// Copyright (c) 2016, 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.

library fasta.field_builder;

import 'package:_fe_analyzer_shared/src/scanner/scanner.dart' show Token;

import 'package:kernel/ast.dart';
import 'package:kernel/class_hierarchy.dart';
import 'package:kernel/core_types.dart';
import 'package:kernel/src/legacy_erasure.dart';
import 'package:kernel/type_algebra.dart';

import '../constant_context.dart' show ConstantContext;

import '../fasta_codes.dart' show messageInternalProblemAlreadyInitialized;

import '../kernel/body_builder.dart' show BodyBuilder;
import '../kernel/hierarchy/class_member.dart';
import '../kernel/hierarchy/members_builder.dart';
import '../kernel/implicit_field_type.dart';
import '../kernel/kernel_helper.dart';
import '../kernel/late_lowering.dart' as late_lowering;
import '../kernel/member_covariance.dart';

import '../modifier.dart' show covariantMask, hasInitializerMask, lateMask;

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

import '../scope.dart' show Scope;

import '../source/name_scheme.dart';
import '../source/source_extension_builder.dart';
import '../source/source_library_builder.dart' show SourceLibraryBuilder;
import '../source/source_loader.dart' show SourceLoader;

import '../type_inference/type_inference_engine.dart'
    show IncludesTypeParametersNonCovariantly;

import '../util/helpers.dart' show DelayedActionPerformer;

import 'class_builder.dart';
import 'member_builder.dart';
import 'metadata_builder.dart';
import 'type_builder.dart';

abstract class FieldBuilder implements MemberBuilder {
  Field get field;

  List<MetadataBuilder>? get metadata;

  TypeBuilder? get type;

  bool get isCovariantByDeclaration;

  bool get isLate;

  bool get hasInitializer;

  /// Whether the body of this field has been built.
  ///
  /// Constant fields have their initializer built in the outline so we avoid
  /// building them twice as part of the non-outline build.
  bool get hasBodyBeenBuilt;

  /// Builds the body of this field using [initializer] as the initializer
  /// expression.
  void buildBody(CoreTypes coreTypes, Expression? initializer);

  /// Builds the field initializers for each field used to encode this field
  /// using the [fileOffset] for the created nodes and [value] as the initial
  /// field value.
  List<Initializer> buildInitializer(int fileOffset, Expression value,
      {required bool isSynthetic});

  bool get isEligibleForInference;

  DartType get builtType;

  DartType inferType();

  DartType get fieldType;
}

class SourceFieldBuilder extends SourceMemberBuilderImpl
    implements FieldBuilder {
  @override
  final String name;

  @override
  final int modifiers;

  late FieldEncoding _fieldEncoding;

  @override
  final List<MetadataBuilder>? metadata;

  @override
  final TypeBuilder? type;

  Token? _constInitializerToken;

  bool hadTypesInferred = false;

  @override
  bool hasBodyBeenBuilt = false;

  // TODO(johnniwinther): [parent] is not trust-worthy for determining
  //  properties since it is changed after the creation of the builder. For now
  //  we require it has an argument here. A follow-up should clean up the
  //  misuse of parent.
  @override
  final bool isTopLevel;

  SourceFieldBuilder(
      this.metadata,
      this.type,
      this.name,
      this.modifiers,
      this.isTopLevel,
      SourceLibraryBuilder libraryBuilder,
      int charOffset,
      int charEndOffset,
      NameScheme fieldNameScheme,
      {Reference? fieldReference,
      Reference? fieldGetterReference,
      Reference? fieldSetterReference,
      Reference? lateIsSetFieldReference,
      Reference? lateIsSetGetterReference,
      Reference? lateIsSetSetterReference,
      Reference? lateGetterReference,
      Reference? lateSetterReference,
      Token? constInitializerToken})
      : _constInitializerToken = constInitializerToken,
        super(libraryBuilder, charOffset) {
    bool isInstanceMember = fieldNameScheme.isInstanceMember;

    Uri fileUri = libraryBuilder.fileUri;
    // If in mixed mode, late lowerings cannot use `null` as a sentinel on
    // non-nullable fields since they can be assigned from legacy code.
    late_lowering.IsSetStrategy isSetStrategy =
        late_lowering.computeIsSetStrategy(libraryBuilder);

    if (isAbstract || isExternal) {
      assert(fieldReference == null);
      assert(lateIsSetFieldReference == null);
      assert(lateIsSetGetterReference == null);
      assert(lateIsSetSetterReference == null);
      assert(lateGetterReference == null);
      assert(lateSetterReference == null);
      _fieldEncoding = new AbstractOrExternalFieldEncoding(
          this,
          name,
          fieldNameScheme,
          fileUri,
          charOffset,
          charEndOffset,
          fieldGetterReference,
          fieldSetterReference,
          isAbstract: isAbstract,
          isExternal: isExternal,
          isFinal: isFinal,
          isCovariantByDeclaration: isCovariantByDeclaration,
          isNonNullableByDefault: library.isNonNullableByDefault);
    } else if (isLate &&
        libraryBuilder.loader.target.backendTarget.isLateFieldLoweringEnabled(
            hasInitializer: hasInitializer,
            isFinal: isFinal,
            isStatic: !isInstanceMember)) {
      if (hasInitializer) {
        if (isFinal) {
          _fieldEncoding = new LateFinalFieldWithInitializerEncoding(
              name,
              fieldNameScheme,
              fileUri,
              charOffset,
              charEndOffset,
              fieldReference,
              fieldGetterReference,
              fieldSetterReference,
              lateIsSetFieldReference,
              lateIsSetGetterReference,
              lateIsSetSetterReference,
              lateGetterReference,
              lateSetterReference,
              isCovariantByDeclaration,
              isSetStrategy);
        } else {
          _fieldEncoding = new LateFieldWithInitializerEncoding(
              name,
              fieldNameScheme,
              fileUri,
              charOffset,
              charEndOffset,
              fieldReference,
              fieldGetterReference,
              fieldSetterReference,
              lateIsSetFieldReference,
              lateIsSetGetterReference,
              lateIsSetSetterReference,
              lateGetterReference,
              lateSetterReference,
              isCovariantByDeclaration,
              isSetStrategy);
        }
      } else {
        if (isFinal) {
          _fieldEncoding = new LateFinalFieldWithoutInitializerEncoding(
              name,
              fieldNameScheme,
              fileUri,
              charOffset,
              charEndOffset,
              fieldReference,
              fieldGetterReference,
              fieldSetterReference,
              lateIsSetFieldReference,
              lateIsSetGetterReference,
              lateIsSetSetterReference,
              lateGetterReference,
              lateSetterReference,
              isCovariantByDeclaration,
              isSetStrategy);
        } else {
          _fieldEncoding = new LateFieldWithoutInitializerEncoding(
              name,
              fieldNameScheme,
              fileUri,
              charOffset,
              charEndOffset,
              fieldReference,
              fieldGetterReference,
              fieldSetterReference,
              lateIsSetFieldReference,
              lateIsSetGetterReference,
              lateIsSetSetterReference,
              lateGetterReference,
              lateSetterReference,
              isCovariantByDeclaration,
              isSetStrategy);
        }
      }
    } else if (libraryBuilder.isNonNullableByDefault &&
        libraryBuilder.loader.target.backendTarget.useStaticFieldLowering &&
        !isInstanceMember &&
        !isConst &&
        hasInitializer) {
      if (isFinal) {
        _fieldEncoding = new LateFinalFieldWithInitializerEncoding(
            name,
            fieldNameScheme,
            fileUri,
            charOffset,
            charEndOffset,
            fieldReference,
            fieldGetterReference,
            fieldSetterReference,
            lateIsSetFieldReference,
            lateIsSetGetterReference,
            lateIsSetSetterReference,
            lateGetterReference,
            lateSetterReference,
            isCovariantByDeclaration,
            isSetStrategy);
      } else {
        _fieldEncoding = new LateFieldWithInitializerEncoding(
            name,
            fieldNameScheme,
            fileUri,
            charOffset,
            charEndOffset,
            fieldReference,
            fieldGetterReference,
            fieldSetterReference,
            lateIsSetFieldReference,
            lateIsSetGetterReference,
            lateIsSetSetterReference,
            lateGetterReference,
            lateSetterReference,
            isCovariantByDeclaration,
            isSetStrategy);
      }
    } else {
      assert(lateIsSetFieldReference == null);
      assert(lateIsSetGetterReference == null);
      assert(lateIsSetSetterReference == null);
      assert(lateGetterReference == null);
      assert(lateSetterReference == null);
      _fieldEncoding = new RegularFieldEncoding(
          name, fieldNameScheme, fileUri, charOffset, charEndOffset,
          isFinal: isFinal,
          isConst: isConst,
          isLate: isLate,
          hasInitializer: hasInitializer,
          isNonNullableByDefault: library.isNonNullableByDefault,
          fieldReference: fieldReference,
          getterReference: fieldGetterReference,
          setterReference: fieldSetterReference);
    }
  }

