pkg/analyzer/lib/src/task/strong_mode.dart - sdk.git - Git at Google

 // Copyright (c) 2015, 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 file.

 import 'dart:collection';

 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/inheritance_manager2.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
 import 'package:analyzer/src/summary/link.dart'
     show FieldElementForLink_ClassField, ParameterElementForLink;

 /**
  * Sets the type of the field. The types in implicit accessors are updated
  * implicitly, and the types of explicit accessors should be updated separately.
  */
 void setFieldType(VariableElement field, DartType newType) {
   (field as VariableElementImpl).type = newType;
 }

 /**
  * A function that returns `true` if the given [element] passes the filter.
  */
 typedef bool VariableFilter(VariableElement element);

 /**
  * An object used to infer the type of instance fields and the return types of
  * instance methods within a single compilation unit.
  */
 class InstanceMemberInferrer {
   final TypeProvider typeProvider;
   final InheritanceManager2 inheritance;
   final Set<ClassElement> elementsBeingInferred = new HashSet<ClassElement>();

   InterfaceType interfaceType;

   /**
    * Initialize a newly create inferrer.
    */
   InstanceMemberInferrer(this.typeProvider, this.inheritance);

   /**
    * Infer type information for all of the instance members in the given
    * compilation [unit].
    */
   void inferCompilationUnit(CompilationUnitElement unit) {
     _inferClasses(unit.mixins);
     _inferClasses(unit.types);
   }

   /**
    * Return `true` if the elements corresponding to the [types] have the same
    * kind as the [element].
    */
   bool _allSameElementKind(
       ExecutableElement element, List<FunctionType> types) {
     var elementKind = element.kind;
     for (int i = 0; i < types.length; i++) {
       if (types[i].element.kind != elementKind) {
         return false;
       }
     }
     return true;
   }

   /**
    * Compute the inferred type for the given property [accessor]. The returned
    * value is never `null`, but might be an error, and/or have the `null` type.
    */
   _FieldOverrideInferenceResult _computeFieldOverrideType(
       PropertyAccessorElement accessor) {
     String name = accessor.displayName;

     var overriddenGetters = inheritance.getOverridden(
       interfaceType,
       new Name(accessor.library.source.uri, name),
     );

     List<FunctionType> overriddenSetters;
     if (overriddenGetters == null || !accessor.variable.isFinal) {
       overriddenSetters = inheritance.getOverridden(
         interfaceType,
         new Name(accessor.library.source.uri, '$name='),
       );
     }

     // Choose overridden types from getters or/and setters.
     List<FunctionType> overriddenTypes = <FunctionType>[];
     if (overriddenGetters == null && overriddenSetters == null) {
       overriddenTypes = const <FunctionType>[];
     } else if (overriddenGetters == null && overriddenSetters != null) {
       overriddenTypes = overriddenSetters;
     } else if (overriddenGetters != null && overriddenSetters == null) {
       overriddenTypes = overriddenGetters;
     } else {
       overriddenTypes = <FunctionType>[]
         ..addAll(overriddenGetters)
         ..addAll(overriddenSetters);
     }

     bool isCovariant = false;
     DartType impliedType;
     for (FunctionType overriddenType in overriddenTypes) {
       var overriddenElementKind = overriddenType.element.kind;
       if (overriddenType == null) {
         return new _FieldOverrideInferenceResult(false, null, true);
       }

       DartType type;
       if (overriddenElementKind == ElementKind.GETTER) {
         type = overriddenType.returnType;
       } else if (overriddenElementKind == ElementKind.SETTER) {
         if (overriddenType.parameters.length == 1) {
           ParameterElement parameter = overriddenType.parameters[0];
           type = parameter.type;
           isCovariant = isCovariant || parameter.isCovariant;
         }
       } else {
         return new _FieldOverrideInferenceResult(false, null, true);
       }

       if (impliedType == null) {
         impliedType = type;
       } else if (type != impliedType) {
         return new _FieldOverrideInferenceResult(false, null, true);
       }
     }

