| // Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file |
| // for details. All rights reserved. Use of this source code is governed by a |
| // BSD-style license that can be found in the LICENSE.md file. |
| |
| import 'dart:math' as math; |
| |
| import 'package:front_end/src/fasta/type_inference/type_constraint_gatherer.dart'; |
| import 'package:front_end/src/fasta/type_inference/type_schema.dart'; |
| import 'package:front_end/src/fasta/type_inference/type_schema_elimination.dart'; |
| import 'package:kernel/ast.dart'; |
| import 'package:kernel/class_hierarchy.dart'; |
| import 'package:kernel/core_types.dart'; |
| import 'package:kernel/type_algebra.dart'; |
| import 'package:kernel/type_environment.dart'; |
| |
| /// A constraint on a type parameter that we're inferring. |
| class TypeConstraint { |
| /// The lower bound of the type being constrained. This bound must be a |
| /// subtype of the type being constrained. |
| DartType lower; |
| |
| /// The upper bound of the type being constrained. The type being constrained |
| /// must be a subtype of this bound. |
| DartType upper; |
| |
| TypeConstraint() |
| : lower = const UnknownType(), |
| upper = const UnknownType(); |
| |
| TypeConstraint._(this.lower, this.upper); |
| |
| TypeConstraint clone() => new TypeConstraint._(lower, upper); |
| |
| String toString() => |
| '${typeSchemaToString(lower)} <: <type> <: ${typeSchemaToString(upper)}'; |
| } |
| |
| class TypeSchemaEnvironment extends TypeEnvironment { |
| TypeSchemaEnvironment(CoreTypes coreTypes, ClassHierarchy hierarchy) |
| : super(coreTypes, hierarchy); |
| |
| /// Modify the given [constraint]'s lower bound to include [lower]. |
| void addLowerBound(TypeConstraint constraint, DartType lower) { |
| constraint.lower = getLeastUpperBound(constraint.lower, lower); |
| } |
| |
| /// Modify the given [constraint]'s upper bound to include [upper]. |
| void addUpperBound(TypeConstraint constraint, DartType upper) { |
| constraint.upper = getGreatestLowerBound(constraint.upper, upper); |
| } |
| |
| /// Implements the function "flatten" defined in the spec, where T is [type]: |
| /// |
| /// If T = Future<S> then flatten(T) = flatten(S). |
| /// |
| /// Otherwise if T <: Future then let S be a type such that T << Future<S> |
| /// and for all R, if T << Future<R> then S << R. Then flatten(T) = S. |
| /// |
| /// In any other circumstance, flatten(T) = T. |
| DartType flattenFutures(DartType type) { |
| if (type is InterfaceType) { |
| if (identical(type.classNode, coreTypes.futureClass)) { |
| return flattenFutures(type.typeArguments[0]); |
| } |
| InterfaceType futureBase = |
| hierarchy.getTypeAsInstanceOf(type, coreTypes.futureClass); |
| if (futureBase != null) { |
| return futureBase.typeArguments[0]; |
| } |
| } |
| return type; |
| } |
| |
| /// Computes the greatest lower bound of [type1] and [type2]. |
| DartType getGreatestLowerBound(DartType type1, DartType type2) { |
| // The greatest lower bound relation is reflexive. Note that we don't test |
| // for equality because we don't want to make the algorithm quadratic. This |
| // is ok because the check is not needed for correctness; it's just a speed |
| // optimization. |
| if (identical(type1, type2)) { |
| return type1; |
| } |
| |
| // For any type T, GLB(?, T) == T. |
| if (type1 is UnknownType) { |
| return type2; |
| } |
| if (type2 is UnknownType) { |
| return type1; |
| } |
| |
| // The GLB of top and any type is just that type. |
| // Also GLB of bottom and any type is bottom. |
| if (isTop(type1) || isBottom(type2)) { |
| return type2; |
| } |
| if (isTop(type2) || isBottom(type1)) { |
| return type1; |
| } |
| |
| // Function types have structural GLB. |
| if (type1 is FunctionType && type2 is FunctionType) { |
| return _functionGreatestLowerBound(type1, type2); |
| } |
| |
| // Otherwise, the GLB of two types is one of them it if it is a subtype of |
| // the other. |
| if (isSubtypeOf(type1, type2)) { |
| return type1; |
| } |
| |
| if (isSubtypeOf(type2, type1)) { |