  bool get isLateLowered => _fieldEncoding.isLateLowering;

  bool _typeEnsured = false;
  Set<ClassMember>? _overrideDependencies;

  void registerOverrideDependency(Set<ClassMember> overriddenMembers) {
    assert(
        overriddenMembers.every((overriddenMember) =>
            overriddenMember.classBuilder != classBuilder),
        "Unexpected override dependencies for $this: $overriddenMembers");
    _overrideDependencies ??= {};
    _overrideDependencies!.addAll(overriddenMembers);
  }

  void _ensureType(ClassMembersBuilder membersBuilder) {
    if (_typeEnsured) return;
    if (_overrideDependencies != null) {
      membersBuilder.inferFieldType(this, _overrideDependencies!);
      _overrideDependencies = null;
    } else {
      inferType();
    }
    _typeEnsured = true;
  }

  @override
  SourceLibraryBuilder get library => super.library as SourceLibraryBuilder;

  @override
  Member get member => _fieldEncoding.field;

  @override
  String get debugName => "FieldBuilder";

  @override
  bool get isField => true;

  @override
  bool get isLate => (modifiers & lateMask) != 0;

  @override
  bool get isCovariantByDeclaration => (modifiers & covariantMask) != 0;

  @override
  bool get hasInitializer => (modifiers & hasInitializerMask) != 0;

  @override
  void buildBody(CoreTypes coreTypes, Expression? initializer) {
    assert(!hasBodyBeenBuilt);
    hasBodyBeenBuilt = true;
    if (!hasInitializer &&
        initializer != null &&
        initializer is! NullLiteral &&
        !isConst &&
        !isFinal) {
      internalProblem(
          messageInternalProblemAlreadyInitialized, charOffset, fileUri);
    }
    _fieldEncoding.createBodies(coreTypes, initializer);
  }

  @override
  List<Initializer> buildInitializer(int fileOffset, Expression value,
      {required bool isSynthetic}) {
    return _fieldEncoding.createInitializer(fileOffset, value,
        isSynthetic: isSynthetic);
  }

  @override
  bool get isEligibleForInference {
    return type == null && (hasInitializer || isClassInstanceMember);
  }

  @override
  bool get isAssignable {
    if (isConst) return false;
    if (isFinal) {
      if (isLate) {
        return !hasInitializer;
      }
      return false;
    }
    return true;
  }

  @override
  Field get field => _fieldEncoding.field;

  @override
  Member get readTarget => _fieldEncoding.readTarget;

  @override
  Member? get writeTarget {
    return isAssignable ? _fieldEncoding.writeTarget : null;
  }

  @override
  Member get invokeTarget => readTarget;

  @override
  Iterable<Member> get exportedMembers => _fieldEncoding.exportedMembers;

  @override
  void buildMembers(
      SourceLibraryBuilder library, void Function(Member, BuiltMemberKind) f) {
    build(library);
    _fieldEncoding.registerMembers(library, this, f);
  }

  /// Builds the core AST structures for this field as needed for the outline.
  void build(SourceLibraryBuilder libraryBuilder) {
    if (type != null) {
      fieldType = type!.build(libraryBuilder);
    }
    _fieldEncoding.build(libraryBuilder, this);
  }

  @override
  void buildOutlineExpressions(
      SourceLibraryBuilder library,
      ClassHierarchy classHierarchy,
      List<DelayedActionPerformer> delayedActionPerformers,
      List<SynthesizedFunctionNode> synthesizedFunctionNodes) {
    _fieldEncoding.completeSignature(classHierarchy.coreTypes);

    for (Annotatable annotatable in _fieldEncoding.annotatables) {
      MetadataBuilder.buildAnnotations(
          annotatable,
          metadata,
          library,
          declarationBuilder,
          this,
          fileUri,
          declarationBuilder?.scope ?? library.scope);
    }

    // For modular compilation we need to include initializers of all const
    // fields and all non-static final fields in classes with const constructors
    // into the outline.
    if ((isConst ||
            (isFinal &&
                !isStatic &&
                isClassMember &&
                classBuilder!.declaresConstConstructor)) &&
        _constInitializerToken != null) {
      Scope scope = declarationBuilder?.scope ?? library.scope;
      BodyBuilder bodyBuilder = library.loader
          .createBodyBuilderForOutlineExpression(
              library, declarationBuilder, this, scope, fileUri);
      bodyBuilder.constantContext =
          isConst ? ConstantContext.inferred : ConstantContext.required;
      Expression initializer = bodyBuilder.typeInferrer.inferFieldInitializer(
          bodyBuilder,
          fieldType,
          bodyBuilder.parseFieldInitializer(_constInitializerToken!));
      if (bodyBuilder.transformSetLiterals ||
          bodyBuilder.transformCollections) {
        // Wrap the initializer in a temporary parent expression; the
        // transformations need a parent relation.
        Not wrapper = new Not(initializer);
        SourceLoader loader = library.loader;
        loader.transformPostInference(wrapper, bodyBuilder.transformSetLiterals,
            bodyBuilder.transformCollections, library.library);
        initializer = wrapper.operand;
      }
      buildBody(classHierarchy.coreTypes, initializer);
      bodyBuilder.performBacklogComputations(delayedActionPerformers);
    }
    _constInitializerToken = null;
  }

  @override
  DartType get fieldType => _fieldEncoding.type;

  void set fieldType(DartType value) {
    _fieldEncoding.type = value;
    if (!isFinal && !isConst && parent is ClassBuilder) {
      Class enclosingClass = classBuilder!.cls;
      if (enclosingClass.typeParameters.isNotEmpty) {
        IncludesTypeParametersNonCovariantly needsCheckVisitor =
            new IncludesTypeParametersNonCovariantly(
                enclosingClass.typeParameters,
                // We are checking the field type as if it is the type of the
                // parameter of the implicit setter and this is a contravariant
                // position.
                initialVariance: Variance.contravariant);
        if (value.accept(needsCheckVisitor)) {
          _fieldEncoding.setGenericCovariantImpl();
        }
      }
    }
  }

