pkg/front_end/lib/src/fasta/type_inference/type_inferrer.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:_fe_analyzer_shared/src/flow_analysis/flow_analysis.dart';

import 'package:_fe_analyzer_shared/src/testing/id.dart';
import 'package:_fe_analyzer_shared/src/util/link.dart';

import 'package:kernel/ast.dart';
import 'package:kernel/canonical_name.dart' as kernel;
import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
import 'package:kernel/core_types.dart' show CoreTypes;
import 'package:kernel/type_algebra.dart';
import 'package:kernel/type_environment.dart';
import 'package:kernel/src/bounds_checks.dart' show calculateBounds;
import 'package:kernel/src/future_value_type.dart';
import 'package:kernel/src/legacy_erasure.dart';

import '../../base/instrumentation.dart'
    show
        Instrumentation,
        InstrumentationValueForMember,
        InstrumentationValueForType,
        InstrumentationValueForTypeArgs;

import '../../base/nnbd_mode.dart';

import '../../testing/id_extractor.dart';
import '../../testing/id_testing_utils.dart';

import '../builder/constructor_builder.dart';
import '../builder/extension_builder.dart';
import '../builder/member_builder.dart';

import '../fasta_codes.dart';

import '../kernel/class_hierarchy_builder.dart' show ClassMember;
import '../kernel/constructor_tearoff_lowering.dart';
import '../kernel/kernel_helper.dart';
import '../kernel/inference_visitor.dart';
import '../kernel/internal_ast.dart';
import '../kernel/invalid_type.dart';
import '../kernel/type_algorithms.dart' show hasAnyTypeVariables;

import '../names.dart';

import '../problems.dart' show internalProblem, unexpected, unhandled;

import '../source/source_library_builder.dart' show SourceLibraryBuilder;

import 'inference_helper.dart' show InferenceHelper;
import 'type_constraint_gatherer.dart' show TypeConstraintGatherer;
import 'type_demotion.dart';
import 'type_inference_engine.dart';
import 'type_schema.dart' show isKnown, UnknownType;
import 'type_schema_elimination.dart' show greatestClosure;
import 'type_schema_environment.dart'
    show
        getNamedParameterType,
        getPositionalParameterType,
        TypeConstraint,
        TypeVariableEliminator,
        TypeSchemaEnvironment;

part 'closure_context.dart';

/// Given a [FunctionNode], gets the named parameter identified by [name], or
/// `null` if there is no parameter with the given name.
VariableDeclaration? getNamedFormal(FunctionNode function, String name) {
  for (VariableDeclaration formal in function.namedParameters) {
    if (formal.name == name) return formal;
  }
  return null;
}

/// Given a [FunctionNode], gets the [i]th positional formal parameter, or
/// `null` if there is no parameter with that index.
VariableDeclaration? getPositionalFormal(FunctionNode function, int i) {
  if (i < function.positionalParameters.length) {
    return function.positionalParameters[i];
  } else {
    return null;
  }
}

bool isOverloadableArithmeticOperator(String name) {
  return identical(name, '+') ||
      identical(name, '-') ||
      identical(name, '*') ||
      identical(name, '%');
}

/// Enum denoting the kinds of contravariance check that might need to be
/// inserted for a method call.
enum MethodContravarianceCheckKind {
  /// No contravariance check is needed.
  none,

  /// The return value from the method call needs to be checked.
  checkMethodReturn,

  /// The method call needs to be desugared into a getter call, followed by an
  /// "as" check, followed by an invocation of the resulting function object.
  checkGetterReturn,
}

/// Keeps track of the local state for the type inference that occurs during
/// compilation of a single method body or top level initializer.
///
/// This class describes the interface for use by clients of type inference
/// (e.g. BodyBuilder).  Derived classes should derive from [TypeInferrerImpl].
abstract class TypeInferrer {
  SourceLibraryBuilder get library;

  /// Gets the [TypeSchemaEnvironment] being used for type inference.
  TypeSchemaEnvironment get typeSchemaEnvironment;

  /// Returns the [FlowAnalysis] used during inference.
  FlowAnalysis<TreeNode, Statement, Expression, VariableDeclaration, DartType>
      get flowAnalysis;

  /// The URI of the code for which type inference is currently being
  /// performed--this is used for testing.
  Uri get uriForInstrumentation;

  AssignedVariables<TreeNode, VariableDeclaration> get assignedVariables;

  /// Performs full type inference on the given field initializer.
  Expression inferFieldInitializer(
      InferenceHelper helper, DartType declaredType, Expression initializer);

  /// Returns the type used as the inferred type of a variable declaration,
  /// based on the static type of the initializer expression, given by
  /// [initializerType].
  DartType inferDeclarationType(DartType initializerType);

  /// Performs type inference on [expression].
  ExpressionInferenceResult inferExpression(
      Expression expression, DartType typeContext, bool typeNeeded,
      {bool isVoidAllowed: false, bool forEffect: false});

  /// Performs type inference on the given function body.
  InferredFunctionBody inferFunctionBody(InferenceHelper helper, int fileOffset,
      DartType returnType, AsyncMarker asyncMarker, Statement body);

  /// Performs type inference on the given constructor initializer.
  InitializerInferenceResult inferInitializer(
      InferenceHelper helper, Initializer initializer);

  /// Performs type inference on the given metadata annotations.
  void inferMetadata(
      InferenceHelper helper, TreeNode? parent, List<Expression>? annotations);

  /// Performs type inference on the given metadata annotations keeping the
  /// existing helper if possible.
  void inferMetadataKeepingHelper(
      TreeNode parent, List<Expression> annotations);

  /// Performs type inference on the given function parameter initializer
  /// expression.
  Expression inferParameterInitializer(
      InferenceHelper helper,
      Expression initializer,
      DartType declaredType,
      bool hasDeclaredInitializer);

  /// Ensures that all parameter types of [constructor] have been inferred.
  // TODO(johnniwinther): We are still parameters on synthesized mixin
  //  application constructors.
  void inferConstructorParameterTypes(Constructor constructor);
}

/// Concrete implementation of [TypeInferrer] specialized to work with kernel
/// objects.
class TypeInferrerImpl implements TypeInferrer {
  /// Marker object to indicate that a function takes an unknown number
  /// of arguments.
  final FunctionType unknownFunction;

  final TypeInferenceEngine engine;

  @override
  final FlowAnalysis<TreeNode, Statement, Expression, VariableDeclaration,
      DartType> flowAnalysis;

  @override
  final AssignedVariables<TreeNode, VariableDeclaration> assignedVariables;

  final InferenceDataForTesting? dataForTesting;

  @override
  final Uri uriForInstrumentation;

  /// Indicates whether the construct we are currently performing inference for
  /// is outside of a method body, and hence top level type inference rules
  /// should apply.
  final bool isTopLevel;

  final ClassHierarchy classHierarchy;

  final Instrumentation? instrumentation;

  @override
  final TypeSchemaEnvironment typeSchemaEnvironment;

  final InterfaceType? thisType;

  @override
  final SourceLibraryBuilder library;

  InferenceHelper? helper;