     return new _FieldOverrideInferenceResult(isCovariant, impliedType, false);
   }

   /**
    * Compute the best type for the [parameter] at the given [index] that must be
    * compatible with the types of the corresponding parameters of the given
    * [overriddenTypes].
    *
    * At the moment, this method will only return a type other than 'dynamic' if
    * the types of all of the parameters are the same. In the future we might
    * want to be smarter about it, such as by returning the least upper bound of
    * the parameter types.
    */
   DartType _computeParameterType(ParameterElement parameter, int index,
       List<FunctionType> overriddenTypes) {
     DartType parameterType = null;
     int length = overriddenTypes.length;
     for (int i = 0; i < length; i++) {
       ParameterElement matchingParameter = _getCorrespondingParameter(
           parameter, index, overriddenTypes[i].parameters);
       DartType type = matchingParameter?.type ?? typeProvider.dynamicType;
       if (parameterType == null) {
         if (type is FunctionType &&
             type.element != null &&
             type.element is! TypeDefiningElement &&
             type.element.enclosingElement is! TypeDefiningElement) {
           // The resulting parameter's type element has an `enclosingElement` of
           // the overridden parameter. Change it to the overriding parameter.
           parameterType = new FunctionTypeImpl.fresh(type, force: true);
           (parameterType.element as ElementImpl).enclosingElement = parameter;
           // TODO(mfairhurst) handle cases where non-functions contain functions
           // See test_inferredType_parameter_genericFunctionType_asTypeArgument
         } else {
           parameterType = type;
         }
       } else if (parameterType != type) {
         if (parameter is ParameterElementForLink) {
           parameter.setInferenceError(new TopLevelInferenceErrorBuilder(
               kind: TopLevelInferenceErrorKind.overrideConflictParameterType));
         }
         return typeProvider.dynamicType;
       }
     }
     return parameterType ?? typeProvider.dynamicType;
   }

   /**
    * Compute the best return type for a method that must be compatible with the
    * return types of each of the given [overriddenReturnTypes].
    *
    * At the moment, this method will only return a type other than 'dynamic' if
    * the return types of all of the methods are the same. In the future we might
    * want to be smarter about it.
    */
   DartType _computeReturnType(Iterable<DartType> overriddenReturnTypes) {
     DartType returnType = null;
     for (DartType type in overriddenReturnTypes) {
       if (type == null) {
         type = typeProvider.dynamicType;
       }
       if (returnType == null) {
         returnType = type;
       } else if (returnType != type) {
         return typeProvider.dynamicType;
       }
     }
     return returnType ?? typeProvider.dynamicType;
   }

   /**
    * Given a method, return the parameter in the method that corresponds to the
    * given [parameter]. If the parameter is positional, then
    * it appears at the given [index] in its enclosing element's list of
    * parameters.
    */
   ParameterElement _getCorrespondingParameter(ParameterElement parameter,
       int index, List<ParameterElement> methodParameters) {
     //
     // Find the corresponding parameter.
     //
     if (parameter.isNamed) {
       //
       // If we're looking for a named parameter, only a named parameter with
       // the same name will be matched.
       //
       return methodParameters.lastWhere(
           (ParameterElement methodParameter) =>
               methodParameter.isNamed && methodParameter.name == parameter.name,
           orElse: () => null);
     }
     //
     // If we're looking for a positional parameter we ignore the difference
     // between required and optional parameters.
     //
     if (index < methodParameters.length) {
       var matchingParameter = methodParameters[index];
       if (!matchingParameter.isNamed) {
         return matchingParameter;
       }
     }
     return null;
   }

   /**
    * If the given [element] represents a non-synthetic instance property
    * accessor for which no type was provided, infer its types.
    */
   void _inferAccessor(PropertyAccessorElement element) {
     if (element.isSynthetic || element.isStatic) {
       return;
     }

     if (element.kind == ElementKind.GETTER && !element.hasImplicitReturnType) {
       return;
     }

     _FieldOverrideInferenceResult typeResult =
         _computeFieldOverrideType(element);
     if (typeResult.isError == null || typeResult.type == null) {
       return;
     }

     if (element.kind == ElementKind.GETTER) {
       (element as ExecutableElementImpl).returnType = typeResult.type;
     } else if (element.kind == ElementKind.SETTER) {
       List<ParameterElement> parameters = element.parameters;
       if (parameters.isNotEmpty) {
         var parameter = parameters[0] as ParameterElementImpl;
         if (parameter.hasImplicitType) {
           parameter.type = typeResult.type;
         }
         parameter.inheritsCovariant = typeResult.isCovariant;
       }
     }
     setFieldType(element.variable, typeResult.type);
   }