| return type2; |
| } |
| |
| // No subtype relation, so no known GLB. |
| return const BottomType(); |
| } |
| |
| /// Compute the least upper bound of two types. |
| DartType getLeastUpperBound(DartType type1, DartType type2) { |
| // The least upper bound relation is reflexive. Note that we don't test |
| // for equality because we don't want to make the algorithm quadratic. This |
| // is ok because the check is not needed for correctness; it's just a speed |
| // optimization. |
| if (identical(type1, type2)) { |
| return type1; |
| } |
| |
| // For any type T, LUB(?, T) == T. |
| if (type1 is UnknownType) { |
| return type2; |
| } |
| if (type2 is UnknownType) { |
| return type1; |
| } |
| |
| // The least upper bound of void and any type T != dynamic is void. |
| if (type1 is VoidType) { |
| return type2 is DynamicType ? type2 : type1; |
| } |
| if (type2 is VoidType) { |
| return type1 is DynamicType ? type1 : type2; |
| } |
| |
| // The least upper bound of top and any type T is top. |
| // The least upper bound of bottom and any type T is T. |
| if (isTop(type1) || isBottom(type2)) { |
| return type1; |
| } |
| if (isTop(type2) || isBottom(type1)) { |
| return type2; |
| } |
| |
| if (type1 is TypeParameterType || type2 is TypeParameterType) { |
| return _typeParameterLeastUpperBound(type1, type2); |
| } |
| |
| // The least upper bound of a function type and an interface type T is the |
| // least upper bound of Function and T. |
| if (type1 is FunctionType && type2 is InterfaceType) { |
| type1 = rawFunctionType; |
| } |
| if (type2 is FunctionType && type1 is InterfaceType) { |
| type2 = rawFunctionType; |
| } |
| |
| // At this point type1 and type2 should both either be interface types or |
| // function types. |
| if (type1 is InterfaceType && type2 is InterfaceType) { |
| return _interfaceLeastUpperBound(type1, type2); |
| } |
| |
| if (type1 is FunctionType && type2 is FunctionType) { |
| return _functionLeastUpperBound(type1, type2); |
| } |
| |
| // Should never happen. As a defensive measure, return the dynamic type. |
| assert(false); |
| return const DynamicType(); |
| } |
| |
| @override |
| DartType getTypeOfOverloadedArithmetic(DartType type1, DartType type2) { |
| // TODO(paulberry): this matches what is defined in the spec. It would be |
| // nice if we could change kernel to match the spec and not have to |
| // override. |
| if (type1 == intType) { |
| if (type2 == intType) return type2; |
| if (type2 == doubleType) return type2; |
| } |
| return numType; |
| } |
| |
| /// Infers a generic type, function, method, or list/map literal |
| /// instantiation, using the downward context type as well as the argument |
| /// types if available. |
| /// |
| /// For example, given a function type with generic type parameters, this |
| /// infers the type parameters from the actual argument types. |
| /// |
| /// Concretely, given a function type with parameter types P0, P1, ... Pn, |
| /// result type R, and generic type parameters T0, T1, ... Tm, use the |
| /// argument types A0, A1, ... An to solve for the type parameters. |
| /// |
| /// For each parameter Pi, we want to ensure that Ai <: Pi. We can do this by |
| /// running the subtype algorithm, and when we reach a type parameter Tj, |
| /// recording the lower or upper bound it must satisfy. At the end, all |
| /// constraints can be combined to determine the type. |
| /// |
| /// All constraints on each type parameter Tj are tracked, as well as where |
| /// they originated, so we can issue an error message tracing back to the |
| /// argument values, type parameter "extends" clause, or the return type |
| /// context. |
| /// |
| /// If non-null values for [formalTypes] and [actualTypes] are provided, this |
| /// is upwards inference. Otherwise it is downward inference. |
| void inferGenericFunctionOrType( |
| DartType declaredReturnType, |
| List<TypeParameter> typeParametersToInfer, |
| List<DartType> formalTypes, |
| List<DartType> actualTypes, |
