pkg/front_end/lib/src/fasta/type_inference/interface_resolver.dart

// 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 'package:kernel/ast.dart'
    show
        Arguments,
        Class,
        DartType,
        DynamicType,
        Expression,
        Field,
        FunctionNode,
        FunctionType,
        Member,
        Name,
        NamedExpression,
        Procedure,
        ProcedureKind,
        ReturnStatement,
        SuperMethodInvocation,
        SuperPropertyGet,
        SuperPropertySet,
        TypeParameter,
        TypeParameterType,
        VariableDeclaration,
        VariableGet,
        VoidType;

import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;

import 'package:kernel/transformations/flags.dart' show TransformerFlag;

import 'package:kernel/type_algebra.dart' show Substitution;

import 'package:kernel/type_environment.dart' show TypeEnvironment;

import '../../base/instrumentation.dart'
    show
        Instrumentation,
        InstrumentationValueForForwardingStub,
        InstrumentationValueLiteral;

import '../builder/builder.dart' show LibraryBuilder;

import '../kernel/kernel_library_builder.dart' show KernelLibraryBuilder;

import '../kernel/kernel_shadow_ast.dart'
    show
        ShadowClass,
        ShadowField,
        ShadowMember,
        ShadowProcedure,
        VariableDeclarationJudgment;

import '../messages.dart'
    show
        messageDeclaredMemberConflictsWithInheritedMember,
        messageDeclaredMemberConflictsWithInheritedMemberCause,
        messageInheritedMembersConflict,
        messageInheritedMembersConflictCause1,
        messageInheritedMembersConflictCause2,
        noLength,
        templateCantInferTypeDueToCircularity,
        templateCantInferTypeDueToInconsistentOverrides;

import '../names.dart' show indexSetName;

import '../problems.dart' show unhandled;

import 'type_inference_engine.dart'
    show
        FieldInitializerInferenceNode,
        IncludesTypeParametersCovariantly,
        InferenceNode,
        TypeInferenceEngine;

import 'type_inferrer.dart' show getNamedFormal;

import 'type_schema_environment.dart'
    show
        getNamedParameterType,
        getPositionalParameterType,
        substituteTypeParams;

/// Concrete class derived from [InferenceNode] to represent type inference of
/// getters, setters, and fields based on inheritance.
class AccessorInferenceNode extends InferenceNode {
  final InterfaceResolver _interfaceResolver;

  /// The method whose return type and/or parameter types should be inferred.
  final Procedure _declaredMethod;

  /// A list containing the methods overridden by [_declaredMethod], if any.
  final List<Member> _candidates;

  /// The index of the first method in [_candidates] overridden by
  /// [_declaredMethod].
  final int _start;

  /// The past-the-end index of the last method in [_candidates] overridden by
  /// [_declaredMethod].
  final int _end;

  final LibraryBuilder _library;

  final Uri _fileUri;

  AccessorInferenceNode(this._interfaceResolver, this._declaredMethod,
      this._candidates, this._start, this._end, this._library, this._fileUri);

  String get _name {
    if (_declaredMethod is! SyntheticAccessor && _declaredMethod.isSetter) {
      return _declaredMethod.function.positionalParameters[0].name;
    }
    return _declaredMethod.name.name;
  }

  int get _offset {
    if (_declaredMethod is! SyntheticAccessor && _declaredMethod.isSetter) {
      return _declaredMethod.function.positionalParameters[0].fileOffset;
    }
    return _declaredMethod.fileOffset;
  }

  @override
  void resolveInternal() {
    var declaredMethod = _declaredMethod;
    var kind = declaredMethod.kind;
    var overriddenTypes = _computeAccessorOverriddenTypes();
    if (isCircular) {
      _library.addProblem(
          templateCantInferTypeDueToCircularity.withArguments(_name),
          _offset,
          noLength,
          _fileUri);
    } else {
      var inferredType = _interfaceResolver.matchTypes(
          overriddenTypes, _library, _name, _fileUri, _offset);
      if (declaredMethod is SyntheticAccessor) {
        declaredMethod._field.type = inferredType;
      } else {
        if (kind == ProcedureKind.Getter) {
          declaredMethod.function.returnType = inferredType;
        } else {
          declaredMethod.function.positionalParameters[0].type = inferredType;
        }
      }
    }
  }

  /// Computes the types of the getters and setters overridden by
  /// [_declaredMethod], with appropriate type parameter substitutions.
  List<DartType> _computeAccessorOverriddenTypes() {
    var overriddenTypes = <DartType>[];
    for (int i = _start; i < _end; i++) {
      var candidate = _candidates[i];
      Procedure resolvedCandidate;
      if (candidate is ForwardingNode) {
        resolvedCandidate = candidate.resolve();
      } else {
        resolvedCandidate = candidate;
      }
      DartType overriddenType;
      if (resolvedCandidate is SyntheticAccessor) {
        var field = resolvedCandidate._field;
        ShadowMember.resolveInferenceNode(field);
        overriddenType = field.type;
      } else if (resolvedCandidate.function != null) {
        switch (resolvedCandidate.kind) {
          case ProcedureKind.Getter:
            overriddenType = resolvedCandidate.function.returnType;
            break;
          case ProcedureKind.Setter:
            overriddenType =
                resolvedCandidate.function.positionalParameters[0].type;
            break;
          default:
            // Illegal override (error will be reported elsewhere).  Just skip
            // this override.
            continue;
        }
      } else {
        // This can happen if there are errors.  Just skip this override.
        continue;
      }
      overriddenTypes.add(_interfaceResolver
          ._substitutionFor(resolvedCandidate, _declaredMethod.enclosingClass)
          .substituteType(overriddenType));
    }
    return overriddenTypes;
  }
}

/// A [ForwardingNode] represents a method, getter, or setter within a class's
/// interface that is either implemented in the class directly or inherited from
/// a superclass.
///
/// This class allows us to defer the determination of exactly which member is
/// inherited, as well as the propagation of covariance annotations, and
/// the creation of forwarding stubs, until type inference.
class ForwardingNode extends Procedure {
  /// The [InterfaceResolver] that created this [ForwardingNode].
  final InterfaceResolver _interfaceResolver;

  /// A list containing the directly implemented and directly inherited
  /// procedures of the class in question.
  ///
  /// Note that many [ForwardingNode]s share the same [_candidates] list;
  /// consult [_start] and [_end] to see which entries in this list are relevant
  /// to this [ForwardingNode].
  final List<Member> _candidates;