   /**
    * Infer type information for all of the instance members in the given
    * [classElement].
    */
   void _inferClass(ClassElement classElement) {
     if (classElement is ClassElementImpl) {
       if (classElement.hasBeenInferred) {
         return;
       }
       if (!elementsBeingInferred.add(classElement)) {
         // We have found a circularity in the class hierarchy. For now we just
         // stop trying to infer any type information for any classes that
         // inherit from any class in the cycle. We could potentially limit the
         // algorithm to only not inferring types in the classes in the cycle,
         // but it isn't clear that the results would be significantly better.
         throw new _CycleException();
       }
       try {
         //
         // Ensure that all of instance members in the supertypes have had types
         // inferred for them.
         //
         _inferType(classElement.supertype);
         classElement.mixins.forEach(_inferType);
         classElement.interfaces.forEach(_inferType);
         classElement.superclassConstraints.forEach(_inferType);
         //
         // Then infer the types for the members.
         //
         this.interfaceType = classElement.type;
         for (FieldElement field in classElement.fields) {
           _inferField(field);
         }
         for (PropertyAccessorElement accessor in classElement.accessors) {
           _inferAccessor(accessor);
         }
         for (MethodElement method in classElement.methods) {
           _inferExecutable(method);
         }
         //
         // Infer initializing formal parameter types. This must happen after
         // field types are inferred.
         //
         classElement.constructors.forEach(_inferConstructorFieldFormals);
         classElement.hasBeenInferred = true;
       } finally {
         elementsBeingInferred.remove(classElement);
       }
     }
   }

   void _inferClasses(List<ClassElement> elements) {
     for (ClassElement element in elements) {
       try {
         _inferClass(element);
       } on _CycleException {
         // This is a short circuit return to prevent types that inherit from
         // types containing a circular reference from being inferred.
       }
     }
   }

   void _inferConstructorFieldFormals(ConstructorElement constructor) {
     for (ParameterElement parameter in constructor.parameters) {
       if (parameter.hasImplicitType &&
           parameter is FieldFormalParameterElementImpl) {
         FieldElement field = parameter.field;
         if (field != null) {
           parameter.type = field.type;
         }
       }
     }
   }

   /**
    * If the given [element] represents a non-synthetic instance method,
    * getter or setter, infer the return type and any parameter type(s) where
    * they were not provided.
    */
   void _inferExecutable(ExecutableElement element) {
     if (element.isSynthetic || element.isStatic) {
       return;
     }

     // TODO(scheglov) If no implicit types, don't ask inherited.

     List<FunctionType> overriddenTypes = inheritance.getOverridden(
       interfaceType,
       new Name(element.library.source.uri, element.name),
     );
     if (overriddenTypes == null ||
         !_allSameElementKind(element, overriddenTypes)) {
       return;
     }
     overriddenTypes = _toOverriddenFunctionTypes(element, overriddenTypes);
     if (overriddenTypes.isEmpty) {
       return;
     }

     //
     // Infer the return type.
     //
     if (element.hasImplicitReturnType && element.displayName != '[]=') {
       (element as ExecutableElementImpl).returnType =
           _computeReturnType(overriddenTypes.map((t) => t.returnType));
       if (element is PropertyAccessorElement) {
         _updateSyntheticVariableType(element);
       }
     }
     //
     // Infer the parameter types.
     //
     List<ParameterElement> parameters = element.parameters;
     int length = parameters.length;
     for (int i = 0; i < length; ++i) {
       ParameterElement parameter = parameters[i];
       if (parameter is ParameterElementImpl) {
         _inferParameterCovariance(parameter, i, overriddenTypes);

         if (parameter.hasImplicitType) {
           parameter.type = _computeParameterType(parameter, i, overriddenTypes);
           if (element is PropertyAccessorElement) {
             _updateSyntheticVariableType(element);
           }
         }
       }
     }
   }

   /**
    * If the given [field] represents a non-synthetic instance field for
    * which no type was provided, infer the type of the field.
    */
   void _inferField(FieldElement field) {
     if (field.isSynthetic || field.isStatic) {
       return;
     }

     _FieldOverrideInferenceResult typeResult =
         _computeFieldOverrideType(field.getter);
     if (typeResult.isError) {
       if (field is FieldElementForLink_ClassField) {
         field.setInferenceError(new TopLevelInferenceErrorBuilder(
             kind: TopLevelInferenceErrorKind.overrideConflictFieldType));
       }
       return;
     }

     if (field.hasImplicitType) {
       DartType newType = typeResult.type;
       if (newType == null && field.initializer != null) {
         newType = field.initializer.returnType;
       }

       if (newType == null || newType.isBottom || newType.isDartCoreNull) {
         newType = typeProvider.dynamicType;
       }

       setFieldType(field, newType);
     }

     if (field.setter != null) {
       var parameter = field.setter.parameters[0] as ParameterElementImpl;
       parameter.inheritsCovariant = typeResult.isCovariant;
     }
   }

   /**
    * If a parameter is covariant, any parameters that override it are too.
    */
   void _inferParameterCovariance(ParameterElementImpl parameter, int index,
       Iterable<FunctionType> overriddenTypes) {
     parameter.inheritsCovariant = overriddenTypes.any((f) {
       var param = _getCorrespondingParameter(parameter, index, f.parameters);
       return param != null && param.isCovariant;
     });
   }

