// Copyright (c) 2018, 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.

// @dart = 2.9

import 'package:kernel/ast.dart' hide MapEntry;

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

import 'package:kernel/core_types.dart' show CoreTypes;

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

import 'package:kernel/src/legacy_erasure.dart';
import 'package:kernel/src/nnbd_top_merge.dart';
import 'package:kernel/src/norm.dart';
import 'package:kernel/src/types.dart' show Types;

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

import '../source/source_class_builder.dart';

import 'class_hierarchy_builder.dart';
import 'member_covariance.dart';

/// Class used for computing and inspecting the combined member signature for
/// a set of overridden/inherited members.
abstract class CombinedMemberSignatureBase<T> {
  ClassHierarchyBase get hierarchy;

  Name get name;

  /// The target class for the combined member signature.
  ///
  /// The [_memberTypes] are computed in terms of each member is inherited into
  /// [classBuilder].
  ///
  /// [classBuilder] is also used for determining whether the combined member
  /// signature should be computed using nnbd or legacy semantics.
  final SourceClassBuilder classBuilder;

  /// The list of members from which the combined member signature is computed.
  List<T> get members;

  /// If `true` the combined member signature is for the setter aspect of the
  /// members. Otherwise it is for the getter/method aspect of the members.
  final bool forSetter;

  /// The index within [members] for the member whose type is the most specific
  /// among [members]. If `null`, the combined member signature is not defined
  /// for [members] in [classBuilder].
  ///
  /// For the legacy computation, the type of this member defines the combined
  /// member signature.
  ///
  /// For the nnbd computation, this is one of the members whose type define
  /// the combined member signature, and the indices of the remaining members
  /// are stored in [_mutualSubtypes].
  int _canonicalMemberIndex;

  /// For the nnbd computation, this maps each distinct but most specific member
  /// type to the index of one of the [members] with that type.
  ///
  /// If there is only one most specific member type, this is `null`.
  Map<DartType, int> _mutualSubtypes;

  /// Cache for the types of [members] as inherited into [classBuilder].
  List<DartType> _memberTypes;

  List<Covariance> _memberCovariances;

  /// Cache for the this type of [classBuilder].
  DartType _thisType;

  /// If `true` the combined member signature type has been computed.
  ///
  /// Note that the combined member signature type might be undefined in which
  /// case [_combinedMemberSignatureType] is `null`.
  bool _isCombinedMemberSignatureTypeComputed = false;

  /// Cache the computed combined member signature type.
  ///
  /// If the combined member signature type is undefined this is set to `null`.
  DartType _combinedMemberSignatureType;

  /// The indices for the members whose type needed legacy erasure.
  ///
  /// This is fully computed when [combinedMemberSignatureType] has been
  /// computed.
  Set<int> _neededLegacyErasureIndices;

  bool _neededNnbdTopMerge = false;

  bool _containsNnbdTypes = false;

  bool _needsCovarianceMerging = false;

  bool _isCombinedMemberSignatureCovarianceComputed = false;

  Covariance _combinedMemberSignatureCovariance;

  /// Creates a [CombinedClassMemberSignature] whose canonical member is already
  /// defined.
  CombinedMemberSignatureBase.internal(
      this.classBuilder, this._canonicalMemberIndex, this.forSetter)
      : assert(forSetter != null);

  /// Creates a [CombinedClassMemberSignature] for [members] inherited into
  /// [classBuilder].
  ///
  /// If [forSetter] is `true`, contravariance of the setter types is used to
  /// compute the most specific member type. Otherwise covariance of the getter
  /// types or function types is used.
  CombinedMemberSignatureBase(this.classBuilder, {this.forSetter}) {
    assert(forSetter != null);
    int bestSoFarIndex;
    if (members.length == 1) {
      bestSoFarIndex = 0;
    } else {
      bool isNonNullableByDefault = classBuilder.library.isNonNullableByDefault;