  @override
  DartType inferType() {
    SourceLibraryBuilder library = this.library;
    if (fieldType is! ImplicitFieldType) {
      // We have already inferred a type.
      return fieldType;
    }

    ImplicitFieldType implicitFieldType = fieldType as ImplicitFieldType;
    DartType inferredType = implicitFieldType.computeType();
    if (fieldType is ImplicitFieldType) {
      // `fieldType` may have changed if a circularity was detected when
      // [inferredType] was computed.
      if (!library.isNonNullableByDefault) {
        inferredType = legacyErasure(inferredType);
      }
      fieldType = implicitFieldType.checkInferred(inferredType);

      IncludesTypeParametersNonCovariantly? needsCheckVisitor;
      if (parent is ClassBuilder) {
        Class enclosingClass = classBuilder!.cls;
        if (enclosingClass.typeParameters.isNotEmpty) {
          needsCheckVisitor = new IncludesTypeParametersNonCovariantly(
              enclosingClass.typeParameters,
              // We are checking the field type as if it is the type of the
              // parameter of the implicit setter and this is a contravariant
              // position.
              initialVariance: Variance.contravariant);
        }
      }
      if (needsCheckVisitor != null) {
        if (fieldType.accept(needsCheckVisitor)) {
          _fieldEncoding.setGenericCovariantImpl();
        }
      }
    }
    return fieldType;
  }

  @override
  DartType get builtType => fieldType;

  List<ClassMember>? _localMembers;
  List<ClassMember>? _localSetters;

  @override
  List<ClassMember> get localMembers =>
      _localMembers ??= _fieldEncoding.getLocalMembers(this);

  @override
  List<ClassMember> get localSetters =>
      _localSetters ??= _fieldEncoding.getLocalSetters(this);
}

/// Strategy pattern for creating different encodings of a declared field.
///
/// This is used to provide lowerings for late fields using synthesized getters
/// and setters.
abstract class FieldEncoding {
  /// The type of the declared field.
  abstract DartType type;

  /// Creates the bodies needed for the field encoding using [initializer] as
  /// the declared initializer expression.
  ///
  /// This method is not called for fields in outlines unless their are constant
  /// or part of a const constructor.
  void createBodies(CoreTypes coreTypes, Expression? initializer);

  List<Initializer> createInitializer(int fileOffset, Expression value,
      {required bool isSynthetic});

  /// Registers that the (implicit) setter associated with this field needs to
  /// contain a runtime type check to deal with generic covariance.
  void setGenericCovariantImpl();

  /// Returns the field that holds the field value at runtime.
  Field get field;

  /// Returns the members that holds the field annotations.
  Iterable<Annotatable> get annotatables;

  /// Returns the member used to read the field value.
  Member get readTarget;

  /// Returns the member used to write to the field.
  Member? get writeTarget;

  /// Returns the generated members that are visible through exports.
  Iterable<Member> get exportedMembers;

  /// Creates the members necessary for this field encoding.
  ///
  /// This method is called for both outline and full compilation so the created
  /// members should be without body. The member bodies are created through
  /// [createBodies].
  void build(
      SourceLibraryBuilder libraryBuilder, SourceFieldBuilder fieldBuilder);

  /// Calls [f] for each member needed for this field encoding.
  void registerMembers(
      SourceLibraryBuilder library,
      SourceFieldBuilder fieldBuilder,
      void Function(Member, BuiltMemberKind) f);

  /// Returns a list of the field, getters and methods created by this field
  /// encoding.
  List<ClassMember> getLocalMembers(SourceFieldBuilder fieldBuilder);

  /// Returns a list of the setters created by this field encoding.
  List<ClassMember> getLocalSetters(SourceFieldBuilder fieldBuilder);

  /// Ensures that the signatures all members created by this field encoding
  /// are fully typed.
  void completeSignature(CoreTypes coreTypes);

  /// Returns `true` if this encoding is a late lowering.
  bool get isLateLowering;
}

class RegularFieldEncoding implements FieldEncoding {
  late final Field _field;

  RegularFieldEncoding(String name, NameScheme nameScheme, Uri fileUri,
      int charOffset, int charEndOffset,
      {required bool isFinal,
      required bool isConst,
      required bool isLate,
      required bool hasInitializer,
      required bool isNonNullableByDefault,
      required Reference? fieldReference,
      required Reference? getterReference,
      required Reference? setterReference}) {
    // ignore: unnecessary_null_comparison
    assert(isFinal != null);
    // ignore: unnecessary_null_comparison
    assert(isConst != null);
    // ignore: unnecessary_null_comparison
    assert(isLate != null);
    // ignore: unnecessary_null_comparison
    assert(hasInitializer != null);
    bool isImmutable =
        isLate ? (isFinal && hasInitializer) : (isFinal || isConst);
    _field = isImmutable
        ? new Field.immutable(
            nameScheme.getFieldName(FieldNameType.Field, name,
                isSynthesized: false),
            isFinal: isFinal,
            isConst: isConst,
            isLate: isLate,
            fileUri: fileUri,
            fieldReference: fieldReference,
            getterReference: getterReference)
        : new Field.mutable(
            nameScheme.getFieldName(FieldNameType.Field, name,
                isSynthesized: false),
            isFinal: isFinal,
            isLate: isLate,
            fileUri: fileUri,
            fieldReference: fieldReference,
            getterReference: getterReference,
            setterReference: setterReference);
    _field
      ..fileOffset = charOffset
      ..fileEndOffset = charEndOffset
      ..isNonNullableByDefault = isNonNullableByDefault;
  }

  @override
  DartType get type => _field.type;

  @override
  void set type(DartType value) {
    _field.type = value;
  }

  @override
  void completeSignature(CoreTypes coreTypes) {}

  @override
  void createBodies(CoreTypes coreTypes, Expression? initializer) {
    if (initializer != null) {
      _field.initializer = initializer..parent = _field;
    }
  }

  @override
  List<Initializer> createInitializer(int fileOffset, Expression value,
      {required bool isSynthetic}) {
    return <Initializer>[
      new FieldInitializer(_field, value)
        ..fileOffset = fileOffset
        ..isSynthetic = isSynthetic
    ];
  }

  @override
  void build(
      SourceLibraryBuilder libraryBuilder, SourceFieldBuilder fieldBuilder) {
    _field..isCovariantByDeclaration = fieldBuilder.isCovariantByDeclaration;
    if (fieldBuilder.isExtensionMember) {
      _field
        ..isStatic = true
        ..isExtensionMember = true;
    } else {
      bool isInstanceMember =
          !fieldBuilder.isStatic && !fieldBuilder.isTopLevel;
      _field
        ..isStatic = !isInstanceMember
        ..isExtensionMember = false;
    }
    _field.isLate = fieldBuilder.isLate;
    updatePrivateMemberName(_field, libraryBuilder);
  }

  @override
  void registerMembers(
      SourceLibraryBuilder library,
      SourceFieldBuilder fieldBuilder,
      void Function(Member, BuiltMemberKind) f) {
    f(
        _field,
        fieldBuilder.isExtensionMember
            ? BuiltMemberKind.ExtensionField
            : BuiltMemberKind.Field);
  }