  /// Index of the first entry in [_candidates] relevant to this
  /// [ForwardingNode].
  final int _start;

  /// Index just beyond the last entry in [_candidates] relevant to this
  /// [ForwardingNode].
  final int _end;

  /// The member this node resolves to (if it has been computed); otherwise
  /// `null`.
  Member _resolution;

  /// The result of finalizing this node (if the node has been finalized);
  /// otherwise `null`.
  Member _finalResolution;

  /// If this forwarding node represents a member that needs type inference, the
  /// corresponding [InferenceNode]; otherwise `null`.
  InferenceNode _inferenceNode;

  ForwardingNode(this._interfaceResolver, this._inferenceNode, Class class_,
      Name name, ProcedureKind kind, this._candidates, this._start, this._end)
      : super(name, kind, null) {
    parent = class_;
  }

  /// Finishes handling of this node by propagating covariance and creating
  /// forwarding stubs if necessary.
  Procedure finalize() => _finalResolution ??= _finalize();

  /// Returns the declared or inherited member this node resolves to.
  ///
  /// Does not create forwarding stubs.
  Procedure resolve() => _resolution ??= _resolve();

  /// Tag the parameters of [interfaceMember] that need type checks
  ///
  /// Parameters can need type checks for calls coming from statically typed
  /// call sites, due to covariant generics and overrides with explicit
  /// `covariant` parameters.
  ///
  /// Tag parameters of [interfaceMember] that need such checks when the member
  /// occurs in [enclosingClass]'s interface.  If parameters need checks but
  /// they would not be checked in an inherited implementation, a forwarding
  /// stub is introduced as a place to put the checks.
  Procedure _computeCovarianceFixes(Procedure interfaceMember) {
    assert(_interfaceResolver.strongMode);
    var substitution =
        _interfaceResolver._substitutionFor(interfaceMember, enclosingClass);
    // We always create a forwarding stub when we've inherited a member from an
    // interface other than the first override candidate.  This is to work
    // around a bug in the Kernel type checker where it chooses the first
    // override candidate.
    //
    // TODO(kmillikin): Fix the Kernel type checker and stop creating these
    // extra stubs.
    var stub = interfaceMember.enclosingClass == enclosingClass ||
            interfaceMember == _resolvedCandidate(_start)
        ? interfaceMember
        : _createForwardingStub(substitution, interfaceMember);

    var interfaceFunction = interfaceMember.function;
    var interfacePositionalParameters = interfaceFunction.positionalParameters;
    var interfaceNamedParameters = interfaceFunction.namedParameters;
    var interfaceTypeParameters = interfaceFunction.typeParameters;

    void createStubIfNeeded() {
      if (stub != interfaceMember) return;
      if (interfaceMember.enclosingClass == enclosingClass) return;
      stub = _createForwardingStub(substitution, interfaceMember);
    }

    bool isImplCreated = false;
    void createImplIfNeeded() {
      if (isImplCreated) return;
      createStubIfNeeded();
      _createForwardingImplIfNeeded(stub.function);
      isImplCreated = true;
    }

    IncludesTypeParametersCovariantly needsCheckVisitor =
        enclosingClass.typeParameters.isEmpty
            ? null
            : ShadowClass.getClassInferenceInfo(enclosingClass)
                    .needsCheckVisitor ??=
                new IncludesTypeParametersCovariantly(
                    enclosingClass.typeParameters);
    bool needsCheck(DartType type) => needsCheckVisitor == null
        ? false
        : substitution.substituteType(type).accept(needsCheckVisitor);
    for (int i = 0; i < interfacePositionalParameters.length; i++) {
      var parameter = interfacePositionalParameters[i];
      var isGenericCovariantImpl =
          parameter.isGenericCovariantImpl || needsCheck(parameter.type);
      var isCovariant = parameter.isCovariant;
      var superParameter = parameter;
      for (int j = _start; j < _end; j++) {
        var otherMember = _finalizedCandidate(j);
        if (otherMember is ForwardingNode) continue;
        var otherPositionalParameters =
            otherMember.function.positionalParameters;
        if (otherPositionalParameters.length <= i) continue;
        var otherParameter = otherPositionalParameters[i];
        if (j == _start) superParameter = otherParameter;
        if (identical(otherMember, interfaceMember)) continue;
        if (otherParameter.isGenericCovariantImpl) {
          isGenericCovariantImpl = true;
        }
        if (otherParameter.isCovariant) {
          isCovariant = true;
        }
      }
      if (isGenericCovariantImpl) {
        if (!superParameter.isGenericCovariantImpl) {
          createImplIfNeeded();
        }
        if (!parameter.isGenericCovariantImpl) {
          createStubIfNeeded();
          stub.function.positionalParameters[i].isGenericCovariantImpl = true;
        }
      }
      if (isCovariant) {
        if (!superParameter.isCovariant) {
          createImplIfNeeded();
        }
        if (!parameter.isCovariant) {
          createStubIfNeeded();
          stub.function.positionalParameters[i].isCovariant = true;
        }
      }
    }
    for (int i = 0; i < interfaceNamedParameters.length; i++) {
      var parameter = interfaceNamedParameters[i];
      var isGenericCovariantImpl =
          parameter.isGenericCovariantImpl || needsCheck(parameter.type);
      var isCovariant = parameter.isCovariant;
      var superParameter = parameter;
      for (int j = _start; j < _end; j++) {
        var otherMember = _finalizedCandidate(j);
        if (otherMember is ForwardingNode) continue;
        var otherParameter =
            getNamedFormal(otherMember.function, parameter.name);
        if (otherParameter == null) continue;
        if (j == _start) superParameter = otherParameter;
        if (identical(otherMember, interfaceMember)) continue;
        if (otherParameter.isGenericCovariantImpl) {
          isGenericCovariantImpl = true;
        }
        if (otherParameter.isCovariant) {
          isCovariant = true;
        }
      }
      if (isGenericCovariantImpl) {
        if (!superParameter.isGenericCovariantImpl) {
          createImplIfNeeded();
        }
        if (!parameter.isGenericCovariantImpl) {
          createStubIfNeeded();
          stub.function.namedParameters[i].isGenericCovariantImpl = true;
        }
      }
      if (isCovariant) {
        if (!superParameter.isCovariant) {
          createImplIfNeeded();
        }
        if (!parameter.isCovariant) {
          createStubIfNeeded();
          stub.function.namedParameters[i].isCovariant = true;
        }
      }
    }
    for (int i = 0; i < interfaceTypeParameters.length; i++) {
      var typeParameter = interfaceTypeParameters[i];
      var isGenericCovariantImpl = typeParameter.isGenericCovariantImpl ||
          needsCheck(typeParameter.bound);
      var superTypeParameter = typeParameter;
      for (int j = _start; j < _end; j++) {
        var otherMember = _finalizedCandidate(j);
        if (otherMember is ForwardingNode) continue;
        var otherTypeParameters = otherMember.function.typeParameters;
        if (otherTypeParameters.length <= i) continue;
        var otherTypeParameter = otherTypeParameters[i];
        if (j == _start) superTypeParameter = otherTypeParameter;
        if (identical(otherMember, interfaceMember)) continue;
        if (otherTypeParameter.isGenericCovariantImpl) {
          isGenericCovariantImpl = true;
        }
      }
      if (isGenericCovariantImpl) {
        if (!superTypeParameter.isGenericCovariantImpl) {
          createImplIfNeeded();
        }
        if (!typeParameter.isGenericCovariantImpl) {
          createStubIfNeeded();
          stub.function.typeParameters[i].isGenericCovariantImpl = true;
        }
      }
    }
    return stub;
  }