      DartType bestTypeSoFar;
      for (int candidateIndex = members.length - 1;
          candidateIndex >= 0;
          candidateIndex--) {
        DartType candidateType = getMemberType(candidateIndex);
        if (bestSoFarIndex == null) {
          bestTypeSoFar = candidateType;
          bestSoFarIndex = candidateIndex;
        } else {
          if (_isMoreSpecific(candidateType, bestTypeSoFar, forSetter)) {
            if (isNonNullableByDefault &&
                _isMoreSpecific(bestTypeSoFar, candidateType, forSetter)) {
              if (_mutualSubtypes == null) {
                _mutualSubtypes = {
                  bestTypeSoFar: bestSoFarIndex,
                  candidateType: candidateIndex
                };
              } else {
                _mutualSubtypes[candidateType] = candidateIndex;
              }
            } else {
              _mutualSubtypes = null;
            }
            bestSoFarIndex = candidateIndex;
            bestTypeSoFar = candidateType;
          }
        }
      }
      if (_mutualSubtypes?.length == 1) {
        /// If all mutual subtypes have the same type, the type should not
        /// be normalized.
        _mutualSubtypes = null;
      }
      if (bestSoFarIndex != null) {
        for (int candidateIndex = 0;
            candidateIndex < members.length;
            candidateIndex++) {
          DartType candidateType = getMemberType(candidateIndex);
          if (!_isMoreSpecific(bestTypeSoFar, candidateType, forSetter)) {
            int favoredIndex = getOverlookedOverrideProblemChoice(classBuilder);
            bestSoFarIndex = favoredIndex;
            _mutualSubtypes = null;
            break;
          }
        }
      }
    }

    _canonicalMemberIndex = bestSoFarIndex;
  }

  /// The member within [members] type is the most specific among [members].
  /// If `null`, the combined member signature is not defined for [members] in
  /// [classBuilder].
  ///
  /// For the legacy computation, the type of this member defines the combined
  /// member signature.
  ///
  /// For the nnbd computation, this is one of the members whose type define
  /// the combined member signature, and the indices of the all members whose
  /// type define the combined member signature are in [mutualSubtypeIndices].
  T get canonicalMember =>
      _canonicalMemberIndex != null ? members[_canonicalMemberIndex] : null;

  /// The index within [members] for the member whose type is the most specific
  /// among [members]. If `null`, the combined member signature is not defined
  /// for [members] in [classBuilder].
  ///
  /// For the legacy computation, the type of this member defines the combined
  /// member signature.
  ///
  /// For the nnbd computation, this is one of the members whose type define
  /// the combined member signature, and the indices of the all members whose
  /// type define the combined member signature are in [mutualSubtypeIndices].
  int get canonicalMemberIndex => _canonicalMemberIndex;

  /// For the nnbd computation, the indices of the [members] with most specific
  /// member type.
  ///
  /// If there is only one most specific member type, this is `null`.
  Set<int> get mutualSubtypeIndices => _mutualSubtypes?.values?.toSet();

  Member _getMember(int index);

  CoreTypes get _coreTypes => hierarchy.coreTypes;

  Types get _types;

  /// Returns `true` if legacy erasure was needed to compute the combined
  /// member signature type.
  ///
  /// Legacy erasure is considered need of if the used of it resulted in a
  /// different type.
  bool get neededLegacyErasure {
    _ensureCombinedMemberSignatureType();
    return _neededLegacyErasureIndices?.contains(canonicalMemberIndex) ?? false;
  }

  /// Returns `true` if nnbd top merge and normalization was needed to compute
  /// the combined member signature type.
  bool get neededNnbdTopMerge {
    _ensureCombinedMemberSignatureType();
    return _neededNnbdTopMerge;
  }