  /// Context information for the current closure, or `null` if we are not
  /// inside a closure.
  ClosureContext? closureContext;

  TypeInferrerImpl(this.engine, this.uriForInstrumentation, bool topLevel,
      this.thisType, this.library, this.assignedVariables, this.dataForTesting)
      // ignore: unnecessary_null_comparison
      : assert(library != null),
        unknownFunction = new FunctionType(
            const [], const DynamicType(), library.nonNullable),
        classHierarchy = engine.classHierarchy,
        instrumentation = topLevel ? null : engine.instrumentation,
        typeSchemaEnvironment = engine.typeSchemaEnvironment,
        isTopLevel = topLevel,
        flowAnalysis = library.isNonNullableByDefault
            ? new FlowAnalysis(
                new TypeOperationsCfe(engine.typeSchemaEnvironment),
                assignedVariables,
                respectImplicitlyTypedVarInitializers:
                    library.enableConstructorTearOffsInLibrary)
            : new FlowAnalysis.legacy(
                new TypeOperationsCfe(engine.typeSchemaEnvironment),
                assignedVariables) {}

  CoreTypes get coreTypes => engine.coreTypes;

  bool get isNonNullableByDefault => library.isNonNullableByDefault;

  NnbdMode get nnbdMode => library.loader.nnbdMode;

  DartType get bottomType =>
      isNonNullableByDefault ? const NeverType.nonNullable() : const NullType();

  DartType computeGreatestClosure(DartType type) {
    return greatestClosure(type, const DynamicType(), bottomType);
  }

  DartType computeGreatestClosure2(DartType type) {
    return greatestClosure(
        type,
        isNonNullableByDefault
            ? coreTypes.objectNullableRawType
            : const DynamicType(),
        bottomType);
  }

  DartType computeNullable(DartType type) {
    if (type is NullType || type is NeverType) {
      return const NullType();
    }
    return type.withDeclaredNullability(library.nullable);
  }

  Expression createReachabilityError(
      int fileOffset, Message errorMessage, Message warningMessage) {
    if (library.loader.target.context.options.warnOnReachabilityCheck &&
        // ignore: unnecessary_null_comparison
        warningMessage != null) {
      helper?.addProblem(warningMessage, fileOffset, noLength);
    }
    Arguments arguments;
    // ignore: unnecessary_null_comparison
    if (errorMessage != null) {
      arguments = new Arguments([
        new StringLiteral(errorMessage.problemMessage)..fileOffset = fileOffset
      ])
        ..fileOffset = fileOffset;
    } else {
      arguments = new Arguments([])..fileOffset = fileOffset;
    }
    // ignore: unnecessary_null_comparison
    assert(coreTypes.reachabilityErrorConstructor != null);
    return new Throw(
        new ConstructorInvocation(
            coreTypes.reachabilityErrorConstructor, arguments)
          ..fileOffset = fileOffset)
      ..fileOffset = fileOffset;
  }

  /// Computes a list of context messages explaining why [receiver] was not
  /// promoted, to be used when reporting an error for a larger expression
  /// containing [receiver].  [node] is the containing tree node.
  List<LocatedMessage>? getWhyNotPromotedContext(
      Map<DartType, NonPromotionReason>? whyNotPromoted,
      TreeNode node,
      bool Function(DartType) typeFilter) {
    List<LocatedMessage>? context;
    if (whyNotPromoted != null && whyNotPromoted.isNotEmpty) {
      _WhyNotPromotedVisitor whyNotPromotedVisitor =
          new _WhyNotPromotedVisitor(this);
      for (MapEntry<DartType, NonPromotionReason> entry
          in whyNotPromoted.entries) {
        if (!typeFilter(entry.key)) continue;
        LocatedMessage? message = entry.value.accept(whyNotPromotedVisitor);
        if (dataForTesting != null) {
          String nonPromotionReasonText = entry.value.shortName;
          List<String> args = <String>[];
          if (whyNotPromotedVisitor.propertyReference != null) {
            Id id = computeMemberId(whyNotPromotedVisitor.propertyReference!);
            args.add('target: $id');
          }
          if (whyNotPromotedVisitor.propertyType != null) {
            String typeText = typeToText(whyNotPromotedVisitor.propertyType!,
                TypeRepresentation.analyzerNonNullableByDefault);
            args.add('type: $typeText');
          }
          if (args.isNotEmpty) {
            nonPromotionReasonText += '(${args.join(', ')})';
          }
          TreeNode origNode = node;
          while (origNode is VariableGet &&
              origNode.variable.name == null &&
              origNode.variable.initializer != null) {
            // This is a read of a synthetic variable, presumably from a "let".
            // Find the original expression.
            // TODO(johnniwinther): add a general solution for getting the
            // original node for testing.
            origNode = origNode.variable.initializer!;
          }
          dataForTesting!.flowAnalysisResult.nonPromotionReasons[origNode] =
              nonPromotionReasonText;
        }
        // Note: this will always pick the first viable reason (only).  I
        // (paulberry) believe this is the one that will be the most relevant,
        // but I need to do more testing to validate that.  I can't do that
        // additional testing yet because at the moment we only handle failed
        // promotions to non-nullable.
        // TODO(paulberry): do more testing and then expand on the comment
        // above.
        if (message != null) {
          context = [message];
        }
        break;
      }
    }
    return context;
  }

  /// Returns `true` if exceptions should be thrown in paths reachable only due
  /// to unsoundness in flow analysis in mixed mode.
  bool get shouldThrowUnsoundnessException =>
      isNonNullableByDefault && nnbdMode != NnbdMode.Strong;

  void registerIfUnreachableForTesting(TreeNode node, {bool? isReachable}) {
    if (dataForTesting == null) return;
    isReachable ??= flowAnalysis.isReachable;
    if (!isReachable) {
      dataForTesting!.flowAnalysisResult.unreachableNodes.add(node);
    }
  }

  /// Ensures that the type of [member] has been computed.
  void ensureMemberType(Member member) {
    if (member is Constructor) {
      inferConstructorParameterTypes(member);
    }
    TypeDependency? typeDependency = engine.typeDependencies.remove(member);
    if (typeDependency != null) {
      ensureMemberType(typeDependency.original);
      typeDependency.copyInferred();
    }
  }

  @override
  void inferConstructorParameterTypes(Constructor target) {
    ConstructorBuilder? constructor = engine.beingInferred[target];
    if (constructor != null) {
      // There is a cyclic dependency where inferring the types of the
      // initializing formals of a constructor required us to infer the
      // corresponding field type which required us to know the type of the
      // constructor.
      String name = target.enclosingClass.name;
      if (target.name.text.isNotEmpty) {
        // TODO(ahe): Use `inferrer.helper.constructorNameForDiagnostics`
        // instead. However, `inferrer.helper` may be null.
        name += ".${target.name.text}";
      }
      constructor.library.addProblem(
          templateCantInferTypeDueToCircularity.withArguments(name),
          target.fileOffset,
          name.length,
          target.fileUri);
      // TODO(johnniwinther): Is this needed? VariableDeclaration.type is
      // non-nullable so the loops have no effect.
      /*for (VariableDeclaration declaration
          in target.function.positionalParameters) {
        declaration.type ??= const InvalidType();
      }
      for (VariableDeclaration declaration in target.function.namedParameters) {
        declaration.type ??= const InvalidType();
      }*/
    } else if ((constructor = engine.toBeInferred[target]) != null) {
      engine.toBeInferred.remove(target);
      engine.beingInferred[target] = constructor!;
      constructor.inferFormalTypes();
      engine.beingInferred.remove(target);
    }
  }