  void _createForwardingImplIfNeeded(FunctionNode function) {
    if (function.body != null) {
      // There is already an implementation; nothing further needs to be done.
      return;
    }
    // Find the concrete implementation in the superclass; this is what we need
    // to forward to.  If we can't find one, then the method is fully abstract
    // and we don't need to do anything.
    var superclass = enclosingClass.superclass;
    if (superclass == null) return;
    Procedure procedure = function.parent;
    var superTarget = _interfaceResolver._typeEnvironment.hierarchy
        .getDispatchTarget(superclass, procedure.name,
            setter: kind == ProcedureKind.Setter);
    if (superTarget == null) return;
    if (superTarget is Procedure && superTarget.isForwardingStub) {
      superTarget = _getForwardingStubSuperTarget(superTarget);
    }
    procedure.isAbstract = false;
    if (!procedure.isForwardingStub) {
      // This procedure exists abstractly in the source code; we need to make it
      // concrete and give it a body that is a forwarding stub.  This situation
      // is called a "forwarding semi-stub".
      procedure.isForwardingStub = true;
      procedure.isForwardingSemiStub = true;
      _interfaceResolver._instrumentation?.record(
          procedure.fileUri,
          procedure.fileOffset,
          'forwardingStub',
          new InstrumentationValueLiteral('semi-stub'));
    }
    var positionalArguments = function.positionalParameters
        .map<Expression>((parameter) => new VariableGet(parameter))
        .toList();
    var namedArguments = function.namedParameters
        .map((parameter) =>
            new NamedExpression(parameter.name, new VariableGet(parameter)))
        .toList();
    var typeArguments = function.typeParameters
        .map<DartType>((typeParameter) => new TypeParameterType(typeParameter))
        .toList();
    var arguments = new Arguments(positionalArguments,
        types: typeArguments, named: namedArguments);
    Expression superCall;
    switch (kind) {
      case ProcedureKind.Method:
      case ProcedureKind.Operator:
        superCall = new SuperMethodInvocation(name, arguments, superTarget);
        break;
      case ProcedureKind.Getter:
        superCall = new SuperPropertyGet(
            name,
            superTarget is SyntheticAccessor
                ? superTarget._field
                : superTarget);
        break;
      case ProcedureKind.Setter:
        superCall = new SuperPropertySet(
            name,
            positionalArguments[0],
            superTarget is SyntheticAccessor
                ? superTarget._field
                : superTarget);
        break;
      default:
        unhandled('$kind', '_createForwardingImplIfNeeded', -1, null);
        break;
    }
    function.body = new ReturnStatement(superCall)..parent = function;
    procedure.transformerFlags |= TransformerFlag.superCalls;
    procedure.forwardingStubSuperTarget = superTarget;
  }

  /// Creates a forwarding stub based on the given [target].
  Procedure _createForwardingStub(Substitution substitution, Procedure target) {
    VariableDeclaration copyParameter(VariableDeclaration parameter) {
      return new VariableDeclaration(parameter.name,
          type: substitution.substituteType(parameter.type),
          isCovariant: parameter.isCovariant)
        ..isGenericCovariantImpl = parameter.isGenericCovariantImpl;
    }

    var targetTypeParameters = target.function.typeParameters;
    List<TypeParameter> typeParameters;
    if (targetTypeParameters.isNotEmpty) {
      typeParameters =
          new List<TypeParameter>.filled(targetTypeParameters.length, null);
      var additionalSubstitution = <TypeParameter, DartType>{};
      for (int i = 0; i < targetTypeParameters.length; i++) {
        var targetTypeParameter = targetTypeParameters[i];
        var typeParameter = new TypeParameter(targetTypeParameter.name, null)
          ..isGenericCovariantImpl = targetTypeParameter.isGenericCovariantImpl;
        typeParameters[i] = typeParameter;
        additionalSubstitution[targetTypeParameter] =
            new TypeParameterType(typeParameter);
      }
      substitution = Substitution.combine(
          substitution, Substitution.fromMap(additionalSubstitution));
      for (int i = 0; i < typeParameters.length; i++) {
        typeParameters[i].bound =
            substitution.substituteType(targetTypeParameters[i].bound);
      }
    }
    var positionalParameters =
        target.function.positionalParameters.map(copyParameter).toList();
    var namedParameters =
        target.function.namedParameters.map(copyParameter).toList();
    var function = new FunctionNode(null,
        positionalParameters: positionalParameters,
        namedParameters: namedParameters,
        typeParameters: typeParameters,
        requiredParameterCount: target.function.requiredParameterCount,
        returnType: substitution.substituteType(target.function.returnType));
    Member finalTarget;
    if (target is Procedure && target.isForwardingStub) {
      finalTarget = target.forwardingStubInterfaceTarget;
    } else if (target is SyntheticAccessor) {
      finalTarget = target._field;
    } else {
      finalTarget = target;
    }
    return new Procedure(name, kind, function,
        isAbstract: true,
        isForwardingStub: true,
        fileUri: enclosingClass.fileUri,
        forwardingStubInterfaceTarget: finalTarget)
      ..startFileOffset = enclosingClass.fileOffset
      ..fileOffset = enclosingClass.fileOffset
      ..parent = enclosingClass;
  }