  @override
  void setGenericCovariantImpl() {
    _field.isCovariantByClass = true;
  }

  @override
  Field get field => _field;

  @override
  Iterable<Annotatable> get annotatables => [_field];

  @override
  Member get readTarget => _field;

  @override
  Member get writeTarget => _field;

  @override
  Iterable<Member> get exportedMembers => [_field];

  @override
  List<ClassMember> getLocalMembers(SourceFieldBuilder fieldBuilder) =>
      <ClassMember>[new SourceFieldMember(fieldBuilder, forSetter: false)];

  @override
  List<ClassMember> getLocalSetters(SourceFieldBuilder fieldBuilder) =>
      fieldBuilder.isAssignable
          ? <ClassMember>[new SourceFieldMember(fieldBuilder, forSetter: true)]
          : const <ClassMember>[];

  @override
  bool get isLateLowering => false;
}

class SourceFieldMember extends BuilderClassMember {
  @override
  final SourceFieldBuilder memberBuilder;

  Covariance? _covariance;

  @override
  final bool forSetter;

  SourceFieldMember(this.memberBuilder, {required this.forSetter})
      // ignore: unnecessary_null_comparison
      : assert(forSetter != null);

  @override
  void inferType(ClassMembersBuilder membersBuilder) {
    memberBuilder._ensureType(membersBuilder);
  }

  @override
  void registerOverrideDependency(Set<ClassMember> overriddenMembers) {
    memberBuilder.registerOverrideDependency(overriddenMembers);
  }

  @override
  Member getMember(ClassMembersBuilder membersBuilder) {
    memberBuilder._ensureType(membersBuilder);
    return memberBuilder.field;
  }

  @override
  Covariance getCovariance(ClassMembersBuilder membersBuilder) {
    return _covariance ??= forSetter
        ? new Covariance.fromMember(getMember(membersBuilder),
            forSetter: forSetter)
        : const Covariance.empty();
  }

  @override
  bool get isSourceDeclaration => true;

  @override
  bool get isProperty => true;

  @override
  bool isSameDeclaration(ClassMember other) {
    return other is SourceFieldMember && memberBuilder == other.memberBuilder;
  }
}

abstract class AbstractLateFieldEncoding implements FieldEncoding {
  final String name;
  final int fileOffset;
  final int fileEndOffset;
  DartType? _type;
  late final Field _field;
  Field? _lateIsSetField;
  late Procedure _lateGetter;
  Procedure? _lateSetter;

  // If `true`, an isSet field is used even when the type of the field is
  // not potentially nullable.
  //
  // This is used to force use isSet fields in mixed mode encoding since
  // we cannot trust non-nullable fields to be initialized with non-null values.
  final late_lowering.IsSetStrategy _isSetStrategy;
  late_lowering.IsSetEncoding? _isSetEncoding;

  // If `true`, the is-set field was register before the type was known to be
  // nullable or non-nullable. In this case we do not try to remove it from
  // the generated AST to avoid inconsistency between the class hierarchy used
  // during and after inference.
  //
  // This is also used to force use isSet fields in mixed mode encoding since
  // we cannot trust non-nullable fields to be initialized with non-null values.
  bool _forceIncludeIsSetField;

  AbstractLateFieldEncoding(
      this.name,
      NameScheme nameScheme,
      Uri fileUri,
      int charOffset,
      int charEndOffset,
      Reference? fieldReference,
      Reference? fieldGetterReference,
      Reference? fieldSetterReference,
      Reference? lateIsSetFieldReference,
      Reference? lateIsSetGetterReference,
      Reference? lateIsSetSetterReference,
      Reference? lateGetterReference,
      Reference? lateSetterReference,
      bool isCovariantByDeclaration,
      late_lowering.IsSetStrategy isSetStrategy)
      : fileOffset = charOffset,
        fileEndOffset = charEndOffset,
        _isSetStrategy = isSetStrategy,
        _forceIncludeIsSetField =
            isSetStrategy == late_lowering.IsSetStrategy.forceUseIsSetField {
    _field = new Field.mutable(
        nameScheme.getFieldName(FieldNameType.Field, name, isSynthesized: true),
        fileUri: fileUri,
        fieldReference: fieldReference,
        getterReference: fieldGetterReference,
        setterReference: fieldSetterReference)
      ..fileOffset = charOffset
      ..fileEndOffset = charEndOffset
      ..isNonNullableByDefault = true
      ..isInternalImplementation = true;
    switch (_isSetStrategy) {
      case late_lowering.IsSetStrategy.useSentinelOrNull:
      case late_lowering.IsSetStrategy.forceUseSentinel:
        // [_lateIsSetField] is never needed.
        break;
      case late_lowering.IsSetStrategy.forceUseIsSetField:
      case late_lowering.IsSetStrategy.useIsSetFieldOrNull:
        _lateIsSetField = new Field.mutable(
            nameScheme.getFieldName(FieldNameType.IsSetField, name,
                isSynthesized: true),
            fileUri: fileUri,
            fieldReference: lateIsSetFieldReference,
            getterReference: lateIsSetGetterReference,
            setterReference: lateIsSetSetterReference)
          ..fileOffset = charOffset
          ..fileEndOffset = charEndOffset
          ..isNonNullableByDefault = true
          ..isInternalImplementation = true;
        break;
    }
    _lateGetter = new Procedure(
        nameScheme.getFieldName(FieldNameType.Getter, name,
            isSynthesized: true),
        ProcedureKind.Getter,
        new FunctionNode(null)
          ..fileOffset = charOffset
          ..fileEndOffset = charEndOffset,
        fileUri: fileUri,
        reference: lateGetterReference)
      ..fileOffset = charOffset
      ..fileEndOffset = charEndOffset
      ..isNonNullableByDefault = true;
    _lateSetter = _createSetter(
        nameScheme.getFieldName(FieldNameType.Setter, name,
            isSynthesized: true),
        fileUri,
        charOffset,
        lateSetterReference,
        isCovariantByDeclaration: isCovariantByDeclaration);
  }

  late_lowering.IsSetEncoding get isSetEncoding {
    assert(_type != null, "Type has not been computed for field $name.");
    return _isSetEncoding ??=
        late_lowering.computeIsSetEncoding(_type!, _isSetStrategy);
  }

  @override
  void completeSignature(CoreTypes coreTypes) {
    _lateIsSetField?.type = coreTypes.boolRawType(Nullability.nonNullable);
  }

  @override
  void createBodies(CoreTypes coreTypes, Expression? initializer) {
    assert(_type != null, "Type has not been computed for field $name.");
    if (isSetEncoding == late_lowering.IsSetEncoding.useSentinel) {
      _field.initializer = new StaticInvocation(coreTypes.createSentinelMethod,
          new Arguments([], types: [_type!])..fileOffset = fileOffset)
        ..fileOffset = fileOffset
        ..parent = _field;
    } else {
      _field.initializer = new NullLiteral()
        ..fileOffset = fileOffset
        ..parent = _field;
    }
    if (_lateIsSetField != null) {
      _lateIsSetField!.initializer = new BoolLiteral(false)
        ..fileOffset = fileOffset
        ..parent = _lateIsSetField;
    }
    _lateGetter.function.body = _createGetterBody(coreTypes, name, initializer)
      ..parent = _lateGetter.function;
    if (_lateSetter != null) {
      _lateSetter!.function.body = _createSetterBody(
          coreTypes, name, _lateSetter!.function.positionalParameters.first)
        ..parent = _lateSetter!.function;
    }
  }