  @override
  InitializerInferenceResult inferInitializer(
      InferenceHelper helper, Initializer initializer) {
    this.helper = helper;
    // Use polymorphic dispatch on [KernelInitializer] to perform whatever
    // kind of type inference is correct for this kind of initializer.
    // TODO(paulberry): experiment to see if dynamic dispatch would be better,
    // so that the type hierarchy will be simpler (which may speed up "is"
    // checks).
    InitializerInferenceResult inferenceResult;
    if (initializer is InitializerJudgment) {
      inferenceResult = initializer.acceptInference(new InferenceVisitor(this));
    } else {
      inferenceResult = initializer.accept(new InferenceVisitor(this));
    }
    this.helper = null;
    return inferenceResult;
  }

  bool isDoubleContext(DartType typeContext) {
    // A context is a double context if double is assignable to it but int is
    // not.  That is the type context is a double context if it is:
    //   * double
    //   * FutureOr<T> where T is a double context
    //
    // We check directly, rather than using isAssignable because it's simpler.
    while (typeContext is FutureOrType) {
      FutureOrType type = typeContext;
      typeContext = type.typeArgument;
    }
    return typeContext is InterfaceType &&
        typeContext.classNode == coreTypes.doubleClass;
  }

  bool isAssignable(DartType contextType, DartType expressionType) {
    if (isNonNullableByDefault) {
      if (expressionType is DynamicType) return true;
      return typeSchemaEnvironment
          .performNullabilityAwareSubtypeCheck(expressionType, contextType)
          .isSubtypeWhenUsingNullabilities();
    }
    return typeSchemaEnvironment
        .performNullabilityAwareSubtypeCheck(expressionType, contextType)
        .orSubtypeCheckFor(contextType, expressionType, typeSchemaEnvironment)
        .isSubtypeWhenIgnoringNullabilities();
  }

  Expression ensureAssignableResult(
      DartType expectedType, ExpressionInferenceResult result,
      {int? fileOffset,
      bool isVoidAllowed: false,
      Template<Message Function(DartType, DartType, bool)>? errorTemplate,
      Template<Message Function(DartType, DartType, bool)>?
          nullabilityErrorTemplate,
      Template<Message Function(DartType, bool)>? nullabilityNullErrorTemplate,
      Template<Message Function(DartType, DartType, bool)>?
          nullabilityNullTypeErrorTemplate,
      Template<Message Function(DartType, DartType, DartType, DartType, bool)>?
          nullabilityPartErrorTemplate}) {
    return ensureAssignable(
        expectedType, result.inferredType, result.expression,
        fileOffset: fileOffset,
        isVoidAllowed: isVoidAllowed,
        errorTemplate: errorTemplate,
        nullabilityErrorTemplate: nullabilityErrorTemplate,
        nullabilityNullErrorTemplate: nullabilityNullErrorTemplate,
        nullabilityNullTypeErrorTemplate: nullabilityNullTypeErrorTemplate,
        nullabilityPartErrorTemplate: nullabilityPartErrorTemplate);
  }

  /// Ensures that [expressionType] is assignable to [contextType].
  ///
  /// Checks whether [expressionType] can be assigned to the greatest closure of
  /// [contextType], and inserts an implicit downcast, inserts a tear-off, or
  /// reports an error if appropriate.
  ///
  /// If [declaredContextType] is provided, this is used instead of
  /// [contextType] for reporting the type against which [expressionType] isn't
  /// assignable. This is used when checking the assignability of return
  /// statements in async functions in which the assignability is checked
  /// against the future value type but the reporting should refer to the
  /// declared return type.
  ///
  /// If [runtimeCheckedType] is provided, this is used for the implicit cast,
  /// otherwise [contextType] is used. This is used for return from async
  /// where the returned expression is wrapped in a `Future`, if necessary,
  /// before returned and therefore shouldn't be checked to be a `Future`
  /// directly.
  Expression ensureAssignable(
      DartType contextType, DartType expressionType, Expression expression,
      {int? fileOffset,
      DartType? declaredContextType,
      DartType? runtimeCheckedType,
      bool isVoidAllowed: false,
      Template<Message Function(DartType, DartType, bool)>? errorTemplate,
      Template<Message Function(DartType, DartType, bool)>?
          nullabilityErrorTemplate,
      Template<Message Function(DartType, bool)>? nullabilityNullErrorTemplate,
      Template<Message Function(DartType, DartType, bool)>?
          nullabilityNullTypeErrorTemplate,
      Template<Message Function(DartType, DartType, DartType, DartType, bool)>?
          nullabilityPartErrorTemplate,
      Map<DartType, NonPromotionReason> Function()? whyNotPromoted}) {
    // ignore: unnecessary_null_comparison
    assert(contextType != null);

    // [errorTemplate], [nullabilityErrorTemplate], and
    // [nullabilityPartErrorTemplate] should be provided together.
    assert((errorTemplate == null) == (nullabilityErrorTemplate == null) &&
        (nullabilityErrorTemplate == null) ==
            (nullabilityPartErrorTemplate == null));
    // [nullabilityNullErrorTemplate] and [nullabilityNullTypeErrorTemplate]
    // should be provided together.
    assert((nullabilityNullErrorTemplate == null) ==
        (nullabilityNullTypeErrorTemplate == null));
    errorTemplate ??= templateInvalidAssignmentError;
    if (nullabilityErrorTemplate == null) {
      // Use [templateInvalidAssignmentErrorNullabilityNull] only if no
      // specific [nullabilityErrorTemplate] template was passed.
      nullabilityNullErrorTemplate ??=
          templateInvalidAssignmentErrorNullabilityNull;
    }
    nullabilityNullTypeErrorTemplate ??= nullabilityErrorTemplate ??
        templateInvalidAssignmentErrorNullabilityNullType;
    nullabilityErrorTemplate ??= templateInvalidAssignmentErrorNullability;
    nullabilityPartErrorTemplate ??=
        templateInvalidAssignmentErrorPartNullability;

    // We don't need to insert assignability checks when doing top level type
    // inference since top level type inference only cares about the type that
    // is inferred (the kernel code is discarded).
    if (isTopLevel) return expression;

    fileOffset ??= expression.fileOffset;
    contextType = computeGreatestClosure(contextType);

    DartType initialContextType = runtimeCheckedType ?? contextType;