  /// Creates a forwarding stub for this node if necessary, and propagates
  /// covariance information.
  Procedure _finalize() {
    return _interfaceResolver.strongMode
        ? _computeCovarianceFixes(resolve())
        : resolve();
  }

  /// Returns the [i]th element of [_candidates], finalizing it if necessary.
  Procedure _finalizedCandidate(int i) {
    Procedure candidate = _candidates[i];
    return candidate is ForwardingNode &&
            _interfaceResolver.isTypeInferencePrepared
        ? candidate.finalize()
        : candidate;
  }

  /// Determines which inherited member this node resolves to, and also performs
  /// type inference.
  Procedure _resolve() {
    Procedure inheritedMember = _candidates[_start];
    bool isDeclaredInThisClass =
        identical(inheritedMember.enclosingClass, enclosingClass);
    if (isDeclaredInThisClass) {
      if (_inferenceNode != null) {
        _inferenceNode.resolve();
        _inferenceNode = null;
      }
    } else {
      // If there are multiple inheritance candidates, the inherited member is
      // the member whose type is a subtype of all the others.  We can find it
      // by two passes over the list of members.  For the first pass, we step
      // through the candidates, updating inheritedMember each time we find a
      // member whose type is a subtype of the previous inheritedMember.  As we
      // do this, we also work out the necessary substitution for matching up
      // type parameters between this class and the corresponding superclass.
      //
      // Since the subtyping relation is reflexive, we will favor the most
      // recently visited candidate in the case where the types are the same.
      // We want to favor earlier candidates, so we visit the candidate list
      // backwards.
      inheritedMember = _resolvedCandidate(_end - 1);
      var inheritedMemberSubstitution =
          _interfaceResolver._substitutionFor(inheritedMember, enclosingClass);
      var inheritedMemberType = inheritedMember is ForwardingNode
          ? const DynamicType()
          : inheritedMemberSubstitution.substituteType(
              kind == ProcedureKind.Setter
                  ? inheritedMember.setterType
                  : inheritedMember.getterType);
      for (int i = _end - 2; i >= _start; i--) {
        var candidate = _resolvedCandidate(i);
        var substitution =
            _interfaceResolver._substitutionFor(candidate, enclosingClass);
        bool isBetter;
        DartType type;
        if (kind == ProcedureKind.Setter) {
          type = candidate is ForwardingNode
              ? const DynamicType()
              : substitution.substituteType(candidate.setterType);
          // Setters are contravariant in their setter type, so we have to
          // reverse the check.
          isBetter = _interfaceResolver._typeEnvironment
              .isSubtypeOf(inheritedMemberType, type);
        } else {
          type = candidate is ForwardingNode
              ? const DynamicType()
              : substitution.substituteType(candidate.getterType);
          isBetter = _interfaceResolver._typeEnvironment
              .isSubtypeOf(type, inheritedMemberType);
        }
        if (isBetter) {
          inheritedMember = candidate;
          inheritedMemberSubstitution = substitution;
          inheritedMemberType = type;
        }
      }
      // For the second pass, we verify that inheritedMember is a subtype of all
      // the other potentially inherited members.
      // TODO(paulberry): implement this.
    }
    return inheritedMember;
  }

  /// Returns the [i]th element of [_candidates], resolving it if necessary.
  Procedure _resolvedCandidate(int i) {
    Procedure candidate = _candidates[i];
    return candidate is ForwardingNode &&
            _interfaceResolver.isTypeInferencePrepared
        ? candidate.resolve()
        : candidate;
  }

  static void createForwardingImplIfNeededForTesting(
      ForwardingNode node, FunctionNode function) {
    node._createForwardingImplIfNeeded(function);
  }

  /// Public method allowing tests to access [_createForwardingStub].
  ///
  /// This method is static so that it can be easily eliminated by tree shaking
  /// when not needed.
  static Procedure createForwardingStubForTesting(
      ForwardingNode node, Substitution substitution, Procedure target) {
    return node._createForwardingStub(substitution, target);
  }

  /// For testing: get the list of candidates relevant to a given node.
  static List<Procedure> getCandidates(ForwardingNode node) {
    return node._candidates.sublist(node._start, node._end);
  }

  static Member _getForwardingStubSuperTarget(Procedure forwardingStub) {
    // TODO(paulberry): when dartbug.com/31562 is fixed, this should become
    // easier.
    ReturnStatement body = forwardingStub.function.body;
    var expression = body.expression;
    if (expression is SuperMethodInvocation) {
      return expression.interfaceTarget;
    } else if (expression is SuperPropertySet) {
      return expression.interfaceTarget;
    } else {
      return unhandled('${expression.runtimeType}',
          '_getForwardingStubSuperTarget', -1, null);
    }
  }
}

/// An [InterfaceResolver] keeps track of the information necessary to resolve
/// method calls, gets, and sets within a chunk of code being compiled, to
/// infer covariance annotations, and to create forwarwding stubs when necessary
/// to meet covariance requirements.
class InterfaceResolver {
  final TypeInferenceEngine _typeInferenceEngine;

  final TypeEnvironment _typeEnvironment;

  final Instrumentation _instrumentation;

  final bool strongMode;

  InterfaceResolver(this._typeInferenceEngine, this._typeEnvironment,
      this._instrumentation, this.strongMode);

  /// Indicates whether the "prepare" phase of type inference is complete.
  bool get isTypeInferencePrepared =>
      _typeInferenceEngine.isTypeInferencePrepared;

  /// Report an error if all types in [types] are not equal using `==`.
  ///
  /// Returns the type if there is at least one and they are all equal,
  /// otherwise the type `dynamic`.  [library], [name], [fileUri], and
  /// [charOffset] are used to report the error.
  DartType matchTypes(Iterable<DartType> types, LibraryBuilder library,
      String name, Uri fileUri, int charOffset) {
    DartType first;
    for (var type in types) {
      if (first == null) {
        first = type;
      } else if (first != type) {
        // Types don't match.  Report an error.
        library.addProblem(
            templateCantInferTypeDueToInconsistentOverrides.withArguments(name),
            charOffset,
            noLength,
            fileUri);
        return const DynamicType();
      }
    }
    // If there are no overridden types, infer `dynamic`.
    return first ?? const DynamicType();
  }

