dart tag=pre+code %} Iterable get zero sync* { yield 0; } {% endprettify %}
If the expression following the yield is correct, then change the return type of the function to allow it:
{% prettify dart tag=pre+code %} Iterable get zero sync* { yield ‘0’; } {% endprettify %}