| DartType returnContextType, |
| List<DartType> inferredTypes) { |
| if (typeParametersToInfer.isEmpty) { |
| return; |
| } |
| |
| // Create a TypeConstraintGatherer that will allow certain type parameters |
| // to be inferred. It will optimistically assume these type parameters can |
| // be subtypes (or supertypes) as necessary, and track the constraints that |
| // are implied by this. |
| var gatherer = new TypeConstraintGatherer(this, typeParametersToInfer); |
| |
| if (returnContextType != null) { |
| gatherer.trySubtypeMatch(declaredReturnType, returnContextType); |
| } |
| |
| if (formalTypes != null) { |
| for (int i = 0; i < formalTypes.length; i++) { |
| // Try to pass each argument to each parameter, recording any type |
| // parameter bounds that were implied by this assignment. |
| gatherer.trySubtypeMatch(actualTypes[i], formalTypes[i]); |
| } |
| } |
| |
| inferTypeFromConstraints( |
| gatherer.computeConstraints(), typeParametersToInfer, inferredTypes, |
| downwardsInferPhase: formalTypes == null); |
| } |
| |
| /// Use the given [constraints] to substitute for type variables.. |
| /// |
| /// [typeParametersToInfer] is the set of type parameters that should be |
| /// substituted for. [inferredTypes] should be a list of the same length. |
| /// |
| /// If [downwardsInferPhase] is `true`, then we are in the first pass of |
| /// inference, pushing context types down. This means we are allowed to push |
| /// down `?` to precisely represent an unknown type. [inferredTypes] should |
| /// be initially populated with `?`. These `?`s will be replaced, if |
| /// appropriate, with the types that were inferred by downwards inference. |
| /// |
| /// If [downwardsInferPhase] is `false`, then we are in the second pass of |
| /// inference, and must not conclude `?` for any type formal. In this pass, |
| /// [inferredTypes] should contain the values from the first pass. They will |
| /// be replaced with the final inferred types. |
| void inferTypeFromConstraints(Map<TypeParameter, TypeConstraint> constraints, |
| List<TypeParameter> typeParametersToInfer, List<DartType> inferredTypes, |
| {bool downwardsInferPhase: false}) { |
| List<DartType> typesFromDownwardsInference = |
| downwardsInferPhase ? null : inferredTypes.toList(growable: false); |
| |
| for (int i = 0; i < typeParametersToInfer.length; i++) { |
| TypeParameter typeParam = typeParametersToInfer[i]; |
| |
| var typeParamBound = typeParam.bound; |
| DartType extendsConstraint; |
| if (!_isObjectOrDynamic(typeParamBound)) { |
| extendsConstraint = Substitution |
| .fromPairs(typeParametersToInfer, inferredTypes) |
| .substituteType(typeParamBound); |
| } |
| |
| var constraint = constraints[typeParam]; |
| if (downwardsInferPhase) { |
| inferredTypes[i] = |
| _inferTypeParameterFromContext(constraint, extendsConstraint); |
| } else { |
| inferredTypes[i] = _inferTypeParameterFromAll( |
| typesFromDownwardsInference[i], constraint, extendsConstraint); |
| } |
| } |
| |
| // If the downwards infer phase has failed, we'll catch this in the upwards |
| // phase later on. |
| if (downwardsInferPhase) { |
| return; |
| } |
| |
| // Check the inferred types against all of the constraints. |
| var knownTypes = <TypeParameter, DartType>{}; |
| for (int i = 0; i < typeParametersToInfer.length; i++) { |
| TypeParameter typeParam = typeParametersToInfer[i]; |
| var constraint = constraints[typeParam]; |
| var typeParamBound = Substitution |
| .fromPairs(typeParametersToInfer, inferredTypes) |
| .substituteType(typeParam.bound); |
| |
| var inferred = inferredTypes[i]; |
| bool success = typeSatisfiesConstraint(inferred, constraint); |
| if (success && !_isObjectOrDynamic(typeParamBound)) { |
| // If everything else succeeded, check the `extends` constraint. |
| var extendsConstraint = typeParamBound; |
| success = isSubtypeOf(inferred, extendsConstraint); |
| } |
| |
| if (!success) { |
| // TODO(paulberry): report error. |
| |
| // Heuristic: even if we failed, keep the erroneous type. |