  /// Computes the types of the methods overridden by [method] in [class_].
  ///
  /// The types have the type parameters of [class_] substituted appropriately.
  ///
  /// [candidates] has the list of inherited interface methods with the same
  /// name as [method] as a sublist from [start] inclusive to [end] exclusive.
  List<FunctionType> _computeMethodOverriddenTypes(Class class_,
      Procedure method, List<Member> candidates, int start, int end) {
    var overriddenTypes = <FunctionType>[];
    var declaredTypeParameters = method.function.typeParameters;
    for (int i = start; i < end; ++i) {
      var candidate = candidates[i];
      if (candidate is SyntheticAccessor) {
        // This can happen if there are errors.  Just skip this override.
        continue;
      }
      var candidateFunction = candidate.function;
      if (candidateFunction == null) {
        // This can happen if there are errors.  Just skip this override.
        continue;
      }
      var substitution = _substitutionFor(candidate, class_);
      FunctionType overriddenType =
          substitution.substituteType(candidateFunction.functionType);
      var overriddenTypeParameters = overriddenType.typeParameters;
      if (overriddenTypeParameters.length != declaredTypeParameters.length) {
        // Generic arity mismatch.  Don't do any inference for this method.
        // TODO(paulberry): report an error.
        overriddenTypes.clear();
        break;
      } else if (overriddenTypeParameters.isNotEmpty) {
        var substitutionMap = <TypeParameter, DartType>{};
        for (int i = 0; i < declaredTypeParameters.length; ++i) {
          substitutionMap[overriddenTypeParameters[i]] =
              new TypeParameterType(declaredTypeParameters[i]);
        }
        overriddenType = substituteTypeParams(
            overriddenType, substitutionMap, declaredTypeParameters);
      }
      overriddenTypes.add(overriddenType);
    }
    return overriddenTypes;
  }

  void inferMethodType(LibraryBuilder library, Class class_, Procedure method,
      List<Member> candidates, int start, int end) {
    var overriddenTypes =
        _computeMethodOverriddenTypes(class_, method, candidates, start, end);
    if (ShadowProcedure.hasImplicitReturnType(method) &&
        method.name != indexSetName) {
      method.function.returnType = matchTypes(
          overriddenTypes.map((type) => type.returnType),
          library,
          method.name.name,
          class_.fileUri,
          method.fileOffset);
    }
    var positionalParameters = method.function.positionalParameters;
    for (int i = 0; i < positionalParameters.length; ++i) {
      if (VariableDeclarationJudgment.isImplicitlyTyped(
          positionalParameters[i])) {
        // Note that if the parameter is not present in the overridden method,
        // getPositionalParameterType treats it as dynamic.  This is consistent
        // with the behavior called for in the informal top level type inference
        // spec, which says:
        //
        //     If there is no corresponding parameter position in the overridden
        //     method to infer from and the signatures are compatible, it is
        //     treated as dynamic (e.g. overriding a one parameter method with a
        //     method that takes a second optional parameter).  Note: if there
        //     is no corresponding parameter position in the overriden method to
        //     infer from and the signatures are incompatible (e.g. overriding a
        //     one parameter method with a method that takes a second
        //     non-optional parameter), the inference result is not defined and
        //     tools are free to either emit an error, or to defer the error to
        //     override checking.
        positionalParameters[i].type = matchTypes(
            overriddenTypes.map((type) => getPositionalParameterType(type, i)),
            library,
            positionalParameters[i].name,
            class_.fileUri,
            positionalParameters[i].fileOffset);
      }
    }
    var namedParameters = method.function.namedParameters;
    for (int i = 0; i < namedParameters.length; i++) {
      if (VariableDeclarationJudgment.isImplicitlyTyped(namedParameters[i])) {
        var name = namedParameters[i].name;
        namedParameters[i].type = matchTypes(
            overriddenTypes.map((type) => getNamedParameterType(type, name)),
            library,
            namedParameters[i].name,
            class_.fileUri,
            namedParameters[i].fileOffset);
      }
    }
  }

  /// Populates [getters] and [setters] with the members of the given [class_]'s
  /// interface.
  ///
  /// [getters] will contain methods and getters, [setters] will contain
  /// setters.  Some members cannot be resolved immediately.  For instance,
  /// top-level type inference has not yet inferred field types based on
  /// initializers and so we cannot yet do override based resolution of getters
  /// and setters.  Members of the class's interface that need to be resolved
  /// later are represented by a [ForwardingNode] object.
  void createApiMembers(Class class_, List<Member> getters,
      List<Member> setters, LibraryBuilder library) {
    var candidates = ClassHierarchy.mergeSortedLists(
        getCandidates(class_, false), getCandidates(class_, true));
    // Now create getter and perhaps setter forwarding nodes for each unique
    // name.
    getters.length = candidates.length;
    setters.length = candidates.length;
    int getterIndex = 0;
    int setterIndex = 0;
    // To detect conflicts between instance members (possibly inherited ones)
    // and static members, use a map from names to lists of members.  There can
    // be more than one static member with a given name, e.g., if there is a
    // getter and a setter.  We will report both conflicts.
    Map<Name, List<Member>> staticMembers = {};
    for (var procedure in class_.procedures) {
      if (procedure.isStatic) {
        staticMembers.putIfAbsent(procedure.name, () => []).add(procedure);
      }
    }
    for (var field in class_.fields) {
      if (field.isStatic) {
        staticMembers.putIfAbsent(field.name, () => []).add(field);
      }
    }
    forEachApiMember(candidates, (int start, int end, Name name) {
      Procedure member = candidates[start];
      // We should not have a method, getter, or setter in our interface that
      // conflicts with a static method, getter, or setter declared in the
      // class.
      List<Member> conflicts = staticMembers[name];
      if (conflicts != null) {
        for (var conflict in conflicts) {
          library.addProblem(messageDeclaredMemberConflictsWithInheritedMember,
              conflict.fileOffset, noLength, conflict.fileUri,
              context: [
                messageDeclaredMemberConflictsWithInheritedMemberCause
                    .withLocation(member.fileUri, member.fileOffset, noLength)
              ]);
        }
        return;
      }
      ProcedureKind kind = _kindOf(member);
      if (kind != ProcedureKind.Getter && kind != ProcedureKind.Setter) {
        for (int i = start + 1; i < end; ++i) {
          if (_kindOf(candidates[i]) != kind) {
            // We've seen a getter or setter.  If it's a getter conflicting
            // with a method and both are declared in the same class, then that
            // has already been signaled as a duplicated definition.
            Procedure conflict = candidates[i];
            if (conflict.enclosingClass != member.enclosingClass) {
              if (member.enclosingClass == class_) {
                library.addProblem(
                    messageDeclaredMemberConflictsWithInheritedMember,
                    member.fileOffset,
                    noLength,
                    member.fileUri,
                    context: [
                      messageDeclaredMemberConflictsWithInheritedMemberCause
                          .withLocation(
                              conflict.fileUri, conflict.fileOffset, noLength)
                    ]);
              } else if (conflict.enclosingClass == class_) {
                library.addProblem(
                    messageDeclaredMemberConflictsWithInheritedMember,
                    conflict.fileOffset,
                    noLength,
                    conflict.fileUri,
                    context: [
                      messageDeclaredMemberConflictsWithInheritedMemberCause
                          .withLocation(
                              member.fileUri, member.fileOffset, noLength)
                    ]);
              } else {
                library.addProblem(messageInheritedMembersConflict,
                    class_.fileOffset, noLength, class_.fileUri,
                    context: [
                      messageInheritedMembersConflictCause1.withLocation(
                          member.fileUri, member.fileOffset, noLength),
                      messageInheritedMembersConflictCause2.withLocation(
                          conflict.fileUri, conflict.fileOffset, noLength)
                    ]);
              }
            } else {
              // If it's a setter conflicting with a method and both are
              // declared in the same class, it hasn't been signaled as a
              // duplicated definition so it's reported here.
              library.addProblem(
                  messageDeclaredMemberConflictsWithInheritedMember,
                  member.fileOffset,
                  noLength,
                  member.fileUri,
                  context: [
                    messageDeclaredMemberConflictsWithInheritedMemberCause
                        .withLocation(
                            conflict.fileUri, conflict.fileOffset, noLength)
                  ]);
            }
            return;
          }
        }
        if (strongMode &&
            member.enclosingClass == class_ &&
            _requiresTypeInference(member)) {
          inferMethodType(library, class_, member, candidates, start + 1, end);
        }
        var forwardingNode = new ForwardingNode(
            this, null, class_, name, kind, candidates, start, end);
        getters[getterIndex++] = forwardingNode.finalize();
        if (library is KernelLibraryBuilder &&
            forwardingNode.finalize() != forwardingNode.resolve()) {
          library.forwardersOrigins.add(forwardingNode.finalize());
          library.forwardersOrigins.add(forwardingNode.resolve());
        }
        return;
      }