   /**
    * Infer type information for all of the instance members in the given
    * interface [type].
    */
   void _inferType(InterfaceType type) {
     if (type != null) {
       ClassElement element = type.element;
       if (element != null) {
         _inferClass(element);
       }
     }
   }

   /**
    * If the [element] has formal type parameters, then the [overriddenType]
    * must have it as well, but they are different.  Replace type parameters
    * of the [overriddenType] with type formals of the [element].
    *
    * Return `null`, in case of type parameters inconsistency.
    *
    * The overridden element must have the same number of generic type
    * parameters as the target element, or none.
    *
    * If we do have generic type parameters on the element we're inferring,
    * we must express its parameter and return types in terms of its own
    * parameters. For example, given `m<T>(t)` overriding `m<S>(S s)` we
    * should infer this as `m<T>(T t)`.
    */
   FunctionType _toOverriddenFunctionType(
       ExecutableElement element, FunctionType overriddenType) {
     List<DartType> typeFormals =
         TypeParameterTypeImpl.getTypes(element.type.typeFormals);
     if (overriddenType.typeFormals.isNotEmpty) {
       if (overriddenType.typeFormals.length != typeFormals.length) {
         return null;
       }
       overriddenType = overriddenType.instantiate(typeFormals);
     }
     return overriddenType;
   }

   /**
    * Return [overriddenTypes] that override [element].
    * Return the empty list, in case of type parameters inconsistency.
    */
   List<FunctionType> _toOverriddenFunctionTypes(
       ExecutableElement element, List<FunctionType> overriddenTypes) {
     var result = <FunctionType>[];
     for (var overriddenType in overriddenTypes) {
       overriddenType = _toOverriddenFunctionType(element, overriddenType);
       if (overriddenType == null) {
         return const <FunctionType>[];
       }
       result.add(overriddenType);
     }
     return result;
   }

   /**
    * If the given [element] is a non-synthetic getter or setter, update its
    * synthetic variable's type to match the getter's return type, or if no
    * corresponding getter exists, use the setter's parameter type.
    *
    * In general, the type of the synthetic variable should not be used, because
    * getters and setters are independent methods. But this logic matches what
    * `TypeResolverVisitor.visitMethodDeclaration` would fill in there.
    */
   void _updateSyntheticVariableType(PropertyAccessorElement element) {
     assert(!element.isSynthetic);
     PropertyAccessorElement getter = element;
     if (element.isSetter) {
       // See if we can find any getter.
       getter = element.correspondingGetter;
     }
     DartType newType;
     if (getter != null) {
       newType = getter.returnType;
     } else if (element.isSetter && element.parameters.isNotEmpty) {
       newType = element.parameters[0].type;
     }
     if (newType != null) {
       (element.variable as VariableElementImpl).type = newType;
     }
   }
 }

 /**
  * A visitor that will gather all of the variables referenced within a given
  * AST structure. The collection can be restricted to contain only those
  * variables that pass a specified filter.
  */
 class VariableGatherer extends RecursiveAstVisitor {
   /**
    * The filter used to limit which variables are gathered, or `null` if no
    * filtering is to be performed.
    */
   final VariableFilter filter;

   /**
    * The variables that were found.
    */
   final Set<VariableElement> results = new HashSet<VariableElement>();

   /**
    * Initialize a newly created gatherer to gather all of the variables that
    * pass the given [filter] (or all variables if no filter is provided).
    */
   VariableGatherer([this.filter = null]);

   @override
   void visitSimpleIdentifier(SimpleIdentifier node) {
     if (!node.inDeclarationContext()) {
       Element nonAccessor(Element element) {
         if (element is PropertyAccessorElement && element.isSynthetic) {
           return element.variable;
         }
         return element;
       }

       Element element = nonAccessor(node.staticElement);
       if (element is VariableElement && (filter == null || filter(element))) {
         results.add(element);
       }
     }
   }
 }

 /**
  * A class of exception that is not used anywhere else.
  */
 class _CycleException implements Exception {}

 /**
  * The result of field type inference.
  */
 class _FieldOverrideInferenceResult {
   final bool isCovariant;
   final DartType type;
   final bool isError;

   _FieldOverrideInferenceResult(this.isCovariant, this.type, this.isError);
 }
	// Copyright (c) 2015, 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 file.