    Template<Message Function(DartType, DartType, bool)>?
        preciseTypeErrorTemplate = _getPreciseTypeErrorTemplate(expression);
    AssignabilityResult assignabilityResult = _computeAssignabilityKind(
        contextType, expressionType,
        isNonNullableByDefault: isNonNullableByDefault,
        isVoidAllowed: isVoidAllowed,
        isExpressionTypePrecise: preciseTypeErrorTemplate != null);

    if (assignabilityResult.needsTearOff) {
      TypedTearoff typedTearoff =
          _tearOffCall(expression, expressionType as InterfaceType, fileOffset);
      expression = typedTearoff.tearoff;
      expressionType = typedTearoff.tearoffType;
    }
    if (assignabilityResult.implicitInstantiation != null) {
      ExpressionInferenceResult instantiationResult =
          _applyImplicitInstantiation(assignabilityResult.implicitInstantiation,
              expressionType, expression);
      expression = instantiationResult.expression;
      expressionType = instantiationResult.inferredType;
    }

    Expression result;
    switch (assignabilityResult.kind) {
      case AssignabilityKind.assignable:
        result = expression;
        break;
      case AssignabilityKind.assignableCast:
        // Insert an implicit downcast.
        result = new AsExpression(expression, initialContextType)
          ..isTypeError = true
          ..isForNonNullableByDefault = isNonNullableByDefault
          ..isForDynamic = expressionType is DynamicType
          ..fileOffset = fileOffset;
        break;
      case AssignabilityKind.unassignable:
        // Error: not assignable.  Perform error recovery.
        result = _wrapUnassignableExpression(
            expression,
            expressionType,
            contextType,
            errorTemplate.withArguments(expressionType,
                declaredContextType ?? contextType, isNonNullableByDefault));
        break;
      case AssignabilityKind.unassignableVoid:
        // Error: not assignable.  Perform error recovery.
        result = helper!.wrapInProblem(
            expression, messageVoidExpression, expression.fileOffset, noLength);
        break;
      case AssignabilityKind.unassignablePrecise:
        // The type of the expression is known precisely, so an implicit
        // downcast is guaranteed to fail.  Insert a compile-time error.
        result = helper!.wrapInProblem(
            expression,
            preciseTypeErrorTemplate!.withArguments(
                expressionType, contextType, isNonNullableByDefault),
            expression.fileOffset,
            noLength);
        break;
      case AssignabilityKind.unassignableCantTearoff:
        result = _wrapTearoffErrorExpression(
            expression, contextType, templateNullableTearoffError);
        break;
      case AssignabilityKind.unassignableNullability:
        if (expressionType == assignabilityResult.subtype &&
            contextType == assignabilityResult.supertype) {
          if (expression is NullLiteral &&
              nullabilityNullErrorTemplate != null) {
            result = _wrapUnassignableExpression(
                expression,
                expressionType,
                contextType,
                nullabilityNullErrorTemplate.withArguments(
                    declaredContextType ?? contextType,
                    isNonNullableByDefault));
          } else if (expressionType is NullType) {
            result = _wrapUnassignableExpression(
                expression,
                expressionType,
                contextType,
                nullabilityNullTypeErrorTemplate.withArguments(
                    expressionType,
                    declaredContextType ?? contextType,
                    isNonNullableByDefault));
          } else {
            whyNotPromoted ??= flowAnalysis.whyNotPromoted(expression);
            result = _wrapUnassignableExpression(
                expression,
                expressionType,
                contextType,
                nullabilityErrorTemplate.withArguments(expressionType,
                    declaredContextType ?? contextType, isNonNullableByDefault),
                context: getWhyNotPromotedContext(
                    whyNotPromoted.call(),
                    expression,
                    (type) => typeSchemaEnvironment.isSubtypeOf(type,
                        contextType, SubtypeCheckMode.withNullabilities)));
          }
        } else {
          result = _wrapUnassignableExpression(
              expression,
              expressionType,
              contextType,
              nullabilityPartErrorTemplate.withArguments(
                  expressionType,
                  declaredContextType ?? contextType,
                  assignabilityResult.subtype!,
                  assignabilityResult.supertype!,
                  isNonNullableByDefault));
        }
        break;
      default:
        return unhandled("${assignabilityResult}", "ensureAssignable",
            fileOffset, helper!.uri);
    }

    if (!identical(result, expression)) {
      flowAnalysis.forwardExpression(result, expression);
    }
    return result;
  }

  Expression _wrapTearoffErrorExpression(Expression expression,
      DartType contextType, Template<Message Function(String)> template) {
    // ignore: unnecessary_null_comparison
    assert(template != null);
    Expression errorNode = new AsExpression(
        expression,
        // TODO(ahe): The outline phase doesn't correctly remove invalid
        // uses of type variables, for example, on static members. Once
        // that has been fixed, we should always be able to use
        // [contextType] directly here.
        hasAnyTypeVariables(contextType)
            ? const NeverType.nonNullable()
            : contextType)
      ..isTypeError = true
      ..fileOffset = expression.fileOffset;
    if (contextType is! InvalidType) {
      errorNode = helper!.wrapInProblem(
          errorNode,
          template.withArguments(callName.text),
          errorNode.fileOffset,
          noLength);
    }
    return errorNode;
  }

  Expression _wrapUnassignableExpression(Expression expression,
      DartType expressionType, DartType contextType, Message message,
      {List<LocatedMessage>? context}) {
    Expression errorNode = new AsExpression(
        expression,
        // TODO(ahe): The outline phase doesn't correctly remove invalid
        // uses of type variables, for example, on static members. Once
        // that has been fixed, we should always be able to use
        // [contextType] directly here.
        hasAnyTypeVariables(contextType)
            ? const NeverType.nonNullable()
            : contextType)
      ..isTypeError = true
      ..isForNonNullableByDefault = isNonNullableByDefault
      ..fileOffset = expression.fileOffset;
    if (contextType is! InvalidType && expressionType is! InvalidType) {
      errorNode = helper!.wrapInProblem(
          errorNode, message, errorNode.fileOffset, noLength,
          context: context);
    }
    return errorNode;
  }

  TypedTearoff _tearOffCall(
      Expression expression, InterfaceType expressionType, int fileOffset) {
    Class classNode = expressionType.classNode;
    Member callMember = classHierarchy.getInterfaceMember(classNode, callName)!;
    assert(callMember is Procedure && callMember.kind == ProcedureKind.Method);

    // Replace expression with:
    // `let t = expression in t == null ? null : t.call`
    VariableDeclaration t =
        new VariableDeclaration.forValue(expression, type: expressionType)
          ..fileOffset = fileOffset;

    // TODO(johnniwinther): Avoid null-check for non-nullable expressions.
    Expression nullCheck =
        new EqualsNull(new VariableGet(t)..fileOffset = fileOffset)
          ..fileOffset = fileOffset;

    DartType tearoffType =
        getGetterTypeForMemberTarget(callMember, expressionType)
            .withDeclaredNullability(expressionType.nullability);
    Expression tearOff = new InstanceTearOff(
        InstanceAccessKind.Instance, new VariableGet(t), callName,
        interfaceTarget: callMember as Procedure, resultType: tearoffType)
      ..fileOffset = fileOffset;
    ConditionalExpression conditional = new ConditionalExpression(nullCheck,
        new NullLiteral()..fileOffset = fileOffset, tearOff, tearoffType);
    return new TypedTearoff(
        tearoffType, new Let(t, conditional)..fileOffset = fileOffset);
  }