      Procedure declaredGetter;
      int inheritedGetterStart = start;
      int getterEnd = start;
      if (kind == ProcedureKind.Getter) {
        if (member.enclosingClass == class_) {
          declaredGetter = member;
          ++inheritedGetterStart;
        }
        while (++getterEnd < end) {
          ProcedureKind currentKind = _kindOf(candidates[getterEnd]);
          if (currentKind == ProcedureKind.Setter) break;
          if (currentKind != ProcedureKind.Getter) {
            Procedure conflict = candidates[getterEnd];
            if (conflict.enclosingClass != member.enclosingClass) {
              if (member.enclosingClass == class_) {
                library.addProblem(
                    messageDeclaredMemberConflictsWithInheritedMember,
                    member.fileOffset,
                    noLength,
                    member.fileUri,
                    context: [
                      messageDeclaredMemberConflictsWithInheritedMemberCause
                          .withLocation(
                              conflict.fileUri, conflict.fileOffset, noLength)
                    ]);
              } else {
                library.addProblem(messageInheritedMembersConflict,
                    class_.fileOffset, noLength, class_.fileUri,
                    context: [
                      messageInheritedMembersConflictCause1.withLocation(
                          member.fileUri, member.fileOffset, noLength),
                      messageInheritedMembersConflictCause2.withLocation(
                          conflict.fileUri, conflict.fileOffset, noLength)
                    ]);
              }
            }
            return;
          }
        }
      }

      Procedure declaredSetter;
      int inheritedSetterStart = getterEnd;
      if (getterEnd < end) {
        member = candidates[getterEnd];
        if (member.enclosingClass == class_) {
          declaredSetter = member;
          ++inheritedSetterStart;
        }
      }

      InferenceNode getterInferenceNode;
      if (start < getterEnd) {
        if (declaredGetter != null) {
          getterInferenceNode = _createInferenceNode(
              class_,
              declaredGetter,
              candidates,
              inheritedGetterStart,
              getterEnd,
              inheritedSetterStart,
              end,
              library,
              class_.fileUri);
        }
        // Getters need to be resolved later, as part of type inference, so just
        // save the forwarding node for now.
        //
        // Choose a representative to use for error reporting, such as if a
        // class inherits this getter and tries to declare a method with the
        // same name.
        Member representative = candidates[start];
        getters[getterIndex++] = new ForwardingNode(this, getterInferenceNode,
            class_, name, ProcedureKind.Getter, candidates, start, getterEnd)
          ..fileUri = representative.fileUri
          ..fileOffset = representative.fileOffset
          ..fileEndOffset = representative.fileEndOffset;
      }
      if (getterEnd < end) {
        InferenceNode setterInferenceNode;
        if (declaredSetter != null) {
          setterInferenceNode = declaredSetter is SyntheticAccessor
              ? getterInferenceNode
              : _createInferenceNode(
                  class_,
                  declaredSetter,
                  candidates,
                  inheritedSetterStart,
                  end,
                  inheritedGetterStart,
                  getterEnd,
                  library,
                  class_.fileUri);
        }
        Member representative = candidates[getterEnd];
        var forwardingNode = new ForwardingNode(this, setterInferenceNode,
            class_, name, ProcedureKind.Setter, candidates, getterEnd, end)
          ..fileUri = representative.fileUri
          ..fileOffset = representative.fileOffset
          ..fileEndOffset = representative.fileEndOffset;
        // Setters need to be resolved later, as part of type inference, so just
        // save the forwarding node for now.
        setters[setterIndex++] = forwardingNode;
      }
    });
    getters.length = getterIndex;
    setters.length = setterIndex;
  }

  void finalizeCovariance(
      Class class_, List<Member> apiMembers, LibraryBuilder library) {
    for (int i = 0; i < apiMembers.length; i++) {
      var member = apiMembers[i];
      Member resolution;
      if (member is ForwardingNode) {
        apiMembers[i] = resolution = member.finalize();
        if (library is KernelLibraryBuilder &&
            member.finalize() != member.resolve()) {
          library.forwardersOrigins.add(member.finalize());
          library.forwardersOrigins.add(member.resolve());
        }
      } else {
        resolution = member;
      }
      if (resolution is Procedure &&
          resolution.isSyntheticForwarder &&
          identical(resolution.enclosingClass, class_)) {
        if (strongMode) class_.addMember(resolution);
        _instrumentation?.record(
            class_.location.file,
            class_.fileOffset,
            'forwardingStub',
            new InstrumentationValueForForwardingStub(resolution));
      }
    }
  }