	import 'dart:collection';

	import 'package:analyzer/dart/ast/ast.dart';
	import 'package:analyzer/dart/ast/visitor.dart';
	import 'package:analyzer/dart/element/element.dart';
	import 'package:analyzer/dart/element/type.dart';
	import 'package:analyzer/src/dart/element/element.dart';
	import 'package:analyzer/src/dart/element/inheritance_manager2.dart';
	import 'package:analyzer/src/dart/element/type.dart';
	import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
	import 'package:analyzer/src/summary/format.dart';
	import 'package:analyzer/src/summary/idl.dart';
	import 'package:analyzer/src/summary/link.dart'
	show FieldElementForLink_ClassField, ParameterElementForLink;

	/**
	* Sets the type of the field. The types in implicit accessors are updated
	* implicitly, and the types of explicit accessors should be updated separately.
	*/
	void setFieldType(VariableElement field, DartType newType) {
	(field as VariableElementImpl).type = newType;
	}

	/**
	* A function that returns `true` if the given [element] passes the filter.
	*/
	typedef bool VariableFilter(VariableElement element);

	/**
	* An object used to infer the type of instance fields and the return types of
	* instance methods within a single compilation unit.
	*/
	class InstanceMemberInferrer {
	final TypeProvider typeProvider;
	final InheritanceManager2 inheritance;
	final Set<ClassElement> elementsBeingInferred = new HashSet<ClassElement>();

	InterfaceType interfaceType;

	/**
	* Initialize a newly create inferrer.
	*/
	InstanceMemberInferrer(this.typeProvider, this.inheritance);

	/**
	* Infer type information for all of the instance members in the given
	* compilation [unit].
	*/
	void inferCompilationUnit(CompilationUnitElement unit) {
	_inferClasses(unit.mixins);
	_inferClasses(unit.types);
	}

	/**
	* Return `true` if the elements corresponding to the [types] have the same
	* kind as the [element].
	*/
	bool _allSameElementKind(
	ExecutableElement element, List<FunctionType> types) {
	var elementKind = element.kind;
	for (int i = 0; i < types.length; i++) {
	if (types[i].element.kind != elementKind) {
	return false;
	}
	}
	return true;
	}

	/**
	* Compute the inferred type for the given property [accessor]. The returned
	* value is never `null`, but might be an error, and/or have the `null` type.
	*/
	_FieldOverrideInferenceResult _computeFieldOverrideType(
	PropertyAccessorElement accessor) {
	String name = accessor.displayName;

	var overriddenGetters = inheritance.getOverridden(
	interfaceType,
	new Name(accessor.library.source.uri, name),
	);

	List<FunctionType> overriddenSetters;
	if (overriddenGetters == null \|\| !accessor.variable.isFinal) {
	overriddenSetters = inheritance.getOverridden(
	interfaceType,
	new Name(accessor.library.source.uri, '$name='),
	);
	}

	// Choose overridden types from getters or/and setters.
	List<FunctionType> overriddenTypes = <FunctionType>[];
	if (overriddenGetters == null && overriddenSetters == null) {
	overriddenTypes = const <FunctionType>[];
	} else if (overriddenGetters == null && overriddenSetters != null) {
	overriddenTypes = overriddenSetters;
	} else if (overriddenGetters != null && overriddenSetters == null) {
	overriddenTypes = overriddenGetters;
	} else {
	overriddenTypes = <FunctionType>[]
	..addAll(overriddenGetters)
	..addAll(overriddenSetters);
	}

	bool isCovariant = false;
	DartType impliedType;
	for (FunctionType overriddenType in overriddenTypes) {
	var overriddenElementKind = overriddenType.element.kind;
	if (overriddenType == null) {
	return new _FieldOverrideInferenceResult(false, null, true);
	}

	DartType type;
	if (overriddenElementKind == ElementKind.GETTER) {
	type = overriddenType.returnType;
	} else if (overriddenElementKind == ElementKind.SETTER) {
	if (overriddenType.parameters.length == 1) {
	ParameterElement parameter = overriddenType.parameters[0];
	type = parameter.type;
	isCovariant = isCovariant \|\| parameter.isCovariant;
	}
	} else {
	return new _FieldOverrideInferenceResult(false, null, true);
	}

	if (impliedType == null) {
	impliedType = type;
	} else if (type != impliedType) {
	return new _FieldOverrideInferenceResult(false, null, true);
	}
	}