  /// Computes the assignability kind of [expressionType] to [contextType].
  ///
  /// The computation is side-effect free.
  AssignabilityResult _computeAssignabilityKind(
      DartType contextType, DartType expressionType,
      {required bool isNonNullableByDefault,
      required bool isVoidAllowed,
      required bool isExpressionTypePrecise}) {
    // ignore: unnecessary_null_comparison
    assert(isNonNullableByDefault != null);
    // ignore: unnecessary_null_comparison
    assert(isVoidAllowed != null);
    // ignore: unnecessary_null_comparison
    assert(isExpressionTypePrecise != null);

    // If an interface type is being assigned to a function type, see if we
    // should tear off `.call`.
    // TODO(paulberry): use resolveTypeParameter.  See findInterfaceMember.
    bool needsTearoff = false;
    if (expressionType is InterfaceType) {
      Class classNode = expressionType.classNode;
      Member? callMember =
          classHierarchy.getInterfaceMember(classNode, callName);
      if (callMember is Procedure && callMember.kind == ProcedureKind.Method) {
        if (_shouldTearOffCall(contextType, expressionType)) {
          needsTearoff = true;
          if (isNonNullableByDefault && expressionType.isPotentiallyNullable) {
            return const AssignabilityResult(
                AssignabilityKind.unassignableCantTearoff,
                needsTearOff: false);
          }
          expressionType =
              getGetterTypeForMemberTarget(callMember, expressionType)
                  .withDeclaredNullability(expressionType.nullability);
        }
      }
    }
    ImplicitInstantiation? implicitInstantiation;
    if (library.enableConstructorTearOffsInLibrary) {
      implicitInstantiation =
          computeImplicitInstantiation(expressionType, contextType);
      if (implicitInstantiation != null) {
        expressionType = implicitInstantiation.instantiatedType;
      }
    }

    if (expressionType is VoidType && !isVoidAllowed) {
      assert(implicitInstantiation == null);
      assert(!needsTearoff);
      return const AssignabilityResult(AssignabilityKind.unassignableVoid,
          needsTearOff: false);
    }

    IsSubtypeOf isDirectSubtypeResult = typeSchemaEnvironment
        .performNullabilityAwareSubtypeCheck(expressionType, contextType);
    bool isDirectlyAssignable = isNonNullableByDefault
        ? isDirectSubtypeResult.isSubtypeWhenUsingNullabilities()
        : isDirectSubtypeResult.isSubtypeWhenIgnoringNullabilities();
    if (isDirectlyAssignable) {
      return new AssignabilityResult(AssignabilityKind.assignable,
          needsTearOff: needsTearoff,
          implicitInstantiation: implicitInstantiation);
    }

    bool isIndirectlyAssignable = isNonNullableByDefault
        ? expressionType is DynamicType
        : typeSchemaEnvironment
            .performNullabilityAwareSubtypeCheck(contextType, expressionType)
            .isSubtypeWhenIgnoringNullabilities();
    if (!isIndirectlyAssignable) {
      if (isNonNullableByDefault &&
          isDirectSubtypeResult.isSubtypeWhenIgnoringNullabilities()) {
        return new AssignabilityResult.withTypes(
            AssignabilityKind.unassignableNullability,
            isDirectSubtypeResult.subtype,
            isDirectSubtypeResult.supertype,
            needsTearOff: needsTearoff,
            implicitInstantiation: implicitInstantiation);
      } else {
        return new AssignabilityResult(AssignabilityKind.unassignable,
            needsTearOff: needsTearoff,
            implicitInstantiation: implicitInstantiation);
      }
    }
    if (isExpressionTypePrecise) {
      // The type of the expression is known precisely, so an implicit
      // downcast is guaranteed to fail.  Insert a compile-time error.
      assert(implicitInstantiation == null);
      assert(!needsTearoff);
      return const AssignabilityResult(AssignabilityKind.unassignablePrecise,
          needsTearOff: false);
    }
    // Insert an implicit downcast.
    return new AssignabilityResult(AssignabilityKind.assignableCast,
        needsTearOff: needsTearoff,
        implicitInstantiation: implicitInstantiation);
  }

  bool isNull(DartType type) {
    return type is NullType;
  }

  /// Computes the type arguments for an access to an extension instance member
  /// on [extension] with the static [receiverType]. If [explicitTypeArguments]
  /// are provided, these are returned, otherwise type arguments are inferred
  /// using [receiverType].
  List<DartType> computeExtensionTypeArgument(Extension extension,
      List<DartType>? explicitTypeArguments, DartType receiverType) {
    if (explicitTypeArguments != null) {
      assert(explicitTypeArguments.length == extension.typeParameters.length);
      return explicitTypeArguments;
    } else if (extension.typeParameters.isEmpty) {
      assert(explicitTypeArguments == null);
      return const <DartType>[];
    } else {
      return inferExtensionTypeArguments(extension, receiverType);
    }
  }

  /// Infers the type arguments for an access to an extension instance member
  /// on [extension] with the static [receiverType].
  List<DartType> inferExtensionTypeArguments(
      Extension extension, DartType receiverType) {
    List<TypeParameter> typeParameters = extension.typeParameters;
    DartType onType = extension.onType;
    List<DartType> inferredTypes =
        new List<DartType>.filled(typeParameters.length, const UnknownType());
    typeSchemaEnvironment.inferGenericFunctionOrType(null, typeParameters,
        [onType], [receiverType], null, inferredTypes, library.library);
    return inferredTypes;
  }

  ObjectAccessTarget _findShownExtensionTypeMember(
      ExtensionType receiverType, Name name, int fileOffset,
      {required ObjectAccessTarget defaultTarget,
      required CallSiteAccessKind callSiteAccessKind,
      required bool isPotentiallyNullable}) {
    Extension extension = receiverType.extension;
    ExtensionTypeShowHideClause? showHideClause = extension.showHideClause;
    if (showHideClause == null) return defaultTarget;

    kernel.Reference? reference = showHideClause.findShownReference(
        name, callSiteAccessKind, classHierarchy);
    if (reference != null) {
      return new ObjectAccessTarget.interfaceMember(reference.asMember,
          isPotentiallyNullable: isPotentiallyNullable);
    } else {
      return defaultTarget;
    }
  }