  /// Gets a list of members implemented or potentially inherited by [class_],
  /// sorted so that members with the same name are contiguous.
  ///
  /// If [setters] is `true`, setters are reported; otherwise getters, methods,
  /// and operators are reported.
  List<Procedure> getCandidates(Class class_, bool setters) {
    // First create a list of candidates for inheritance based on the members
    // declared directly in the class.
    List<Procedure> candidates = _typeEnvironment.hierarchy
        .getDeclaredMembers(class_, setters: setters)
        .map((member) => makeCandidate(member, setters))
        .toList();
    // Merge in candidates from superclasses.
    if (class_.superclass != null) {
      candidates = _mergeCandidates(candidates, class_.superclass, setters);
    }
    for (var supertype in class_.implementedTypes) {
      candidates = _mergeCandidates(candidates, supertype.classNode, setters);
    }
    return candidates;
  }

  /// If instrumentation is enabled, records the covariance bits for the given
  /// [class_] to [_instrumentation].
  void recordInstrumentation(Class class_) {
    if (_instrumentation != null) {
      _recordInstrumentation(class_);
    }
  }

  /// Creates the appropriate [InferenceNode] for inferring [procedure] in the
  /// context of [class_].
  ///
  /// [candidates] a list containing the procedures overridden by [procedure],
  /// if any.  [start] is the index of the first such procedure, and [end] is
  /// the past-the-end index of the last such procedure.
  ///
  /// For getters and setters, [crossStart] and [crossEnd] are the start and end
  /// indices of the corresponding overridden setters/getters, respectively.
  InferenceNode _createInferenceNode(
      Class class_,
      Procedure procedure,
      List<Member> candidates,
      int start,
      int end,
      int crossStart,
      int crossEnd,
      LibraryBuilder library,
      Uri fileUri) {
    InferenceNode node;
    if (procedure.isAccessor && _requiresTypeInference(procedure)) {
      if (strongMode && start < end) {
        node = new AccessorInferenceNode(
            this, procedure, candidates, start, end, library, fileUri);
      } else if (strongMode && crossStart < crossEnd) {
        node = new AccessorInferenceNode(this, procedure, candidates,
            crossStart, crossEnd, library, fileUri);
      } else if (procedure is SyntheticAccessor &&
          procedure._field.initializer != null) {
        node = new FieldInitializerInferenceNode(
            _typeInferenceEngine, procedure._field, library);
      }

      if (node != null && procedure is SyntheticAccessor) {
        ShadowField.setInferenceNode(procedure._field, node);
      }
    }
    return node;
  }

  /// Retrieves a list of the interface members of the given [class_].
  ///
  /// If [setters] is true, setters are retrieved; otherwise getters and methods
  /// are retrieved.
  List<Member> _getInterfaceMembers(Class class_, bool setters) {
    // If class_ is being compiled from source, retrieve its forwarding nodes.
    var inferenceInfo = ShadowClass.getClassInferenceInfo(class_);
    if (inferenceInfo != null) {
      return setters ? inferenceInfo.setters : inferenceInfo.gettersAndMethods;
    } else {
      return _typeEnvironment.hierarchy
          .getInterfaceMembers(class_, setters: setters);
    }
  }

  /// Merges together the list of interface inheritance candidates in
  /// [candidates] with interface inheritance candidates from superclass
  /// [class_].
  ///
  /// Any candidates from [class_] are converted into interface inheritance
  /// candidates using [_makeCandidate].
  List<Procedure> _mergeCandidates(
      List<Procedure> candidates, Class class_, bool setters) {
    List<Member> members = _getInterfaceMembers(class_, setters);
    if (candidates.isEmpty) {
      return members.map((member) => makeCandidate(member, setters)).toList();
    }
    if (members.isEmpty) return candidates;
    List<Procedure> result = <Procedure>[]..length =
        candidates.length + members.length;
    int storeIndex = 0;
    int i = 0, j = 0;
    while (i < candidates.length && j < members.length) {
      Procedure candidate = candidates[i];
      Member member = members[j];
      int compare = ClassHierarchy.compareMembers(candidate, member);
      if (compare <= 0) {
        result[storeIndex++] = candidate;
        ++i;
        // If the same member occurs in both lists, skip the duplicate.
        if (identical(candidate, member)) ++j;
      } else {
        result[storeIndex++] = makeCandidate(member, setters);
        ++j;
      }
    }
    while (i < candidates.length) {
      result[storeIndex++] = candidates[i++];
    }
    while (j < members.length) {
      result[storeIndex++] = makeCandidate(members[j++], setters);
    }
    result.length = storeIndex;
    return result;
  }

  /// Records the covariance bits for the given [class_] to [_instrumentation].
  ///
  /// Caller is responsible for checking whether [_instrumentation] is `null`.
  void _recordInstrumentation(Class class_) {
    var uri = class_.fileUri;
    void recordCovariance(int fileOffset, bool isExplicitlyCovariant,
        bool isGenericCovariantImpl) {
      var covariance = <String>[];
      if (isExplicitlyCovariant) covariance.add('explicit');
      if (!isExplicitlyCovariant && isGenericCovariantImpl) {
        covariance.add('genericImpl');
      }
      if (covariance.isNotEmpty) {
        _instrumentation.record(uri, fileOffset, 'covariance',
            new InstrumentationValueLiteral(covariance.join(', ')));
      }
    }

    for (var procedure in class_.procedures) {
      if (procedure.isStatic) continue;
      // Forwarding stubs are annotated separately
      if (procedure.isSyntheticForwarder) {
        continue;
      }
      void recordFormalAnnotations(VariableDeclaration formal) {
        recordCovariance(formal.fileOffset, formal.isCovariant,
            formal.isGenericCovariantImpl);
      }

      void recordTypeParameterAnnotations(TypeParameter typeParameter) {
        recordCovariance(typeParameter.fileOffset, false,
            typeParameter.isGenericCovariantImpl);
      }

      procedure.function.positionalParameters.forEach(recordFormalAnnotations);
      procedure.function.namedParameters.forEach(recordFormalAnnotations);
      procedure.function.typeParameters.forEach(recordTypeParameterAnnotations);
    }
    for (var field in class_.fields) {
      if (field.isStatic) continue;
      recordCovariance(
          field.fileOffset, field.isCovariant, field.isGenericCovariantImpl);
    }
  }