  /// Returns `true` if the type of combined member signature has nnbd types.
  ///
  /// If the combined member signature for an opt-in class is computed from
  /// identical legacy types, that is, without the need for nnbd top merge, then
  /// the type will be copied over directly and a member created from the
  /// combined member signature will therefore be a legacy member, even though
  /// it is declared in an opt in class.
  ///
  /// To avoid reporting errors as if the member was an opt-in member, it is
  /// marked as nullable-by-default.
  ///
  /// For instance
  ///
  ///    // opt out:
  ///    mixin Mixin {
  ///      void method({int named}) {}
  ///    }
  ///    // opt in:
  ///    class Super {
  ///      void method({required covariant int named}) {}
  ///    }
  ///    class Class extends Super with Mixin {
  ///      // A forwarding stop for Mixin.method will be inserted here:
  ///      // void method({covariant int named}) -> Mixin.method
  ///    }
  ///    class SubClass extends Class {
  ///      // This is a valid override since `Class.method` should should
  ///      // not be considered as _not_ having a required named parameter -
  ///      // it is legacy and doesn't know about required named parameters.
  ///      void method({required int named}) {}
  ///    }
  ///
  bool get containsNnbdTypes {
    _ensureCombinedMemberSignatureType();
    return _containsNnbdTypes;
  }

  /// The this type of [classBuilder].
  DartType get thisType {
    return _thisType ??= _coreTypes.thisInterfaceType(
        classBuilder.cls, classBuilder.library.nonNullable);
  }

  /// Returns `true` if the canonical member is declared in [classBuilder].
  bool get isCanonicalMemberDeclared {
    return _canonicalMemberIndex != null &&
        _getMember(_canonicalMemberIndex).enclosingClass == classBuilder.cls;
  }

  /// Returns `true` if the canonical member is the 0th.
  // TODO(johnniwinther): This is currently used under the assumption that the
  // 0th member is either from the superclass or the one found if looked up
  // the class hierarchy. This is a very brittle assumption.
  bool get isCanonicalMemberFirst => _canonicalMemberIndex == 0;

  /// Returns type of the [index]th member in [members] as inherited in
  /// [classBuilder].
  DartType getMemberType(int index) {
    _memberTypes ??= new List<DartType>.filled(members.length, null);
    DartType candidateType = _memberTypes[index];
    if (candidateType == null) {
      Member target = _getMember(index);
      assert(target != null,
          "No member computed for index ${index} in ${members}");
      candidateType = _computeMemberType(thisType, target);
      if (!classBuilder.library.isNonNullableByDefault) {
        DartType legacyErasure;
        if (target == hierarchy.coreTypes.objectEquals) {
          // In legacy code we special case `Object.==` to infer `dynamic`
          // instead `Object!`.
          legacyErasure = new FunctionType([const DynamicType()],
              hierarchy.coreTypes.boolLegacyRawType, Nullability.legacy);
        } else {
          legacyErasure = rawLegacyErasure(candidateType);
        }
        if (legacyErasure != null) {
          _neededLegacyErasureIndices ??= {};
          _neededLegacyErasureIndices.add(index);
          candidateType = legacyErasure;
        }
      }
      _memberTypes[index] = candidateType;
    }
    return candidateType;
  }

  void _ensureCombinedMemberSignatureType() {
    if (!_isCombinedMemberSignatureTypeComputed) {
      _isCombinedMemberSignatureTypeComputed = true;
      if (_canonicalMemberIndex == null) {
        return null;
      }
      if (classBuilder.library.isNonNullableByDefault) {
        DartType canonicalMemberType =
            _combinedMemberSignatureType = getMemberType(_canonicalMemberIndex);
        _containsNnbdTypes =
            _getMember(_canonicalMemberIndex).isNonNullableByDefault;
        if (_mutualSubtypes != null) {
          _combinedMemberSignatureType =
              norm(_coreTypes, _combinedMemberSignatureType);
          for (int index in _mutualSubtypes.values) {
            if (_canonicalMemberIndex != index) {
              _combinedMemberSignatureType = nnbdTopMerge(
                  _coreTypes,
                  _combinedMemberSignatureType,
                  norm(_coreTypes, getMemberType(index)));
              assert(
                  _combinedMemberSignatureType != null,
                  "No combined member signature found for "
                  "${_mutualSubtypes.values.map((int i) => getMemberType(i))}");
            }
          }
          _neededNnbdTopMerge =
              canonicalMemberType != _combinedMemberSignatureType;
          _containsNnbdTypes = _neededNnbdTopMerge;
        }
      } else {
        _combinedMemberSignatureType = getMemberType(_canonicalMemberIndex);
      }
    }
  }