| // It should satisfy at least some of the constraints (e.g. the return |
| // context). If we fall back to instantiateToBounds, we'll typically get |
| // more errors (e.g. because `dynamic` is the most common bound). |
| } |
| |
| if (isKnown(inferred)) { |
| knownTypes[typeParam] = inferred; |
| } |
| } |
| |
| // TODO(paulberry): report any errors from instantiateToBounds. |
| } |
| |
| /// Given a [DartType] [type], if [type] is an uninstantiated |
| /// parameterized type then instantiate the parameters to their |
| /// bounds. See the issue for the algorithm description. |
| /// |
| /// https://github.com/dart-lang/sdk/issues/27526#issuecomment-260021397 |
| /// |
| /// TODO(paulberry) Compute lazily and cache. |
| DartType instantiateToBounds(DartType type, |
| {Map<TypeParameter, DartType> knownTypes}) { |
| List<TypeParameter> typeFormals = _typeFormalsAsParameters(type); |
| int count = typeFormals.length; |
| if (count == 0) { |
| return type; |
| } |
| var substitution = <TypeParameter, DartType>{}; |
| for (TypeParameter parameter in typeFormals) { |
| // Note: we treat class<T extends Object> as equivalent to class<T>; in |
| // both cases they instantiate to class<dynamic>. See dartbug.com/29561 |
| if (_isObjectOrDynamic(parameter.bound)) { |
| substitution[parameter] = const DynamicType(); |
| } else { |
| substitution[parameter] = parameter.bound; |
| } |
| } |
| if (knownTypes != null) { |
| type = substitute(type, knownTypes); |
| } |
| var result = substituteDeep(type, substitution); |
| if (result != null) return result; |
| |
| // Instantiation failed due to a circularity. |
| // TODO(paulberry): report the error. |
| // Substitute `dynamic` for all parameters to try to allow compilation to |
| // continue. Note that [substituteDeep] is destructive of the |
| // [substitution] so we create a fresh one. |
| substitution = <TypeParameter, DartType>{}; |
| for (TypeParameter parameter in typeFormals) { |
| substitution[parameter] = const DynamicType(); |
| } |
| return substitute(type, substitution); |
| } |
| |
| @override |
| bool isBottom(DartType t) { |
| if (t is UnknownType) { |
| return true; |
| } else if (t is InterfaceType && |
| identical(t.classNode, coreTypes.nullClass)) { |
| return true; |
| } else { |
| return super.isBottom(t); |
| } |
| } |
| |
| @override |
| bool isTop(DartType t) { |
| if (t is UnknownType) { |
| return true; |
| } else { |
| return super.isTop(t); |
| } |
| } |
| |
| /// Computes the constraint solution for a type variable based on a given set |
| /// of constraints. |
| /// |
| /// If [grounded] is `true`, then the returned type is guaranteed to be a |
| /// known type (i.e. it will not contain any instances of `?`). |
| DartType solveTypeConstraint(TypeConstraint constraint, |
| {bool grounded: false}) { |
| // Prefer the known bound, if any. |
| if (isKnown(constraint.lower)) return constraint.lower; |
| if (isKnown(constraint.upper)) return constraint.upper; |
| |
| // Otherwise take whatever bound has partial information, e.g. `Iterable<?>` |
| if (constraint.lower is! UnknownType) { |
| return grounded |
| ? leastClosure(coreTypes, constraint.lower) |
| : constraint.lower; |
| } else { |
| return grounded |
| ? greatestClosure(coreTypes, constraint.upper) |
| : constraint.upper; |
| } |
| } |
| |
| /// Determine if the given [type] satisfies the given type [constraint]. |
| bool typeSatisfiesConstraint(DartType type, TypeConstraint constraint) { |
| return isSubtypeOf(constraint.lower, type) && |
| isSubtypeOf(type, constraint.upper); |
| } |
| |
| /// Compute the greatest lower bound of function types [f] and [g]. |
| /// |
| /// The spec rules for GLB on function types, informally, are pretty simple: |
| /// |
| /// - If a parameter is required in both, it stays required. |
| /// |
| /// - If a positional parameter is optional or missing in one, it becomes |
| /// optional. (This is because we're trying to build a function type which |
| /// is a subtype of both [f] and [g], meaning it accepts all possible inputs |