  @override
  List<Initializer> createInitializer(int fileOffset, Expression value,
      {required bool isSynthetic}) {
    List<Initializer> initializers = <Initializer>[];
    if (_lateIsSetField != null) {
      initializers.add(new FieldInitializer(
          _lateIsSetField!, new BoolLiteral(true)..fileOffset = fileOffset)
        ..fileOffset = fileOffset
        ..isSynthetic = isSynthetic);
    }
    initializers.add(new FieldInitializer(_field, value)
      ..fileOffset = fileOffset
      ..isSynthetic = isSynthetic);
    return initializers;
  }

  /// Creates an [Expression] that reads [_field].
  ///
  /// If [needsPromotion] is `true`, the field will be read through a `let`
  /// expression that promotes the expression to [_type]. This is needed for a
  /// sound encoding of fields with type variable type of undetermined
  /// nullability.
  Expression _createFieldRead({bool needsPromotion: false}) {
    assert(_type != null, "Type has not been computed for field $name.");
    if (needsPromotion) {
      VariableDeclaration variable = new VariableDeclaration.forValue(
          _createFieldGet(_field),
          type: _type!.withDeclaredNullability(Nullability.nullable))
        ..fileOffset = fileOffset;
      return new Let(
          variable, new VariableGet(variable, _type)..fileOffset = fileOffset);
    } else {
      return _createFieldGet(_field);
    }
  }

  /// Creates an [Expression] that reads [field].
  Expression _createFieldGet(Field field) {
    if (field.isStatic) {
      return new StaticGet(field)..fileOffset = fileOffset;
    } else {
      // No substitution needed for the result type, since any type variables
      // in there are also in scope at the access site.
      return new InstanceGet(InstanceAccessKind.Instance,
          new ThisExpression()..fileOffset = fileOffset, field.name,
          interfaceTarget: field, resultType: field.type)
        ..fileOffset = fileOffset;
    }
  }

  /// Creates an [Expression] that writes [value] to [field].
  Expression _createFieldSet(Field field, Expression value) {
    if (field.isStatic) {
      return new StaticSet(field, value)..fileOffset = fileOffset;
    } else {
      return new InstanceSet(InstanceAccessKind.Instance,
          new ThisExpression()..fileOffset = fileOffset, field.name, value,
          interfaceTarget: field)
        ..fileOffset = fileOffset;
    }
  }

  Statement _createGetterBody(
      CoreTypes coreTypes, String name, Expression? initializer);

  Procedure? _createSetter(
      Name name, Uri fileUri, int charOffset, Reference? reference,
      {required bool isCovariantByDeclaration}) {
    // ignore: unnecessary_null_comparison
    assert(isCovariantByDeclaration != null);
    VariableDeclaration parameter = new VariableDeclaration(null)
      ..isCovariantByDeclaration = isCovariantByDeclaration
      ..fileOffset = fileOffset;
    return new Procedure(
        name,
        ProcedureKind.Setter,
        new FunctionNode(null,
            positionalParameters: [parameter], returnType: const VoidType())
          ..fileOffset = charOffset
          ..fileEndOffset = fileEndOffset,
        fileUri: fileUri,
        reference: reference)
      ..fileOffset = charOffset
      ..fileEndOffset = fileEndOffset
      ..isNonNullableByDefault = true;
  }

  Statement _createSetterBody(
      CoreTypes coreTypes, String name, VariableDeclaration parameter);

  @override
  DartType get type {
    assert(_type != null, "Type has not been computed for field $name.");
    return _type!;
  }

  @override
  void set type(DartType value) {
    assert(_type == null || _type is ImplicitFieldType,
        "Type has already been computed for field $name.");
    _type = value;
    if (value is! ImplicitFieldType) {
      _field.type = value.withDeclaredNullability(Nullability.nullable);
      _lateGetter.function.returnType = value;
      if (_lateSetter != null) {
        _lateSetter!.function.positionalParameters.single.type = value;
      }
      if (!_type!.isPotentiallyNullable && !_forceIncludeIsSetField) {
        // We only need the is-set field if the field is potentially nullable.
        //  Otherwise we use `null` to signal that the field is uninitialized.
        _lateIsSetField = null;
      }
    }
  }

  @override
  void setGenericCovariantImpl() {
    _field.isCovariantByClass = true;
    _lateSetter?.function.positionalParameters.single.isCovariantByClass = true;
  }

  @override
  Field get field => _field;

  @override
  Iterable<Annotatable> get annotatables {
    List<Annotatable> list = [_lateGetter];
    if (_lateSetter != null) {
      list.add(_lateSetter!);
    }
    return list;
  }

  @override
  Member get readTarget => _lateGetter;

  @override
  Member? get writeTarget => _lateSetter;

  @override
  Iterable<Member> get exportedMembers {
    if (_lateSetter != null) {
      return [_lateGetter, _lateSetter!];
    }
    return [_lateGetter];
  }

  @override
  void build(
      SourceLibraryBuilder libraryBuilder, SourceFieldBuilder fieldBuilder) {
    bool isInstanceMember;
    bool isExtensionMember = fieldBuilder.isExtensionMember;
    if (isExtensionMember) {
      _field
        ..isStatic = true
        ..isExtensionMember = isExtensionMember;
      isInstanceMember = false;
    } else {
      isInstanceMember = !fieldBuilder.isStatic && !fieldBuilder.isTopLevel;
      _field
        ..isStatic = !isInstanceMember
        ..isExtensionMember = false;
    }
    updatePrivateMemberName(_field, libraryBuilder);
    if (_lateIsSetField != null) {
      _lateIsSetField!
        ..isStatic = !isInstanceMember
        ..isStatic = _field.isStatic
        ..isExtensionMember = isExtensionMember;
      updatePrivateMemberName(_lateIsSetField!, libraryBuilder);
    }
    _lateGetter
      ..isStatic = !isInstanceMember
      ..isExtensionMember = isExtensionMember;
    updatePrivateMemberName(_lateGetter, libraryBuilder);
    if (_lateSetter != null) {
      _lateSetter!
        ..isStatic = !isInstanceMember
        ..isExtensionMember = isExtensionMember;
      updatePrivateMemberName(_lateSetter!, libraryBuilder);
    }
  }

  @override
  void registerMembers(
      SourceLibraryBuilder library,
      SourceFieldBuilder fieldBuilder,
      void Function(Member, BuiltMemberKind) f) {
    f(
        _field,
        fieldBuilder.isExtensionMember
            ? BuiltMemberKind.ExtensionField
            : BuiltMemberKind.Field);
    if (_lateIsSetField != null) {
      _forceIncludeIsSetField = true;
      f(_lateIsSetField!, BuiltMemberKind.LateIsSetField);
    }
    f(_lateGetter, BuiltMemberKind.LateGetter);
    if (_lateSetter != null) {
      f(_lateSetter!, BuiltMemberKind.LateSetter);
    }
  }

  @override
  List<ClassMember> getLocalMembers(SourceFieldBuilder fieldBuilder) {
    List<ClassMember> list = <ClassMember>[
      new _SynthesizedFieldClassMember(
          fieldBuilder, field, _SynthesizedFieldMemberKind.LateField,
          isInternalImplementation: true),
      new _SynthesizedFieldClassMember(fieldBuilder, _lateGetter,
          _SynthesizedFieldMemberKind.LateGetterSetter,
          isInternalImplementation: false)
    ];
    if (_lateIsSetField != null) {
      list.add(new _SynthesizedFieldClassMember(
          fieldBuilder, _lateIsSetField!, _SynthesizedFieldMemberKind.LateIsSet,
          isInternalImplementation: true));
    }
    return list;
  }