  /// Returns the type of the combined member signature, if defined.
  DartType get combinedMemberSignatureType {
    _ensureCombinedMemberSignatureType();
    return _combinedMemberSignatureType;
  }

  Covariance _getMemberCovariance(int index);

  void _ensureCombinedMemberSignatureCovariance() {
    if (!_isCombinedMemberSignatureCovarianceComputed) {
      _isCombinedMemberSignatureCovarianceComputed = true;
      if (canonicalMemberIndex == null) {
        return;
      }
      Covariance canonicalMemberCovariance =
          _combinedMemberSignatureCovariance =
              _getMemberCovariance(canonicalMemberIndex);
      if (members.length == 1) {
        return;
      }
      for (int index = 0; index < members.length; index++) {
        if (index != canonicalMemberIndex) {
          _combinedMemberSignatureCovariance =
              _combinedMemberSignatureCovariance
                  .merge(_getMemberCovariance(index));
        }
      }
      _needsCovarianceMerging =
          canonicalMemberCovariance != _combinedMemberSignatureCovariance;
    }
  }

  // Returns `true` if the covariance of [members] needs to be merged into
  // the combined member signature.
  bool get needsCovarianceMerging {
    if (members.length != 1) {
      _ensureCombinedMemberSignatureCovariance();
    }
    return _needsCovarianceMerging;
  }

  /// Returns [Covariance] for the combined member signature.
  Covariance get combinedMemberSignatureCovariance {
    _ensureCombinedMemberSignatureCovariance();
    return _combinedMemberSignatureCovariance;
  }

  /// Returns the type of the combined member signature, if defined, with
  /// all method type parameters substituted with [typeParameters].
  ///
  /// This is used for inferring types on a declared member from the type of the
  /// combined member signature.
  DartType getCombinedSignatureTypeInContext(
      List<TypeParameter> typeParameters) {
    DartType type = combinedMemberSignatureType;
    if (type == null) {
      return null;
    }
    int typeParameterCount = typeParameters.length;
    if (type is FunctionType) {
      List<TypeParameter> signatureTypeParameters = type.typeParameters;
      if (typeParameterCount != signatureTypeParameters.length) {
        return null;
      }
      if (typeParameterCount == 0) {
        return type;
      }
      List<DartType> types =
          new List<DartType>.filled(typeParameterCount, null);
      for (int i = 0; i < typeParameterCount; i++) {
        types[i] = new TypeParameterType.forAlphaRenaming(
            signatureTypeParameters[i], typeParameters[i]);
      }
      Substitution substitution =
          Substitution.fromPairs(signatureTypeParameters, types);
      for (int i = 0; i < typeParameterCount; i++) {
        DartType typeParameterBound = typeParameters[i].bound;
        DartType signatureTypeParameterBound =
            substitution.substituteType(signatureTypeParameters[i].bound);
        if (!_types
            .performNullabilityAwareMutualSubtypesCheck(
                typeParameterBound, signatureTypeParameterBound)
            .isSubtypeWhenUsingNullabilities()) {
          return null;
        }
      }
      return substitution.substituteType(type.withoutTypeParameters);
    } else if (typeParameterCount != 0) {
      return null;
    }
    return type;
  }