  /// Returns extension member declared immediately for [receiverType].
  ///
  /// If none is found, [defaultTarget] is returned.
  ObjectAccessTarget _findDirectExtensionTypeMember(
      ExtensionType receiverType, Name name, int fileOffset,
      {required ObjectAccessTarget defaultTarget, required bool isSetter}) {
    Member? targetMember;
    Member? targetTearoff;
    ProcedureKind? targetKind;
    for (ExtensionMemberDescriptor descriptor
        in receiverType.extension.members) {
      if (descriptor.name == name) {
        switch (descriptor.kind) {
          case ExtensionMemberKind.Method:
            if (!isSetter) {
              targetMember = descriptor.member.asMember;
              targetTearoff ??= targetMember;
              targetKind = ProcedureKind.Method;
            }
            break;
          case ExtensionMemberKind.TearOff:
            if (!isSetter) {
              targetTearoff = descriptor.member.asMember;
            }
            break;
          case ExtensionMemberKind.Getter:
            if (!isSetter) {
              targetMember = descriptor.member.asMember;
              targetTearoff = null;
              targetKind = ProcedureKind.Getter;
            }
            break;
          case ExtensionMemberKind.Setter:
            if (isSetter) {
              targetMember = descriptor.member.asMember;
              targetTearoff = null;
              targetKind = ProcedureKind.Setter;
            }
            break;
          case ExtensionMemberKind.Operator:
            if (!isSetter) {
              targetMember = descriptor.member.asMember;
              targetTearoff = null;
              targetKind = ProcedureKind.Operator;
            }
            break;
          default:
            unhandled("${descriptor.kind}", "_findDirectExtensionMember",
                fileOffset, library.fileUri);
        }
      }
    }
    if (targetMember != null) {
      assert(targetKind != null);
      return new ObjectAccessTarget.extensionMember(
          targetMember, targetTearoff, targetKind!, receiverType.typeArguments);
    } else {
      return defaultTarget;
    }
  }

  /// Returns the extension member access by the given [name] for a receiver
  /// with the static [receiverType].
  ///
  /// If none is found, [defaultTarget] is returned.
  ///
  /// If multiple are found, none more specific, an
  /// [AmbiguousExtensionAccessTarget] is returned. This access kind results in
  /// a compile-time error, but is used to provide a better message than just
  /// reporting that the receiver does not have a member by the given name.
  ///
  /// If [isPotentiallyNullableAccess] is `true`, the returned extension member
  /// is flagged as a nullable extension member access. This access kind results
  /// in a compile-time error, but is used to provide a better message than just
  /// reporting that the receiver does not have a member by the given name.
  ObjectAccessTarget? _findExtensionMember(
      DartType receiverType, Class classNode, Name name, int fileOffset,
      {bool setter: false,
      ObjectAccessTarget? defaultTarget,
      bool isPotentiallyNullableAccess: false}) {
    Name otherName = name;
    bool otherIsSetter;
    if (name == indexGetName) {
      // [] must be checked against []=.
      otherName = indexSetName;
      otherIsSetter = false;
    } else if (name == indexSetName) {
      // []= must be checked against [].
      otherName = indexGetName;
      otherIsSetter = false;
    } else {
      otherName = name;
      otherIsSetter = !setter;
    }

    Member? otherMember =
        _getInterfaceMember(classNode, otherName, otherIsSetter, fileOffset);
    if (otherMember != null) {
      // If we're looking for `foo` and `foo=` can be found or vice-versa then
      // extension methods should not be found.
      return defaultTarget;
    }

    ExtensionAccessCandidate? bestSoFar;
    List<ExtensionAccessCandidate> noneMoreSpecific = [];
    library.forEachExtensionInScope((ExtensionBuilder extensionBuilder) {
      MemberBuilder? thisBuilder = extensionBuilder
          .lookupLocalMemberByName(name, setter: setter) as MemberBuilder?;
      MemberBuilder? otherBuilder = extensionBuilder.lookupLocalMemberByName(
          otherName,
          setter: otherIsSetter) as MemberBuilder?;
      if ((thisBuilder != null && !thisBuilder.isStatic) ||
          (otherBuilder != null && !otherBuilder.isStatic)) {
        DartType onType;
        DartType onTypeInstantiateToBounds;
        List<DartType> inferredTypeArguments;
        if (extensionBuilder.extension.typeParameters.isEmpty) {
          onTypeInstantiateToBounds =
              onType = extensionBuilder.extension.onType;
          inferredTypeArguments = const <DartType>[];
        } else {
          List<TypeParameter> typeParameters =
              extensionBuilder.extension.typeParameters;
          inferredTypeArguments = inferExtensionTypeArguments(
              extensionBuilder.extension, receiverType);
          Substitution inferredSubstitution =
              Substitution.fromPairs(typeParameters, inferredTypeArguments);

          for (int index = 0; index < typeParameters.length; index++) {
            TypeParameter typeParameter = typeParameters[index];
            DartType typeArgument = inferredTypeArguments[index];
            DartType bound =
                inferredSubstitution.substituteType(typeParameter.bound);
            if (!typeSchemaEnvironment.isSubtypeOf(
                typeArgument, bound, SubtypeCheckMode.withNullabilities)) {
              return;
            }
          }
          onType = inferredSubstitution
              .substituteType(extensionBuilder.extension.onType);
          List<DartType> instantiateToBoundTypeArguments = calculateBounds(
              typeParameters, coreTypes.objectClass, library.library);
          Substitution instantiateToBoundsSubstitution = Substitution.fromPairs(
              typeParameters, instantiateToBoundTypeArguments);
          onTypeInstantiateToBounds = instantiateToBoundsSubstitution
              .substituteType(extensionBuilder.extension.onType);
        }

        if (typeSchemaEnvironment.isSubtypeOf(
            receiverType, onType, SubtypeCheckMode.withNullabilities)) {
          ObjectAccessTarget target = const ObjectAccessTarget.missing();
          if (thisBuilder != null && !thisBuilder.isStatic) {
            if (thisBuilder.isField) {
              if (thisBuilder.isExternal) {
                target = new ObjectAccessTarget.extensionMember(
                    setter ? thisBuilder.writeTarget! : thisBuilder.readTarget!,
                    thisBuilder.readTarget,
                    setter ? ProcedureKind.Setter : ProcedureKind.Getter,
                    inferredTypeArguments,
                    isPotentiallyNullable: isPotentiallyNullableAccess);
              }
            } else {
              target = new ObjectAccessTarget.extensionMember(
                  setter ? thisBuilder.writeTarget! : thisBuilder.invokeTarget!,
                  thisBuilder.readTarget,
                  thisBuilder.kind!,
                  inferredTypeArguments,
                  isPotentiallyNullable: isPotentiallyNullableAccess);
            }
          }
          ExtensionAccessCandidate candidate = new ExtensionAccessCandidate(
              (thisBuilder ?? otherBuilder)!,
              onType,
              onTypeInstantiateToBounds,
              target,
              isPlatform: extensionBuilder.library.importUri.scheme == 'dart');
          if (noneMoreSpecific.isNotEmpty) {
            bool isMostSpecific = true;
            for (ExtensionAccessCandidate other in noneMoreSpecific) {
              bool? isMoreSpecific =
                  candidate.isMoreSpecificThan(typeSchemaEnvironment, other);
              if (isMoreSpecific != true) {
                isMostSpecific = false;
                break;
              }
            }
            if (isMostSpecific) {
              bestSoFar = candidate;
              noneMoreSpecific.clear();
            } else {
              noneMoreSpecific.add(candidate);
            }
          } else if (bestSoFar == null) {
            bestSoFar = candidate;
          } else {
            bool? isMoreSpecific =
                candidate.isMoreSpecificThan(typeSchemaEnvironment, bestSoFar!);
            if (isMoreSpecific == true) {
              bestSoFar = candidate;
            } else if (isMoreSpecific == null) {
              noneMoreSpecific.add(bestSoFar!);
              noneMoreSpecific.add(candidate);
              bestSoFar = null;
            }
          }
        }
      }
    });
    if (bestSoFar != null) {
      return bestSoFar!.target;
    } else {
      if (noneMoreSpecific.isNotEmpty) {
        return new AmbiguousExtensionAccessTarget(noneMoreSpecific);
      }
    }
    return defaultTarget;
  }