| /// that [f] and [g] accept.) |
| /// |
| /// - Named parameters are unioned together. |
| /// |
| /// - For any parameter that exists in both functions, use the LUB of them as |
| /// the resulting parameter type. |
| /// |
| /// - Use the GLB of their return types. |
| DartType _functionGreatestLowerBound(FunctionType f, FunctionType g) { |
| // TODO(rnystrom,paulberry): Right now, this assumes f and g do not have any |
| // type parameters. Revisit that in the presence of generic methods. |
| |
| // Calculate the LUB of each corresponding pair of parameters. |
| int totalPositional = |
| math.max(f.positionalParameters.length, g.positionalParameters.length); |
| var positionalParameters = new List<DartType>(totalPositional); |
| for (int i = 0; i < totalPositional; i++) { |
| if (i < f.positionalParameters.length) { |
| var fType = f.positionalParameters[i]; |
| if (i < g.positionalParameters.length) { |
| var gType = g.positionalParameters[i]; |
| positionalParameters[i] = getLeastUpperBound(fType, gType); |
| } else { |
| positionalParameters[i] = fType; |
| } |
| } else { |
| positionalParameters[i] = g.positionalParameters[i]; |
| } |
| } |
| |
| // Parameters that are required in both functions are required in the |
| // result. Parameters that are optional or missing in either end up |
| // optional. |
| int requiredParameterCount = |
| math.min(f.requiredParameterCount, g.requiredParameterCount); |
| bool hasPositional = requiredParameterCount < totalPositional; |
| |
| // Union the named parameters together. |
| List<NamedType> namedParameters = []; |
| { |
| int i = 0; |
| int j = 0; |
| while (true) { |
| if (i < f.namedParameters.length) { |
| if (j < g.namedParameters.length) { |
| var fName = f.namedParameters[i].name; |
| var gName = g.namedParameters[j].name; |
| int order = fName.compareTo(gName); |
| if (order < 0) { |
| namedParameters.add(f.namedParameters[i++]); |
| } else if (order > 0) { |
| namedParameters.add(g.namedParameters[j++]); |
| } else { |
| namedParameters.add(new NamedType( |
| fName, |
| getLeastUpperBound(f.namedParameters[i++].type, |
| g.namedParameters[j++].type))); |
| } |
| } else { |
| namedParameters.addAll(f.namedParameters.skip(i)); |
| break; |
| } |
| } else { |
| namedParameters.addAll(g.namedParameters.skip(j)); |
| break; |
| } |
| } |
| } |
| bool hasNamed = namedParameters.isNotEmpty; |
| |
| // Edge case. Dart does not support functions with both optional positional |
| // and named parameters. If we would synthesize that, give up. |
| if (hasPositional && hasNamed) return const BottomType(); |
| |
| // Calculate the GLB of the return type. |
| DartType returnType = getGreatestLowerBound(f.returnType, g.returnType); |
| return new FunctionType(positionalParameters, returnType, |
| namedParameters: namedParameters, |
| requiredParameterCount: requiredParameterCount); |
| } |
| |
| /// Compute the least upper bound of function types [f] and [g]. |
| /// |
| /// The rules for LUB on function types, informally, are pretty simple: |
| /// |
| /// - If the functions don't have the same number of required parameters, |
| /// always return `Function`. |
| /// |
| /// - Discard any optional named or positional parameters the two types do not |
| /// have in common. |
| /// |
| /// - Compute the GLB of each corresponding pair of parameter types, and the |
| /// LUB of the return types. Return a function type with those types. |
| DartType _functionLeastUpperBound(FunctionType f, FunctionType g) { |
| // TODO(rnystrom): Right now, this assumes f and g do not have any type |
| // parameters. Revisit that in the presence of generic methods. |
| |
| // If F and G differ in their number of required parameters, then the |
| // least upper bound of F and G is Function. |
| // TODO(paulberry): We could do better here, e.g.: |
| // LUB(([int]) -> void, (int) -> void) = (int) -> void |
| if (f.requiredParameterCount != g.requiredParameterCount) { |
| return coreTypes.functionClass.rawType; |