  /// Create a member signature with the [combinedMemberSignatureType] using the
  /// [canonicalMember] as member signature origin.
  Procedure createMemberFromSignature({bool copyLocation: true}) {
    if (canonicalMemberIndex == null) {
      return null;
    }
    Member member = _getMember(canonicalMemberIndex);
    Procedure combinedMemberSignature;
    if (member is Procedure) {
      switch (member.kind) {
        case ProcedureKind.Getter:
          combinedMemberSignature = _createGetterMemberSignature(
              member, combinedMemberSignatureType,
              copyLocation: copyLocation);
          break;
        case ProcedureKind.Setter:
          VariableDeclaration parameter =
              member.function.positionalParameters.first;
          combinedMemberSignature = _createSetterMemberSignature(
              member, combinedMemberSignatureType,
              isGenericCovariantImpl: parameter.isGenericCovariantImpl,
              isCovariant: parameter.isCovariant,
              parameterName: parameter.name,
              copyLocation: copyLocation);
          break;
        case ProcedureKind.Method:
        case ProcedureKind.Operator:
          combinedMemberSignature = _createMethodSignature(
              member, combinedMemberSignatureType,
              copyLocation: copyLocation);
          break;
        case ProcedureKind.Factory:
        default:
          throw new UnsupportedError(
              'Unexpected canonical member kind ${member.kind} for $member');
      }
    } else if (member is Field) {
      if (forSetter) {
        combinedMemberSignature = _createSetterMemberSignature(
            member, combinedMemberSignatureType,
            isGenericCovariantImpl: member.isGenericCovariantImpl,
            isCovariant: member.isCovariant,
            copyLocation: copyLocation);
      } else {
        combinedMemberSignature = _createGetterMemberSignature(
            member, combinedMemberSignatureType,
            copyLocation: copyLocation);
      }
    } else {
      throw new UnsupportedError(
          'Unexpected canonical member $member (${member.runtimeType})');
    }
    combinedMemberSignatureCovariance.applyCovariance(combinedMemberSignature);
    return combinedMemberSignature;
  }

  /// Creates a getter member signature for [member] with the given
  /// [type].
  Member _createGetterMemberSignature(Member member, DartType type,
      {bool copyLocation}) {
    assert(copyLocation != null);
    Class enclosingClass = classBuilder.cls;
    Reference reference;
    if (classBuilder.referencesFromIndexed != null) {
      reference =
          classBuilder.referencesFromIndexed.lookupGetterReference(member.name);
    }
    Uri fileUri;
    int startFileOffset;
    int fileOffset;
    if (copyLocation) {
      fileUri = member.fileUri;
      startFileOffset =
          member is Procedure ? member.startFileOffset : member.fileOffset;
      fileOffset = member.fileOffset;
    } else {
      fileUri = enclosingClass.fileUri;
      fileOffset = startFileOffset = enclosingClass.fileOffset;
    }
    return new Procedure(
      member.name,
      ProcedureKind.Getter,
      new FunctionNode(null, returnType: type),
      isAbstract: true,
      fileUri: fileUri,
      reference: reference,
      isSynthetic: true,
      stubKind: ProcedureStubKind.MemberSignature,
      stubTarget: member.memberSignatureOrigin ?? member,
    )
      ..startFileOffset = startFileOffset
      ..fileOffset = fileOffset
      ..isNonNullableByDefault = containsNnbdTypes
      ..parent = enclosingClass;
  }

  /// Creates a setter member signature for [member] with the given
  /// [type]. The flags of parameter is set according to [isCovariant] and
  /// [isGenericCovariantImpl] and the [parameterName] is used, if provided.
  Member _createSetterMemberSignature(Member member, DartType type,
      {bool isCovariant,
      bool isGenericCovariantImpl,
      String parameterName,
      bool copyLocation}) {
    assert(isCovariant != null);
    assert(isGenericCovariantImpl != null);
    assert(copyLocation != null);
    Class enclosingClass = classBuilder.cls;
    Reference reference;
    if (classBuilder.referencesFromIndexed != null) {
      reference =
          classBuilder.referencesFromIndexed.lookupSetterReference(member.name);
    }
    Uri fileUri;
    int startFileOffset;
    int fileOffset;
    if (copyLocation) {
      fileUri = member.fileUri;
      startFileOffset =
          member is Procedure ? member.startFileOffset : member.fileOffset;
      fileOffset = member.fileOffset;
    } else {
      fileUri = enclosingClass.fileUri;
      fileOffset = startFileOffset = enclosingClass.fileOffset;
    }
    return new Procedure(
      member.name,
      ProcedureKind.Setter,
      new FunctionNode(null,
          returnType: const VoidType(),
          positionalParameters: [
            new VariableDeclaration(parameterName ?? 'value',
                type: type, isCovariant: isCovariant)
              ..isGenericCovariantImpl = isGenericCovariantImpl
          ]),
      isAbstract: true,
      fileUri: fileUri,
      reference: reference,
      isSynthetic: true,
      stubKind: ProcedureStubKind.MemberSignature,
      stubTarget: member.memberSignatureOrigin ?? member,
    )
      ..startFileOffset = startFileOffset
      ..fileOffset = fileOffset
      ..isNonNullableByDefault = containsNnbdTypes
      ..parent = enclosingClass;
  }