  @override
  List<ClassMember> getLocalSetters(SourceFieldBuilder fieldBuilder) {
    List<ClassMember> list = <ClassMember>[
      new _SynthesizedFieldClassMember(
          fieldBuilder, field, _SynthesizedFieldMemberKind.LateField,
          forSetter: true, isInternalImplementation: true),
    ];
    if (_lateIsSetField != null) {
      list.add(new _SynthesizedFieldClassMember(
          fieldBuilder, _lateIsSetField!, _SynthesizedFieldMemberKind.LateIsSet,
          forSetter: true, isInternalImplementation: true));
    }
    if (_lateSetter != null) {
      list.add(new _SynthesizedFieldClassMember(fieldBuilder, _lateSetter!,
          _SynthesizedFieldMemberKind.LateGetterSetter,
          forSetter: true, isInternalImplementation: false));
    }
    return list;
  }

  @override
  bool get isLateLowering => true;
}

mixin NonFinalLate on AbstractLateFieldEncoding {
  @override
  Statement _createSetterBody(
      CoreTypes coreTypes, String name, VariableDeclaration parameter) {
    assert(_type != null, "Type has not been computed for field $name.");
    return late_lowering.createSetterBody(
        coreTypes, fileOffset, name, parameter, _type!,
        shouldReturnValue: false,
        createVariableWrite: (Expression value) =>
            _createFieldSet(_field, value),
        createIsSetWrite: (Expression value) =>
            _createFieldSet(_lateIsSetField!, value),
        isSetEncoding: isSetEncoding);
  }
}

mixin LateWithoutInitializer on AbstractLateFieldEncoding {
  @override
  Statement _createGetterBody(
      CoreTypes coreTypes, String name, Expression? initializer) {
    assert(_type != null, "Type has not been computed for field $name.");
    return late_lowering.createGetterBodyWithoutInitializer(
        coreTypes, fileOffset, name, type,
        createVariableRead: _createFieldRead,
        createIsSetRead: () => _createFieldGet(_lateIsSetField!),
        isSetEncoding: isSetEncoding,
        forField: true);
  }
}

class LateFieldWithoutInitializerEncoding extends AbstractLateFieldEncoding
    with NonFinalLate, LateWithoutInitializer {
  LateFieldWithoutInitializerEncoding(
      String name,
      NameScheme nameScheme,
      Uri fileUri,
      int charOffset,
      int charEndOffset,
      Reference? fieldReference,
      Reference? fieldGetterReference,
      Reference? fieldSetterReference,
      Reference? lateIsSetFieldReference,
      Reference? lateIsSetGetterReference,
      Reference? lateIsSetSetterReference,
      Reference? lateGetterReference,
      Reference? lateSetterReference,
      bool isCovariantByDeclaration,
      late_lowering.IsSetStrategy isSetStrategy)
      : super(
            name,
            nameScheme,
            fileUri,
            charOffset,
            charEndOffset,
            fieldReference,
            fieldGetterReference,
            fieldSetterReference,
            lateIsSetFieldReference,
            lateIsSetGetterReference,
            lateIsSetSetterReference,
            lateGetterReference,
            lateSetterReference,
            isCovariantByDeclaration,
            isSetStrategy);
}

class LateFieldWithInitializerEncoding extends AbstractLateFieldEncoding
    with NonFinalLate {
  LateFieldWithInitializerEncoding(
      String name,
      NameScheme nameScheme,
      Uri fileUri,
      int charOffset,
      int charEndOffset,
      Reference? fieldReference,
      Reference? fieldGetterReference,
      Reference? fieldSetterReference,
      Reference? lateIsSetFieldReference,
      Reference? lateIsSetGetterReference,
      Reference? lateIsSetSetterReference,
      Reference? lateGetterReference,
      Reference? lateSetterReference,
      bool isCovariantByDeclaration,
      late_lowering.IsSetStrategy isSetStrategy)
      : super(
            name,
            nameScheme,
            fileUri,
            charOffset,
            charEndOffset,
            fieldReference,
            fieldGetterReference,
            fieldSetterReference,
            lateIsSetFieldReference,
            lateIsSetGetterReference,
            lateIsSetSetterReference,
            lateGetterReference,
            lateSetterReference,
            isCovariantByDeclaration,
            isSetStrategy);

  @override
  Statement _createGetterBody(
      CoreTypes coreTypes, String name, Expression? initializer) {
    assert(_type != null, "Type has not been computed for field $name.");
    return late_lowering.createGetterWithInitializer(
        coreTypes, fileOffset, name, _type!, initializer!,
        createVariableRead: _createFieldRead,
        createVariableWrite: (Expression value) =>
            _createFieldSet(_field, value),
        createIsSetRead: () => _createFieldGet(_lateIsSetField!),
        createIsSetWrite: (Expression value) =>
            _createFieldSet(_lateIsSetField!, value),
        isSetEncoding: isSetEncoding);
  }
}

class LateFinalFieldWithoutInitializerEncoding extends AbstractLateFieldEncoding
    with LateWithoutInitializer {
  LateFinalFieldWithoutInitializerEncoding(
      String name,
      NameScheme nameScheme,
      Uri fileUri,
      int charOffset,
      int charEndOffset,
      Reference? fieldReference,
      Reference? fieldGetterReference,
      Reference? fieldSetterReference,
      Reference? lateIsSetFieldReference,
      Reference? lateIsSetGetterReference,
      Reference? lateIsSetSetterReference,
      Reference? lateGetterReference,
      Reference? lateSetterReference,
      bool isCovariantByDeclaration,
      late_lowering.IsSetStrategy isSetStrategy)
      : super(
            name,
            nameScheme,
            fileUri,
            charOffset,
            charEndOffset,
            fieldReference,
            fieldGetterReference,
            fieldSetterReference,
            lateIsSetFieldReference,
            lateIsSetGetterReference,
            lateIsSetSetterReference,
            lateGetterReference,
            lateSetterReference,
            isCovariantByDeclaration,
            isSetStrategy);

  @override
  Statement _createSetterBody(
      CoreTypes coreTypes, String name, VariableDeclaration parameter) {
    assert(_type != null, "Type has not been computed for field $name.");
    return late_lowering.createSetterBodyFinal(
        coreTypes, fileOffset, name, parameter, type,
        shouldReturnValue: false,
        createVariableRead: () => _createFieldGet(_field),
        createVariableWrite: (Expression value) =>
            _createFieldSet(_field, value),
        createIsSetRead: () => _createFieldGet(_lateIsSetField!),
        createIsSetWrite: (Expression value) =>
            _createFieldSet(_lateIsSetField!, value),
        isSetEncoding: isSetEncoding,
        forField: true);
  }
}