| } |
| int requiredParameterCount = f.requiredParameterCount; |
| |
| // Calculate the GLB of each corresponding pair of parameters. |
| // Ignore any extra optional positional parameters if one has more than the |
| // other. |
| int totalPositional = |
| math.min(f.positionalParameters.length, g.positionalParameters.length); |
| var positionalParameters = new List<DartType>(totalPositional); |
| for (int i = 0; i < totalPositional; i++) { |
| positionalParameters[i] = getGreatestLowerBound( |
| f.positionalParameters[i], g.positionalParameters[i]); |
| } |
| |
| // Intersect the named parameters. |
| List<NamedType> namedParameters = []; |
| { |
| int i = 0; |
| int j = 0; |
| while (true) { |
| if (i < f.namedParameters.length) { |
| if (j < g.namedParameters.length) { |
| var fName = f.namedParameters[i].name; |
| var gName = g.namedParameters[j].name; |
| int order = fName.compareTo(gName); |
| if (order < 0) { |
| i++; |
| } else if (order > 0) { |
| j++; |
| } else { |
| namedParameters.add(new NamedType( |
| fName, |
| getGreatestLowerBound(f.namedParameters[i++].type, |
| g.namedParameters[j++].type))); |
| } |
| } else { |
| break; |
| } |
| } else { |
| break; |
| } |
| } |
| } |
| |
| // Calculate the LUB of the return type. |
| DartType returnType = getLeastUpperBound(f.returnType, g.returnType); |
| return new FunctionType(positionalParameters, returnType, |
| namedParameters: namedParameters, |
| requiredParameterCount: requiredParameterCount); |
| } |
| |
| DartType _inferTypeParameterFromAll(DartType typeFromContextInference, |
| TypeConstraint constraint, DartType extendsConstraint) { |
| // See if we already fixed this type from downwards inference. |
| // If so, then we aren't allowed to change it based on argument types. |
| if (isKnown(typeFromContextInference)) { |
| return typeFromContextInference; |
| } |
| |
| if (extendsConstraint != null) { |
| constraint = constraint.clone(); |
| addUpperBound(constraint, extendsConstraint); |
| } |
| |
| return solveTypeConstraint(constraint, grounded: true); |
| } |
| |
| DartType _inferTypeParameterFromContext( |
| TypeConstraint constraint, DartType extendsConstraint) { |
| DartType t = solveTypeConstraint(constraint); |
| if (!isKnown(t)) { |
| return t; |
| } |
| |
| // If we're about to make our final choice, apply the extends clause. |
| // This gives us a chance to refine the choice, in case it would violate |
| // the `extends` clause. For example: |
| // |
| // Object obj = math.min/*<infer Object, error>*/(1, 2); |
| // |
| // If we consider the `T extends num` we conclude `<num>`, which works. |
| if (extendsConstraint != null) { |
| constraint = constraint.clone(); |
| addUpperBound(constraint, extendsConstraint); |
| return solveTypeConstraint(constraint); |
| } |
| return t; |
| } |
| |
| DartType _interfaceLeastUpperBound(InterfaceType type1, InterfaceType type2) { |
| // This currently does not implement a very complete least upper bound |
| // algorithm, but handles a couple of the very common cases that are |
| // causing pain in real code. The current algorithm is: |
| // 1. If either of the types is a supertype of the other, return it. |
| // This is in fact the best result in this case. |
| // 2. If the two types have the same class element, then take the |
| // pointwise least upper bound of the type arguments. This is again |
| // the best result, except that the recursive calls may not return |
| // the true least upper bounds. The result is guaranteed to be a |
| // well-formed type under the assumption that the input types were |
| // well-formed (and assuming that the recursive calls return |
| // well-formed types). |
| // 3. Otherwise return the spec-defined least upper bound. This will |
| // be an upper bound, might (or might not) be least, and might |
| // (or might not) be a well-formed type. |
| if (isSubtypeOf(type1, type2)) { |
| return type2; |
| } |
| if (isSubtypeOf(type2, type1)) { |
| return type1; |
| } |
| if (type1 is InterfaceType && |