  /// Finds a member of [receiverType] called [name], and if it is found,
  /// reports it through instrumentation using [fileOffset].
  ///
  /// For the case where [receiverType] is a [FunctionType], and the name
  /// is `call`, the string 'call' is returned as a sentinel object.
  ///
  /// For the case where [receiverType] is `dynamic`, and the name is declared
  /// in Object, the member from Object is returned though the call may not end
  /// up targeting it if the arguments do not match (the basic principle is that
  /// the Object member is used for inferring types only if noSuchMethod cannot
  /// be targeted due to, e.g., an incorrect argument count).
  ObjectAccessTarget findInterfaceMember(
      DartType receiverType, Name name, int fileOffset,
      {required CallSiteAccessKind callSiteAccessKind,
      bool instrumented: true,
      bool includeExtensionMethods: false}) {
    // ignore: unnecessary_null_comparison
    assert(receiverType != null && isKnown(receiverType));

    bool isSetter = callSiteAccessKind == CallSiteAccessKind.setterInvocation;

    DartType receiverBound = resolveTypeParameter(receiverType);

    bool isReceiverTypePotentiallyNullable = isNonNullableByDefault &&
        receiverType.isPotentiallyNullable &&
        // Calls to `==` are always on a non-null receiver.
        name != equalsName;

    Class classNode = receiverBound is InterfaceType
        ? receiverBound.classNode
        : coreTypes.objectClass;

    if (isReceiverTypePotentiallyNullable) {
      Member? member = _getInterfaceMember(
          coreTypes.objectClass, name, isSetter, fileOffset);
      if (member != null) {
        // Null implements all Object members so this is not considered a
        // potentially nullable access.
        return new ObjectAccessTarget.objectMember(member);
      }
      if (includeExtensionMethods && receiverBound is! DynamicType) {
        ObjectAccessTarget? target = _findExtensionMember(
            isNonNullableByDefault ? receiverType : receiverBound,
            coreTypes.objectClass,
            name,
            fileOffset,
            setter: isSetter);
        if (target != null) {
          return target;
        }
      }
    }

    if (receiverBound is FunctionType && name == callName) {
      return isReceiverTypePotentiallyNullable
          ? const ObjectAccessTarget.nullableCallFunction()
          : const ObjectAccessTarget.callFunction();
    } else if (receiverBound is NeverType) {
      switch (receiverBound.nullability) {
        case Nullability.nonNullable:
          return const ObjectAccessTarget.never();
        case Nullability.nullable:
        case Nullability.legacy:
          // Never? and Never* are equivalent to Null.
          return findInterfaceMember(const NullType(), name, fileOffset,
              callSiteAccessKind: callSiteAccessKind);
        case Nullability.undetermined:
          return internalProblem(
              templateInternalProblemUnsupportedNullability.withArguments(
                  "${receiverBound.nullability}",
                  receiverBound,
                  isNonNullableByDefault),
              fileOffset,
              library.fileUri);
      }
    }

    ObjectAccessTarget? target;
    Member? interfaceMember =
        _getInterfaceMember(classNode, name, isSetter, fileOffset);
    if (interfaceMember != null) {
      target = new ObjectAccessTarget.interfaceMember(interfaceMember,
          isPotentiallyNullable: isReceiverTypePotentiallyNullable);
    } else if (receiverBound is DynamicType) {
      target = const ObjectAccessTarget.dynamic();
    } else if (receiverBound is InvalidType) {
      target = const ObjectAccessTarget.invalid();
    } else if (receiverBound is InterfaceType &&
        receiverBound.classNode == coreTypes.functionClass &&
        name == callName) {
      target = isReceiverTypePotentiallyNullable
          ? const ObjectAccessTarget.nullableCallFunction()
          : const ObjectAccessTarget.callFunction();
    } else if (library.enableExtensionTypesInLibrary &&
        receiverBound is ExtensionType) {
      target = _findDirectExtensionTypeMember(receiverBound, name, fileOffset,
          isSetter: isSetter,
          defaultTarget: const ObjectAccessTarget.missing());
      if (target.kind == ObjectAccessTargetKind.missing) {
        target = _findShownExtensionTypeMember(receiverBound, name, fileOffset,
            callSiteAccessKind: callSiteAccessKind,
            isPotentiallyNullable: isReceiverTypePotentiallyNullable,
            defaultTarget: const ObjectAccessTarget.missing());
      }
    } else {
      target = const ObjectAccessTarget.missing();
    }
    if (instrumented &&
        receiverBound != const DynamicType() &&
        (target.isInstanceMember || target.isObjectMember)) {
      instrumentation?.record(uriForInstrumentation, fileOffset, 'target',
          new InstrumentationValueForMember(target.member!));
    }

    if (target.isMissing && includeExtensionMethods) {
      if (isReceiverTypePotentiallyNullable) {
        // When the receiver type is potentially nullable we would have found
        // the extension member above, if available. Therefore we know that we
        // are in an erroneous case and instead look up the extension member on
        // the non-nullable receiver bound but flag the found target as a
        // nullable extension member access. This is done to provide the better
        // error message that the extension member exists but that the access is
        // invalid.
        target = _findExtensionMember(
            isNonNullableByDefault
                ? receiverType.toNonNull()
                : receiverBound.toNonNull(),
            classNode,
            name,
            fileOffset,
            setter: isSetter,
            defaultTarget: target,
            isPotentiallyNullableAccess: true)!;
      } else {
        target = _findExtensionMember(
            isNonNullableByDefault ? receiverType : receiverBound,
            classNode,
            name,
            fileOffset,
            setter: isSetter,
            defaultTarget: target)!;
      }
    }
    return target;
  }

  /// If target is missing on a non-dynamic receiver, an error is reported
  /// using [errorTemplate] and an invalid expression is returned.
  Expression? reportMissingInterfaceMember(
      ObjectAccessTarget target,
      DartType receiverType,
      Name name,
      int fileOffset,
      Template<Message Function(String, DartType, bool)> errorTemplate) {
    // ignore: unnecessary_null_comparison
    assert(receiverType != null && isKnown(receiverType));
    // ignore: unnecessary_null_comparison
    if (!isTopLevel && target.isMissing && errorTemplate != null) {
      int length = name.text.length;
      if (identical(name.text, callName.text) ||
          identical(name.text, unaryMinusName.text)) {
        length = 1;
      }
      return helper!.buildProblem(
          errorTemplate.withArguments(name.text,
              resolveTypeParameter(receiverType), isNonNullableByDefault),
          fileOffset,
          length);
    }
    return null;
  }