class LateFinalFieldWithInitializerEncoding extends AbstractLateFieldEncoding {
  LateFinalFieldWithInitializerEncoding(
      String name,
      NameScheme nameScheme,
      Uri fileUri,
      int charOffset,
      int charEndOffset,
      Reference? fieldReference,
      Reference? fieldGetterReference,
      Reference? fieldSetterReference,
      Reference? lateIsSetFieldReference,
      Reference? lateIsSetGetterReference,
      Reference? lateIsSetSetterReference,
      Reference? lateGetterReference,
      Reference? lateSetterReference,
      bool isCovariantByDeclaration,
      late_lowering.IsSetStrategy isSetStrategy)
      : super(
            name,
            nameScheme,
            fileUri,
            charOffset,
            charEndOffset,
            fieldReference,
            fieldGetterReference,
            fieldSetterReference,
            lateIsSetFieldReference,
            lateIsSetGetterReference,
            lateIsSetSetterReference,
            lateGetterReference,
            lateSetterReference,
            isCovariantByDeclaration,
            isSetStrategy);
  @override
  Statement _createGetterBody(
      CoreTypes coreTypes, String name, Expression? initializer) {
    assert(_type != null, "Type has not been computed for field $name.");
    return late_lowering.createGetterWithInitializerWithRecheck(
        coreTypes, fileOffset, name, _type!, initializer!,
        createVariableRead: _createFieldRead,
        createVariableWrite: (Expression value) =>
            _createFieldSet(_field, value),
        createIsSetRead: () => _createFieldGet(_lateIsSetField!),
        createIsSetWrite: (Expression value) =>
            _createFieldSet(_lateIsSetField!, value),
        isSetEncoding: isSetEncoding,
        forField: true);
  }

  @override
  Procedure? _createSetter(
          Name name, Uri fileUri, int charOffset, Reference? reference,
          {required bool isCovariantByDeclaration}) =>
      null;

  @override
  Statement _createSetterBody(
          CoreTypes coreTypes, String name, VariableDeclaration parameter) =>
      throw new UnsupportedError(
          '$runtimeType._createSetterBody is not supported.');
}

class _SynthesizedFieldClassMember implements ClassMember {
  final SourceFieldBuilder fieldBuilder;
  final _SynthesizedFieldMemberKind _kind;

  final Member _member;

  Covariance? _covariance;

  @override
  final bool forSetter;

  @override
  final bool isInternalImplementation;

  _SynthesizedFieldClassMember(this.fieldBuilder, this._member, this._kind,
      {this.forSetter: false, required this.isInternalImplementation})
      // ignore: unnecessary_null_comparison
      : assert(isInternalImplementation != null);

  @override
  Member getMember(ClassMembersBuilder membersBuilder) {
    fieldBuilder._ensureType(membersBuilder);
    return _member;
  }

  @override
  Covariance getCovariance(ClassMembersBuilder membersBuilder) {
    return _covariance ??= new Covariance.fromMember(getMember(membersBuilder),
        forSetter: forSetter);
  }

  @override
  void inferType(ClassMembersBuilder membersBuilder) {
    fieldBuilder._ensureType(membersBuilder);
  }

  @override
  void registerOverrideDependency(Set<ClassMember> overriddenMembers) {
    fieldBuilder.registerOverrideDependency(overriddenMembers);
  }

  @override
  bool get isSourceDeclaration => true;

  @override
  bool get isProperty => isField || isGetter || isSetter;

  @override
  ClassBuilder get classBuilder => fieldBuilder.classBuilder!;

  @override
  bool isObjectMember(ClassBuilder objectClass) {
    return classBuilder == objectClass;
  }

  @override
  bool get isDuplicate => fieldBuilder.isDuplicate;

  @override
  bool get isStatic => fieldBuilder.isStatic;

  @override
  bool get isField => _member is Field;

  @override
  bool get isAssignable {
    Member field = _member;
    return field is Field && field.hasSetter;
  }

  @override
  bool get isSetter {
    Member procedure = _member;
    return procedure is Procedure && procedure.kind == ProcedureKind.Setter;
  }

  @override
  bool get isGetter {
    Member procedure = _member;
    return procedure is Procedure && procedure.kind == ProcedureKind.Getter;
  }

  @override
  bool get isFinal {
    Member field = _member;
    return field is Field && field.isFinal;
  }

  @override
  bool get isConst {
    Member field = _member;
    return field is Field && field.isConst;
  }

  @override
  Name get name => _member.name;

  @override
  String get fullName {
    String suffix = isSetter ? "=" : "";
    String className = classBuilder.fullNameForErrors;
    // ignore: unnecessary_null_comparison
    return className == null
        ? "${fullNameForErrors}$suffix"
        : "${className}.${fullNameForErrors}$suffix";
  }

  @override
  String get fullNameForErrors => fieldBuilder.fullNameForErrors;

  @override
  Uri get fileUri => fieldBuilder.fileUri;

  @override
  int get charOffset => fieldBuilder.charOffset;

  @override
  bool get isAbstract => _member.isAbstract;

  @override
  bool get isSynthesized => false;

  @override
  bool get hasDeclarations => false;

  @override
  List<ClassMember> get declarations =>
      throw new UnsupportedError("$runtimeType.declarations");

  @override
  ClassMember get interfaceMember => this;

  @override
  bool isSameDeclaration(ClassMember other) {
    if (identical(this, other)) return true;
    return other is _SynthesizedFieldClassMember &&
        fieldBuilder == other.fieldBuilder &&
        _kind == other._kind;
  }

  @override
  String toString() => '_SynthesizedFieldClassMember('
      '$fieldBuilder,$_member,$_kind,forSetter=${forSetter})';
}

class AbstractOrExternalFieldEncoding implements FieldEncoding {
  final SourceFieldBuilder _fieldBuilder;
  final bool isAbstract;
  final bool isExternal;
  final bool _isExtensionInstanceMember;

  late Procedure _getter;
  Procedure? _setter;
  DartType? _type;

  AbstractOrExternalFieldEncoding(
      this._fieldBuilder,
      String name,
      NameScheme nameScheme,
      Uri fileUri,
      int charOffset,
      int charEndOffset,
      Reference? getterReference,
      Reference? setterReference,
      {required this.isAbstract,
      required this.isExternal,
      required bool isFinal,
      required bool isCovariantByDeclaration,
      required bool isNonNullableByDefault})
      // ignore: unnecessary_null_comparison
      : assert(isAbstract != null),
        // ignore: unnecessary_null_comparison
        assert(isExternal != null),
        // ignore: unnecessary_null_comparison
        assert(isFinal != null),
        // ignore: unnecessary_null_comparison
        assert(isCovariantByDeclaration != null),
        // ignore: unnecessary_null_comparison
        assert(isNonNullableByDefault != null),
        _isExtensionInstanceMember = isExternal &&
            nameScheme.isExtensionMember &&
            nameScheme.isInstanceMember {
    if (_isExtensionInstanceMember) {
      _getter = new Procedure(
          nameScheme.getProcedureName(ProcedureKind.Getter, name),
          ProcedureKind.Method,
          new FunctionNode(null, positionalParameters: [
            new VariableDeclaration(extensionThisName)..fileOffset
          ]),
          fileUri: fileUri,
          reference: getterReference)
        ..fileOffset = charOffset
        ..fileEndOffset = charEndOffset
        ..isNonNullableByDefault = isNonNullableByDefault;
      if (!isFinal) {
        VariableDeclaration parameter =
            new VariableDeclaration("#externalFieldValue")
              ..isCovariantByDeclaration = isCovariantByDeclaration
              ..fileOffset = charOffset;
        _setter = new Procedure(
            nameScheme.getProcedureName(ProcedureKind.Setter, name),
            ProcedureKind.Method,
            new FunctionNode(null,
                positionalParameters: [
                  new VariableDeclaration(extensionThisName)..fileOffset,
                  parameter
                ],
                returnType: const VoidType())
              ..fileOffset = charOffset
              ..fileEndOffset = charEndOffset,
            fileUri: fileUri,
            reference: setterReference)
          ..fileOffset = charOffset
          ..fileEndOffset = charEndOffset
          ..isNonNullableByDefault = isNonNullableByDefault;
      }
    } else {
      _getter = new Procedure(
          nameScheme.getFieldName(FieldNameType.Getter, name,
              isSynthesized: true),
          ProcedureKind.Getter,
          new FunctionNode(null),
          fileUri: fileUri,
          reference: getterReference)
        ..fileOffset = charOffset
        ..fileEndOffset = charEndOffset
        ..isNonNullableByDefault = isNonNullableByDefault;
      if (!isFinal) {
        VariableDeclaration parameter =
            new VariableDeclaration("#externalFieldValue")
              ..isCovariantByDeclaration = isCovariantByDeclaration
              ..fileOffset = charOffset;
        _setter = new Procedure(
            nameScheme.getFieldName(FieldNameType.Setter, name,
                isSynthesized: true),
            ProcedureKind.Setter,
            new FunctionNode(null,
                positionalParameters: [parameter], returnType: const VoidType())
              ..fileOffset = charOffset
              ..fileEndOffset = charEndOffset,
            fileUri: fileUri,
            reference: setterReference)
          ..fileOffset = charOffset
          ..fileEndOffset = charEndOffset
          ..isNonNullableByDefault = isNonNullableByDefault;
      }
    }
  }