| type2 is InterfaceType && |
| identical(type1.classNode, type2.classNode)) { |
| List<DartType> tArgs1 = type1.typeArguments; |
| List<DartType> tArgs2 = type2.typeArguments; |
| |
| assert(tArgs1.length == tArgs2.length); |
| List<DartType> tArgs = new List(tArgs1.length); |
| for (int i = 0; i < tArgs1.length; i++) { |
| tArgs[i] = getLeastUpperBound(tArgs1[i], tArgs2[i]); |
| } |
| return new InterfaceType(type1.classNode, tArgs); |
| } |
| return hierarchy.getClassicLeastUpperBound(type1, type2); |
| } |
| |
| bool _isObjectOrDynamic(DartType type) => |
| type is DynamicType || |
| (type is InterfaceType && |
| identical(type.classNode, coreTypes.objectClass)); |
| |
| /// Given a [type], returns the [TypeParameter]s corresponding to its formal |
| /// type parameters (if any). |
| List<TypeParameter> _typeFormalsAsParameters(DartType type) { |
| if (type is TypedefType) { |
| return type.typedefNode.typeParameters; |
| } else if (type is InterfaceType) { |
| return type.classNode.typeParameters; |
| } else { |
| return const []; |
| } |
| } |
| |
| DartType _typeParameterLeastUpperBound(DartType type1, DartType type2) { |
| // This currently just implements a simple least upper bound to |
| // handle some common cases. It also avoids some termination issues |
| // with the naive spec algorithm. The least upper bound of two types |
| // (at least one of which is a type parameter) is computed here as: |
| // 1. If either type is a supertype of the other, return it. |
| // 2. If the first type is a type parameter, replace it with its bound, |
| // with recursive occurrences of itself replaced with Object. |
| // The second part of this should ensure termination. Informally, |
| // each type variable instantiation in one of the arguments to the |
| // least upper bound algorithm now strictly reduces the number |
| // of bound variables in scope in that argument position. |
| // 3. If the second type is a type parameter, do the symmetric operation |
| // to #2. |
| // |
| // It's not immediately obvious why this is symmetric in the case that both |
| // of them are type parameters. For #1, symmetry holds since subtype |
| // is antisymmetric. For #2, it's clearly not symmetric if upper bounds of |
| // bottom are allowed. Ignoring this (for various reasons, not least |
| // of which that there's no way to write it), there's an informal |
| // argument (that might even be right) that you will always either |
| // end up expanding both of them or else returning the same result no matter |
| // which order you expand them in. A key observation is that |
| // identical(expand(type1), type2) => subtype(type1, type2) |
| // and hence the contra-positive. |
| // |
| // TODO(leafp): Think this through and figure out what's the right |
| // definition. Be careful about termination. |
| // |
| // I suspect in general a reasonable algorithm is to expand the innermost |
| // type variable first. Alternatively, you could probably choose to treat |
| // it as just an instance of the interface type upper bound problem, with |
| // the "inheritance" chain extended by the bounds placed on the variables. |
| if (isSubtypeOf(type1, type2)) { |
| return type2; |
| } |
| if (isSubtypeOf(type2, type1)) { |
| return type1; |
| } |
| if (type1 is TypeParameterType) { |
| // TODO(paulberry): Analyzer collapses simple bounds in one step, i.e. for |
| // C<T extends U, U extends List>, T gets resolved directly to List. Do |
| // we need to replicate that behavior? |
| return getLeastUpperBound( |
| Substitution.fromMap({type1.parameter: objectType}).substituteType( |
| type1.parameter.bound), |
| type2); |
| } else if (type2 is TypeParameterType) { |
| return getLeastUpperBound( |
| type1, |
| Substitution.fromMap({type2.parameter: objectType}).substituteType( |
| type2.parameter.bound)); |
| } else { |
| // We should only be called when at least one of the types is a |
| // TypeParameterType |
| assert(false); |
| return const DynamicType(); |
| } |
| } |
| } |