	return new _FieldOverrideInferenceResult(isCovariant, impliedType, false);
	}

	/**
	* Compute the best type for the [parameter] at the given [index] that must be
	* compatible with the types of the corresponding parameters of the given
	* [overriddenTypes].
	*
	* At the moment, this method will only return a type other than 'dynamic' if
	* the types of all of the parameters are the same. In the future we might
	* want to be smarter about it, such as by returning the least upper bound of
	* the parameter types.
	*/
	DartType _computeParameterType(ParameterElement parameter, int index,
	List<FunctionType> overriddenTypes) {
	DartType parameterType = null;
	int length = overriddenTypes.length;
	for (int i = 0; i < length; i++) {
	ParameterElement matchingParameter = _getCorrespondingParameter(
	parameter, index, overriddenTypes[i].parameters);
	DartType type = matchingParameter?.type ?? typeProvider.dynamicType;
	if (parameterType == null) {
	if (type is FunctionType &&
	type.element != null &&
	type.element is! TypeDefiningElement &&
	type.element.enclosingElement is! TypeDefiningElement) {
	// The resulting parameter's type element has an `enclosingElement` of
	// the overridden parameter. Change it to the overriding parameter.
	parameterType = new FunctionTypeImpl.fresh(type, force: true);
	(parameterType.element as ElementImpl).enclosingElement = parameter;
	// TODO(mfairhurst) handle cases where non-functions contain functions
	// See test_inferredType_parameter_genericFunctionType_asTypeArgument
	} else {
	parameterType = type;
	}
	} else if (parameterType != type) {
	if (parameter is ParameterElementForLink) {
	parameter.setInferenceError(new TopLevelInferenceErrorBuilder(
	kind: TopLevelInferenceErrorKind.overrideConflictParameterType));
	}
	return typeProvider.dynamicType;
	}
	}
	return parameterType ?? typeProvider.dynamicType;
	}

	/**
	* Compute the best return type for a method that must be compatible with the
	* return types of each of the given [overriddenReturnTypes].
	*
	* At the moment, this method will only return a type other than 'dynamic' if
	* the return types of all of the methods are the same. In the future we might
	* want to be smarter about it.
	*/
	DartType _computeReturnType(Iterable<DartType> overriddenReturnTypes) {
	DartType returnType = null;
	for (DartType type in overriddenReturnTypes) {
	if (type == null) {
	type = typeProvider.dynamicType;
	}
	if (returnType == null) {
	returnType = type;
	} else if (returnType != type) {
	return typeProvider.dynamicType;
	}
	}
	return returnType ?? typeProvider.dynamicType;
	}

	/**
	* Given a method, return the parameter in the method that corresponds to the
	* given [parameter]. If the parameter is positional, then
	* it appears at the given [index] in its enclosing element's list of
	* parameters.
	*/
	ParameterElement _getCorrespondingParameter(ParameterElement parameter,
	int index, List<ParameterElement> methodParameters) {
	//
	// Find the corresponding parameter.
	//
	if (parameter.isNamed) {
	//
	// If we're looking for a named parameter, only a named parameter with
	// the same name will be matched.
	//
	return methodParameters.lastWhere(
	(ParameterElement methodParameter) =>
	methodParameter.isNamed && methodParameter.name == parameter.name,
	orElse: () => null);
	}
	//
	// If we're looking for a positional parameter we ignore the difference
	// between required and optional parameters.
	//
	if (index < methodParameters.length) {
	var matchingParameter = methodParameters[index];
	if (!matchingParameter.isNamed) {
	return matchingParameter;
	}
	}
	return null;
	}

	/**
	* If the given [element] represents a non-synthetic instance property
	* accessor for which no type was provided, infer its types.
	*/
	void _inferAccessor(PropertyAccessorElement element) {
	if (element.isSynthetic \|\| element.isStatic) {
	return;
	}

	if (element.kind == ElementKind.GETTER && !element.hasImplicitReturnType) {
	return;
	}

	_FieldOverrideInferenceResult typeResult =
	_computeFieldOverrideType(element);
	if (typeResult.isError == null \|\| typeResult.type == null) {
	return;
	}

	if (element.kind == ElementKind.GETTER) {
	(element as ExecutableElementImpl).returnType = typeResult.type;
	} else if (element.kind == ElementKind.SETTER) {
	List<ParameterElement> parameters = element.parameters;
	if (parameters.isNotEmpty) {
	var parameter = parameters[0] as ParameterElementImpl;
	if (parameter.hasImplicitType) {
	parameter.type = typeResult.type;
	}
	parameter.inheritsCovariant = typeResult.isCovariant;
	}
	}
	setFieldType(element.variable, typeResult.type);
	}