  /// Determines the appropriate substitution to translate type parameters
  /// mentioned in the given [candidate] to type parameters on [class_].
  Substitution _substitutionFor(Procedure candidate, Class class_) {
    return Substitution.fromInterfaceType(_typeEnvironment.hierarchy
        .getTypeAsInstanceOf(class_.thisType, candidate.enclosingClass));
  }

  /// Executes [callback] once for each uniquely named member of [candidates].
  ///
  /// The [start] and [end] values passed to [callback] are the start and
  /// past-the-end indices into [candidates] of a group of members having the
  /// same name.  The [name] value passed to [callback] is the common name.
  static void forEachApiMember(
      List<Member> candidates, void callback(int start, int end, Name name)) {
    int i = 0;
    while (i < candidates.length) {
      var name = candidates[i].name;
      int j = i + 1;
      while (j < candidates.length && candidates[j].name == name) {
        j++;
      }
      callback(i, j, name);
      i = j;
    }
  }

  /// Transforms [member] into a candidate for interface inheritance.
  ///
  /// Fields are transformed into getters and setters; methods are passed
  /// through unchanged.
  static Procedure makeCandidate(Member member, bool setter) {
    if (member is Procedure) return member;
    if (member is Field) {
      // TODO(paulberry): don't set the type or covariance annotations here,
      // since they might not have been inferred yet.  Instead, ensure that this
      // information is propagated to the getter/setter during type inference.
      var type = member.type;
      var isGenericCovariantImpl = member.isGenericCovariantImpl;
      var isCovariant = member.isCovariant;
      if (setter) {
        var valueParam = new VariableDeclaration('_', type: type)
          ..isGenericCovariantImpl = isGenericCovariantImpl
          ..isCovariant = isCovariant;
        var function = new FunctionNode(null,
            positionalParameters: [valueParam], returnType: const VoidType());
        return new SyntheticAccessor(
            member.name, ProcedureKind.Setter, function, member)
          ..parent = member.enclosingClass;
      } else {
        var function = new FunctionNode(null, returnType: type);
        return new SyntheticAccessor(
            member.name, ProcedureKind.Getter, function, member)
          ..parent = member.enclosingClass;
      }
    }
    return unhandled('${member.runtimeType}', 'makeCandidate', -1, null);
  }

  static ProcedureKind _kindOf(Procedure procedure) => procedure.kind;

  /// Determines whether the given [procedure] will require type inference.
  static bool _requiresTypeInference(Procedure procedure) {
    if (procedure is SyntheticAccessor) {
      return ShadowField.isImplicitlyTyped(procedure._field);
    }
    if (procedure.kind != ProcedureKind.Setter &&
        ShadowProcedure.hasImplicitReturnType(procedure)) {
      // Inference of the return type of `[]=` is handled separately by
      // KernelProcedureBuilder.build, since there are no dependencies.
      if (procedure.kind != ProcedureKind.Operator ||
          procedure.name.name != '[]=') {
        return true;
      }
    }
    var function = procedure.function;
    for (var parameter in function.positionalParameters) {
      if (VariableDeclarationJudgment.isImplicitlyTyped(parameter)) return true;
    }
    for (var parameter in function.namedParameters) {
      if (VariableDeclarationJudgment.isImplicitlyTyped(parameter)) return true;
    }
    return false;
  }
}

/// A [SyntheticAccessor] represents the getter or setter implied by a field.
class SyntheticAccessor extends Procedure {
  /// The field associated with the synthetic accessor.
  final Field _field;

  SyntheticAccessor(
      Name name, ProcedureKind kind, FunctionNode function, this._field)
      : super(
            name,
            kind,
            kind == ProcedureKind.Setter
                ? new SyntheticAccessorFunctionNode.setter(_field)
                : new SyntheticAccessorFunctionNode.getter(_field),
            fileUri: _field.fileUri) {
    fileOffset = _field.fileOffset;
  }

  @override
  DartType get getterType => _field.type;

  static getField(SyntheticAccessor accessor) => accessor._field;
}

/// A [SyntheticAccessorFunctionNode] represents the [FunctionNode] part of the
/// getter or setter implied by a field.
///
/// For getters, [returnType] maps to the underlying field's type, so that if
/// type inference fills in the type of the field, the change will automatically
/// be reflected in the synthetic getter.
class SyntheticAccessorFunctionNode extends FunctionNode {
  final Field _field;

  SyntheticAccessorFunctionNode.getter(this._field)
      : super(new ReturnStatement());

  SyntheticAccessorFunctionNode.setter(this._field)
      : super(new ReturnStatement(),
            positionalParameters: [new SyntheticSetterParameter(_field)]);

  @override
  DartType get returnType =>
      positionalParameters.isEmpty ? _field.type : const VoidType();
}

/// A [SyntheticSetterParameter] represents the "value" parameter of the setter
/// implied by a field.
///
/// The getters [isCovariant], [isGenericCovariantImpl],
/// [isGenericCovariantInterface], and [type] map to the underlying field's
/// properties, so that if these properties are modified on the field, the
/// change will automatically be reflected in the synthetic setter.  Similarly,
/// the setters [isCovariant], [isGenericCovariantImpl], and
/// [isGenericCovariantInterface] update the corresponding properties on the
/// field, so that covariance propagation logic can act uniformly on [Procedure]
/// objects without having to have special case handling for fields.
class SyntheticSetterParameter extends VariableDeclaration {
  final Field _field;

  SyntheticSetterParameter(this._field)
      : super('_', isCovariant: _field.isCovariant);

  @override
  bool get isCovariant => _field.isCovariant;

  @override
  void set isCovariant(bool value) {
    _field.isCovariant = value;
  }

  @override
  bool get isGenericCovariantImpl => _field.isGenericCovariantImpl;

  @override
  void set isGenericCovariantImpl(bool value) {
    _field.isGenericCovariantImpl = value;
  }

  @override
  DartType get type => _field.type;
}