  @override
  DartType get type {
    assert(_type != null,
        "Type has not been computed for field ${_fieldBuilder.name}.");
    return _type!;
  }

  @override
  void set type(DartType value) {
    assert(_type == null || _type is ImplicitFieldType,
        "Type has already been computed for field ${_fieldBuilder.name}.");
    _type = value;
    if (value is! ImplicitFieldType) {
      if (_isExtensionInstanceMember) {
        SourceExtensionBuilder extensionBuilder =
            _fieldBuilder.parent as SourceExtensionBuilder;
        DartType onType = extensionBuilder.extension.onType;
        List<TypeParameter> typeParameters =
            extensionBuilder.extension.typeParameters;
        if (typeParameters.isNotEmpty) {
          FreshTypeParameters getterTypeParameters =
              getFreshTypeParameters(typeParameters);
          _getter.function.positionalParameters.first.type =
              getterTypeParameters.substitute(onType);
          _getter.function.returnType = getterTypeParameters.substitute(value);
          _getter.function.typeParameters =
              getterTypeParameters.freshTypeParameters;
          setParents(
              getterTypeParameters.freshTypeParameters, _getter.function);

          Procedure? setter = _setter;
          if (setter != null) {
            FreshTypeParameters setterTypeParameters =
                getFreshTypeParameters(typeParameters);
            setter.function.positionalParameters.first.type =
                setterTypeParameters.substitute(onType);
            setter.function.positionalParameters[1].type =
                setterTypeParameters.substitute(value);
            setter.function.typeParameters =
                setterTypeParameters.freshTypeParameters;
            setParents(
                setterTypeParameters.freshTypeParameters, setter.function);
          }
        } else {
          _getter.function.returnType = value;
          _setter?.function.positionalParameters[1].type = value;
          _getter.function.positionalParameters.first.type = onType;
          _setter?.function.positionalParameters.first.type = onType;
        }
      } else {
        _getter.function.returnType = value;
        Procedure? setter = _setter;
        if (setter != null) {
          if (setter.kind == ProcedureKind.Method) {
            setter.function.positionalParameters[1].type = value;
          } else {
            setter.function.positionalParameters.first.type = value;
          }
        }
      }
    }
  }

  @override
  void completeSignature(CoreTypes coreTypes) {}

  @override
  void createBodies(CoreTypes coreTypes, Expression? initializer) {
    //assert(initializer != null);
  }

  @override
  List<Initializer> createInitializer(int fileOffset, Expression value,
      {required bool isSynthetic}) {
    throw new UnsupportedError('ExternalFieldEncoding.createInitializer');
  }

  @override
  void build(
      SourceLibraryBuilder libraryBuilder, SourceFieldBuilder fieldBuilder) {
    bool isExtensionMember = fieldBuilder.isExtensionMember;
    bool isInstanceMember = !fieldBuilder.isExtensionMember &&
        !fieldBuilder.isStatic &&
        !fieldBuilder.isTopLevel;
    _getter..isConst = fieldBuilder.isConst;
    _getter
      ..isStatic = !isInstanceMember
      ..isExtensionMember = isExtensionMember
      ..isAbstract = isAbstract && !isExternal
      ..isExternal = isExternal;
    updatePrivateMemberName(_getter, libraryBuilder);

    if (_setter != null) {
      _setter!
        ..isStatic = !isInstanceMember
        ..isExtensionMember = isExtensionMember
        ..isAbstract = isAbstract && !isExternal
        ..isExternal = isExternal;
      updatePrivateMemberName(_setter!, libraryBuilder);
    }
  }

  @override
  void registerMembers(
      SourceLibraryBuilder library,
      SourceFieldBuilder fieldBuilder,
      void Function(Member, BuiltMemberKind) f) {
    f(
        _getter,
        fieldBuilder.isExtensionMember
            ? BuiltMemberKind.ExtensionGetter
            : BuiltMemberKind.Method);
    if (_setter != null) {
      f(
          _setter!,
          fieldBuilder.isExtensionMember
              ? BuiltMemberKind.ExtensionSetter
              : BuiltMemberKind.Method);
    }
  }

  @override
  void setGenericCovariantImpl() {
    _setter!.function.positionalParameters.first.isCovariantByClass = true;
  }

  @override
  Field get field {
    throw new UnsupportedError("ExternalFieldEncoding.field");
  }

  @override
  Iterable<Annotatable> get annotatables {
    List<Annotatable> list = [_getter];
    if (_setter != null) {
      list.add(_setter!);
    }
    return list;
  }

  @override
  Member get readTarget => _getter;

  @override
  Member? get writeTarget => _setter;

  @override
  Iterable<Member> get exportedMembers {
    if (_setter != null) {
      return [_getter, _setter!];
    }
    return [_getter];
  }

  @override
  List<ClassMember> getLocalMembers(SourceFieldBuilder fieldBuilder) =>
      <ClassMember>[
        new _SynthesizedFieldClassMember(fieldBuilder, _getter,
            _SynthesizedFieldMemberKind.AbstractExternalGetterSetter,
            forSetter: false, isInternalImplementation: false)
      ];

  @override
  List<ClassMember> getLocalSetters(SourceFieldBuilder fieldBuilder) =>
      _setter != null
          ? <ClassMember>[
              new _SynthesizedFieldClassMember(fieldBuilder, _setter!,
                  _SynthesizedFieldMemberKind.AbstractExternalGetterSetter,
                  forSetter: true, isInternalImplementation: false)
            ]
          : const <ClassMember>[];

  @override
  bool get isLateLowering => false;
}

enum _SynthesizedFieldMemberKind {
  /// A `isSet` field used for late lowering.
  LateIsSet,

  /// A field used for the value of a late lowered field.
  LateField,

  /// A getter or setter used for late lowering.
  LateGetterSetter,

  /// A getter or setter used for abstract or external fields.
  AbstractExternalGetterSetter,
}