	/**
	* Infer type information for all of the instance members in the given
	* [classElement].
	*/
	void _inferClass(ClassElement classElement) {
	if (classElement is ClassElementImpl) {
	if (classElement.hasBeenInferred) {
	return;
	}
	if (!elementsBeingInferred.add(classElement)) {
	// We have found a circularity in the class hierarchy. For now we just
	// stop trying to infer any type information for any classes that
	// inherit from any class in the cycle. We could potentially limit the
	// algorithm to only not inferring types in the classes in the cycle,
	// but it isn't clear that the results would be significantly better.
	throw new _CycleException();
	}
	try {
	//
	// Ensure that all of instance members in the supertypes have had types
	// inferred for them.
	//
	_inferType(classElement.supertype);
	classElement.mixins.forEach(_inferType);
	classElement.interfaces.forEach(_inferType);
	classElement.superclassConstraints.forEach(_inferType);
	//
	// Then infer the types for the members.
	//
	this.interfaceType = classElement.type;
	for (FieldElement field in classElement.fields) {
	_inferField(field);
	}
	for (PropertyAccessorElement accessor in classElement.accessors) {
	_inferAccessor(accessor);
	}
	for (MethodElement method in classElement.methods) {
	_inferExecutable(method);
	}
	//
	// Infer initializing formal parameter types. This must happen after
	// field types are inferred.
	//
	classElement.constructors.forEach(_inferConstructorFieldFormals);
	classElement.hasBeenInferred = true;
	} finally {
	elementsBeingInferred.remove(classElement);
	}
	}
	}

	void _inferClasses(List<ClassElement> elements) {
	for (ClassElement element in elements) {
	try {
	_inferClass(element);
	} on _CycleException {
	// This is a short circuit return to prevent types that inherit from
	// types containing a circular reference from being inferred.
	}
	}
	}

	void _inferConstructorFieldFormals(ConstructorElement constructor) {
	for (ParameterElement parameter in constructor.parameters) {
	if (parameter.hasImplicitType &&
	parameter is FieldFormalParameterElementImpl) {
	FieldElement field = parameter.field;
	if (field != null) {
	parameter.type = field.type;
	}
	}
	}
	}

	/**
	* If the given [element] represents a non-synthetic instance method,
	* getter or setter, infer the return type and any parameter type(s) where
	* they were not provided.
	*/
	void _inferExecutable(ExecutableElement element) {
	if (element.isSynthetic \|\| element.isStatic) {
	return;
	}

	// TODO(scheglov) If no implicit types, don't ask inherited.

	List<FunctionType> overriddenTypes = inheritance.getOverridden(
	interfaceType,
	new Name(element.library.source.uri, element.name),
	);
	if (overriddenTypes == null \|\|
	!_allSameElementKind(element, overriddenTypes)) {
	return;
	}
	overriddenTypes = _toOverriddenFunctionTypes(element, overriddenTypes);
	if (overriddenTypes.isEmpty) {
	return;
	}

	//
	// Infer the return type.
	//
	if (element.hasImplicitReturnType && element.displayName != '[]=') {
	(element as ExecutableElementImpl).returnType =
	_computeReturnType(overriddenTypes.map((t) => t.returnType));
	if (element is PropertyAccessorElement) {
	_updateSyntheticVariableType(element);
	}
	}
	//
	// Infer the parameter types.
	//
	List<ParameterElement> parameters = element.parameters;
	int length = parameters.length;
	for (int i = 0; i < length; ++i) {
	ParameterElement parameter = parameters[i];
	if (parameter is ParameterElementImpl) {
	_inferParameterCovariance(parameter, i, overriddenTypes);

	if (parameter.hasImplicitType) {
	parameter.type = _computeParameterType(parameter, i, overriddenTypes);
	if (element is PropertyAccessorElement) {
	_updateSyntheticVariableType(element);
	}
	}
	}
	}
	}

	/**
	* If the given [field] represents a non-synthetic instance field for
	* which no type was provided, infer the type of the field.
	*/
	void _inferField(FieldElement field) {
	if (field.isSynthetic \|\| field.isStatic) {
	return;
	}

	_FieldOverrideInferenceResult typeResult =
	_computeFieldOverrideType(field.getter);
	if (typeResult.isError) {
	if (field is FieldElementForLink_ClassField) {
	field.setInferenceError(new TopLevelInferenceErrorBuilder(
	kind: TopLevelInferenceErrorKind.overrideConflictFieldType));
	}
	return;
	}

	if (field.hasImplicitType) {
	DartType newType = typeResult.type;
	if (newType == null && field.initializer != null) {
	newType = field.initializer.returnType;
	}

	if (newType == null \|\| newType.isBottom \|\| newType.isDartCoreNull) {
	newType = typeProvider.dynamicType;
	}

	setFieldType(field, newType);
	}

	if (field.setter != null) {
	var parameter = field.setter.parameters[0] as ParameterElementImpl;
	parameter.inheritsCovariant = typeResult.isCovariant;
	}
	}