  Member _createMethodSignature(Procedure procedure, FunctionType functionType,
      {bool copyLocation}) {
    assert(copyLocation != null);
    Class enclosingClass = classBuilder.cls;
    Reference reference = classBuilder.referencesFromIndexed
        ?.lookupGetterReference(procedure.name);
    Uri fileUri;
    int startFileOffset;
    int fileOffset;
    if (copyLocation) {
      fileUri = procedure.fileUri;
      startFileOffset = procedure.startFileOffset;
      fileOffset = procedure.fileOffset;
    } else {
      fileUri = enclosingClass.fileUri;
      fileOffset = startFileOffset = enclosingClass.fileOffset;
    }
    FunctionNode function = procedure.function;
    List<VariableDeclaration> positionalParameters = [];
    for (int i = 0; i < function.positionalParameters.length; i++) {
      VariableDeclaration parameter = function.positionalParameters[i];
      DartType parameterType = functionType.positionalParameters[i];
      positionalParameters.add(new VariableDeclaration(parameter.name,
          type: parameterType, isCovariant: parameter.isCovariant)
        ..isGenericCovariantImpl = parameter.isGenericCovariantImpl);
    }
    List<VariableDeclaration> namedParameters = [];
    int namedParameterCount = function.namedParameters.length;
    if (namedParameterCount == 1) {
      NamedType namedType = functionType.namedParameters.first;
      VariableDeclaration parameter = function.namedParameters.first;
      namedParameters.add(new VariableDeclaration(parameter.name,
          type: namedType.type,
          isRequired: namedType.isRequired,
          isCovariant: parameter.isCovariant)
        ..isGenericCovariantImpl = parameter.isGenericCovariantImpl);
    } else if (namedParameterCount > 1) {
      Map<String, NamedType> namedTypes = {};
      for (NamedType namedType in functionType.namedParameters) {
        namedTypes[namedType.name] = namedType;
      }
      for (int i = 0; i < namedParameterCount; i++) {
        VariableDeclaration parameter = function.namedParameters[i];
        NamedType namedParameterType = namedTypes[parameter.name];
        namedParameters.add(new VariableDeclaration(parameter.name,
            type: namedParameterType.type,
            isRequired: namedParameterType.isRequired,
            isCovariant: parameter.isCovariant)
          ..isGenericCovariantImpl = parameter.isGenericCovariantImpl);
      }
    }
    return new Procedure(
      procedure.name,
      procedure.kind,
      new FunctionNode(null,
          typeParameters: functionType.typeParameters,
          returnType: functionType.returnType,
          positionalParameters: positionalParameters,
          namedParameters: namedParameters,
          requiredParameterCount: function.requiredParameterCount),
      isAbstract: true,
      fileUri: fileUri,
      reference: reference,
      isSynthetic: true,
      stubKind: ProcedureStubKind.MemberSignature,
      stubTarget: procedure.memberSignatureOrigin ?? procedure,
    )
      ..startFileOffset = startFileOffset
      ..fileOffset = fileOffset
      ..isNonNullableByDefault = containsNnbdTypes
      ..parent = enclosingClass;
  }