	/**
	* If a parameter is covariant, any parameters that override it are too.
	*/
	void _inferParameterCovariance(ParameterElementImpl parameter, int index,
	Iterable<FunctionType> overriddenTypes) {
	parameter.inheritsCovariant = overriddenTypes.any((f) {
	var param = _getCorrespondingParameter(parameter, index, f.parameters);
	return param != null && param.isCovariant;
	});
	}

	/**
	* Infer type information for all of the instance members in the given
	* interface [type].
	*/
	void _inferType(InterfaceType type) {
	if (type != null) {
	ClassElement element = type.element;
	if (element != null) {
	_inferClass(element);
	}
	}
	}

	/**
	* If the [element] has formal type parameters, then the [overriddenType]
	* must have it as well, but they are different. Replace type parameters
	* of the [overriddenType] with type formals of the [element].
	*
	* Return `null`, in case of type parameters inconsistency.
	*
	* The overridden element must have the same number of generic type
	* parameters as the target element, or none.
	*
	* If we do have generic type parameters on the element we're inferring,
	* we must express its parameter and return types in terms of its own
	* parameters. For example, given `m<T>(t)` overriding `m<S>(S s)` we
	* should infer this as `m<T>(T t)`.
	*/
	FunctionType _toOverriddenFunctionType(
	ExecutableElement element, FunctionType overriddenType) {
	List<DartType> typeFormals =
	TypeParameterTypeImpl.getTypes(element.type.typeFormals);
	if (overriddenType.typeFormals.isNotEmpty) {
	if (overriddenType.typeFormals.length != typeFormals.length) {
	return null;
	}
	overriddenType = overriddenType.instantiate(typeFormals);
	}
	return overriddenType;
	}

	/**
	* Return [overriddenTypes] that override [element].
	* Return the empty list, in case of type parameters inconsistency.
	*/
	List<FunctionType> _toOverriddenFunctionTypes(
	ExecutableElement element, List<FunctionType> overriddenTypes) {
	var result = <FunctionType>[];
	for (var overriddenType in overriddenTypes) {
	overriddenType = _toOverriddenFunctionType(element, overriddenType);
	if (overriddenType == null) {
	return const <FunctionType>[];
	}
	result.add(overriddenType);
	}
	return result;
	}

	/**
	* If the given [element] is a non-synthetic getter or setter, update its
	* synthetic variable's type to match the getter's return type, or if no
	* corresponding getter exists, use the setter's parameter type.
	*
	* In general, the type of the synthetic variable should not be used, because
	* getters and setters are independent methods. But this logic matches what
	* `TypeResolverVisitor.visitMethodDeclaration` would fill in there.
	*/
	void _updateSyntheticVariableType(PropertyAccessorElement element) {
	assert(!element.isSynthetic);
	PropertyAccessorElement getter = element;
	if (element.isSetter) {
	// See if we can find any getter.
	getter = element.correspondingGetter;
	}
	DartType newType;
	if (getter != null) {
	newType = getter.returnType;
	} else if (element.isSetter && element.parameters.isNotEmpty) {
	newType = element.parameters[0].type;
	}
	if (newType != null) {
	(element.variable as VariableElementImpl).type = newType;
	}
	}
	}

	/**
	* A visitor that will gather all of the variables referenced within a given
	* AST structure. The collection can be restricted to contain only those
	* variables that pass a specified filter.
	*/
	class VariableGatherer extends RecursiveAstVisitor {
	/**
	* The filter used to limit which variables are gathered, or `null` if no
	* filtering is to be performed.
	*/
	final VariableFilter filter;

	/**
	* The variables that were found.
	*/
	final Set<VariableElement> results = new HashSet<VariableElement>();

	/**
	* Initialize a newly created gatherer to gather all of the variables that
	* pass the given [filter] (or all variables if no filter is provided).
	*/
	VariableGatherer([this.filter = null]);

	@override
	void visitSimpleIdentifier(SimpleIdentifier node) {
	if (!node.inDeclarationContext()) {
	Element nonAccessor(Element element) {
	if (element is PropertyAccessorElement && element.isSynthetic) {
	return element.variable;
	}
	return element;
	}

	Element element = nonAccessor(node.staticElement);
	if (element is VariableElement && (filter == null \|\| filter(element))) {
	results.add(element);
	}
	}
	}
	}

	/**
	* A class of exception that is not used anywhere else.
	*/
	class _CycleException implements Exception {}

	/**
	* The result of field type inference.
	*/
	class _FieldOverrideInferenceResult {
	final bool isCovariant;
	final DartType type;
	final bool isError;

	_FieldOverrideInferenceResult(this.isCovariant, this.type, this.isError);
	}