  DartType _computeMemberType(DartType thisType, Member member) {
    DartType type;
    if (member is Procedure) {
      if (member.isGetter) {
        type = member.getterType;
      } else if (member.isSetter) {
        type = member.setterType;
      } else {
        type = member.function
            .computeFunctionType(classBuilder.cls.enclosingLibrary.nonNullable);
      }
    } else if (member is Field) {
      type = member.type;
    } else {
      unhandled("${member.runtimeType}", "$member", classBuilder.charOffset,
          classBuilder.fileUri);
    }
    if (member.enclosingClass.typeParameters.isEmpty) {
      return type;
    }
    InterfaceType instance = hierarchy.getTypeAsInstanceOf(
        thisType, member.enclosingClass, classBuilder.library.library);
    assert(
        instance != null,
        "No instance of $thisType as ${member.enclosingClass} found for "
        "$member.");
    return Substitution.fromInterfaceType(instance).substituteType(type);
  }

  bool _isMoreSpecific(DartType a, DartType b, bool forSetter) {
    if (forSetter) {
      return _types.isSubtypeOf(b, a, SubtypeCheckMode.withNullabilities);
    } else {
      return _types.isSubtypeOf(a, b, SubtypeCheckMode.withNullabilities);
    }
  }
}

/// Class used for computing and inspecting the combined member signature for
/// a set of overridden/inherited [ClassMember]s.
class CombinedClassMemberSignature
    extends CombinedMemberSignatureBase<ClassMember> {
  /// The class hierarchy builder used for building this class.
  @override
  final ClassHierarchyBuilder hierarchy;

  /// The list of the members inherited into or overridden in [classBuilder].
  @override
  final List<ClassMember> members;

  /// Creates a [CombinedClassMemberSignature] whose canonical member is already
  /// defined.
  CombinedClassMemberSignature.internal(this.hierarchy,
      SourceClassBuilder classBuilder, int canonicalMemberIndex, this.members,
      {bool forSetter})
      : super.internal(classBuilder, canonicalMemberIndex, forSetter);

  /// Creates a [CombinedClassMemberSignature] for [members] inherited into
  /// [classBuilder].
  ///
  /// If [forSetter] is `true`, contravariance of the setter types is used to
  /// compute the most specific member type. Otherwise covariance of the getter
  /// types or function types is used.
  CombinedClassMemberSignature(
      this.hierarchy, SourceClassBuilder classBuilder, this.members,
      {bool forSetter})
      : super(classBuilder, forSetter: forSetter);

  @override
  Name get name => members.first.name;

  @override
  Types get _types => hierarchy.types;

  @override
  Member _getMember(int index) {
    ClassMember candidate = members[index];
    Member target = candidate.getMember(hierarchy);
    assert(target != null,
        "No member computed for ${candidate} (${candidate.runtimeType})");
    return target;
  }

  @override
  Covariance _getMemberCovariance(int index) {
    ClassMember candidate = members[index];
    Covariance covariance = candidate.getCovariance(hierarchy);
    assert(covariance != null,
        "No covariance computed for ${candidate} (${candidate.runtimeType})");
    return covariance;
  }
}

/// Class used for computing and inspecting the combined member signature for
/// a set of overridden/inherited [Member]s.
class CombinedMemberSignatureBuilder
    extends CombinedMemberSignatureBase<Member> {
  @override
  final ClassHierarchyBase hierarchy;

  @override
  final Types _types;

  @override
  final List<Member> members;

  CombinedMemberSignatureBuilder(
      this.hierarchy, SourceClassBuilder classBuilder, this.members,
      {bool forSetter})
      : _types = new Types(hierarchy),
        super(classBuilder, forSetter: forSetter);

  @override
  Name get name => members.first.name;

  @override
  Member _getMember(int index) => members[index];

  @override
  Covariance _getMemberCovariance(int index) {
    _memberCovariances ??= new List<Covariance>.filled(members.length, null);
    Covariance covariance = _memberCovariances[index];
    if (covariance == null) {
      _memberCovariances[index] = covariance =
          new Covariance.fromMember(members[index], forSetter: forSetter);
    }
    return covariance;
  }
}