  /// Returns [type] as passed from [superClass] to the current class.
  ///
  /// If a legacy class occurs between the current class and [superClass] then
  /// [type] needs to be legacy erased. For instance
  ///
  ///    // Opt in:
  ///    class Super {
  ///      int extendedMethod(int i, {required int j}) => i;
  ///    }
  ///    class Mixin {
  ///      int mixedInMethod(int i, {required int j}) => i;
  ///    }
  ///    // Opt out:
  ///    class Legacy extends Super with Mixin {}
  ///    // Opt in:
  ///    class Class extends Legacy {
  ///      test() {
  ///        // Ok to call `Legacy.extendedMethod` since its type is
  ///        // `int* Function(int*, {int* j})`.
  ///        super.extendedMethod(null);
  ///        // Ok to call `Legacy.mixedInMethod` since its type is
  ///        // `int* Function(int*, {int* j})`.
  ///        super.mixedInMethod(null);
  ///      }
  ///    }
  ///
  DartType computeTypeFromSuperClass(Class superClass, DartType type) {
    if (needsLegacyErasure(thisType!.classNode, superClass)) {
      type = legacyErasure(type);
    }
    return type;
  }

  /// Returns the type of [target] when accessed as a getter on [receiverType].
  ///
  /// For instance
  ///
  ///    class Class<T> {
  ///      T method() {}
  ///      T getter => null;
  ///    }
  ///
  ///    Class<int> c = ...
  ///    c.method; // The getter type is `int Function()`.
  ///    c.getter; // The getter type is `int`.
  ///
  DartType getGetterType(ObjectAccessTarget target, DartType receiverType) {
    switch (target.kind) {
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
        return receiverType;
      case ObjectAccessTargetKind.invalid:
        return const InvalidType();
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        return const DynamicType();
      case ObjectAccessTargetKind.never:
        return const NeverType.nonNullable();
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        return getGetterTypeForMemberTarget(target.member!, receiverType);
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        switch (target.extensionMethodKind) {
          case ProcedureKind.Method:
          case ProcedureKind.Operator:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            List<TypeParameter> extensionTypeParameters = functionType
                .typeParameters
                .take(target.inferredExtensionTypeArguments.length)
                .toList();
            Substitution substitution = Substitution.fromPairs(
                extensionTypeParameters, target.inferredExtensionTypeArguments);
            DartType resultType = substitution.substituteType(new FunctionType(
                functionType.positionalParameters.skip(1).toList(),
                functionType.returnType,
                library.nonNullable,
                namedParameters: functionType.namedParameters,
                typeParameters: functionType.typeParameters
                    .skip(target.inferredExtensionTypeArguments.length)
                    .toList(),
                requiredParameterCount:
                    functionType.requiredParameterCount - 1));
            if (!isNonNullableByDefault) {
              resultType = legacyErasure(resultType);
            }
            return resultType;
          case ProcedureKind.Getter:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            List<TypeParameter> extensionTypeParameters = functionType
                .typeParameters
                .take(target.inferredExtensionTypeArguments.length)
                .toList();
            Substitution substitution = Substitution.fromPairs(
                extensionTypeParameters, target.inferredExtensionTypeArguments);
            DartType resultType =
                substitution.substituteType(functionType.returnType);
            if (!isNonNullableByDefault) {
              resultType = legacyErasure(resultType);
            }
            return resultType;
          case ProcedureKind.Setter:
          case ProcedureKind.Factory:
            break;
        }
    }
    throw unhandled('$target', 'getGetterType', -1, null);
  }

  /// Returns the getter type of [interfaceMember] on a receiver of type
  /// [receiverType].
  ///
  /// For instance
  ///
  ///    class Class<T> {
  ///      T method() {}
  ///      T getter => null;
  ///    }
  ///
  ///    Class<int> c = ...
  ///    c.method; // The getter type is `int Function()`.
  ///    c.getter; // The getter type is `int`.
  ///
  DartType getGetterTypeForMemberTarget(
      Member interfaceMember, DartType receiverType) {
    Class memberClass = interfaceMember.enclosingClass!;
    assert(interfaceMember is Field || interfaceMember is Procedure,
        "Unexpected interface member $interfaceMember.");
    DartType calleeType = interfaceMember.getterType;
    if (memberClass.typeParameters.isNotEmpty) {
      receiverType = resolveTypeParameter(receiverType);
      if (receiverType is InterfaceType) {
        List<DartType> castedTypeArguments = classHierarchy
            .getTypeArgumentsAsInstanceOf(receiverType, memberClass)!;
        calleeType = Substitution.fromPairs(
                memberClass.typeParameters, castedTypeArguments)
            .substituteType(calleeType);
      }
    }
    if (!isNonNullableByDefault) {
      calleeType = legacyErasure(calleeType);
    }
    return calleeType;
  }

  /// Returns the type of [target] when accessed as an invocation on
  /// [receiverType].
  ///
  /// If the target is known not to be invokable [unknownFunction] is returned.
  ///
  /// For instance
  ///
  ///    class Class<T> {
  ///      T method() {}
  ///      T Function() getter1 => null;
  ///      T getter2 => null;
  ///    }
  ///
  ///    Class<int> c = ...
  ///    c.method; // The getter type is `int Function()`.
  ///    c.getter1; // The getter type is `int Function()`.
  ///    c.getter2; // The getter type is [unknownFunction].
  ///
  FunctionType getFunctionType(
      ObjectAccessTarget target, DartType receiverType) {
    switch (target.kind) {
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
        return _getFunctionType(receiverType);
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.never:
      case ObjectAccessTargetKind.invalid:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        return unknownFunction;
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        return _getFunctionType(
            getGetterTypeForMemberTarget(target.member!, receiverType));
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        switch (target.extensionMethodKind) {
          case ProcedureKind.Method:
          case ProcedureKind.Operator:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            if (!isNonNullableByDefault) {
              functionType = legacyErasure(functionType) as FunctionType;
            }
            return functionType;
          case ProcedureKind.Getter:
            // TODO(johnniwinther): Handle implicit .call on extension getter.
            return _getFunctionType(target.member!.function!.returnType);
          case ProcedureKind.Setter:
          case ProcedureKind.Factory:
            break;
        }
    }
    throw unhandled('$target', 'getFunctionType', -1, null);
  }

  /// Returns the type of the receiver argument in an access to an extension
  /// member on [extension] with the given extension [typeArguments].
  DartType getExtensionReceiverType(
      Extension extension, List<DartType> typeArguments) {
    DartType receiverType = extension.onType;
    if (extension.typeParameters.isNotEmpty) {
      Substitution substitution =
          Substitution.fromPairs(extension.typeParameters, typeArguments);
      return substitution.substituteType(receiverType);
    }
    return receiverType;
  }

  /// Returns the return type of the invocation of [target] on [receiverType].
  // TODO(johnniwinther): Cleanup [getFunctionType], [getReturnType],
  // [getIndexKeyType] and [getIndexSetValueType]. We shouldn't need that many.
  DartType getReturnType(ObjectAccessTarget target, DartType receiverType) {
    switch (target.kind) {
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        FunctionType functionType = _getFunctionType(
            getGetterTypeForMemberTarget(target.member!, receiverType));
        return functionType.returnType;
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        switch (target.extensionMethodKind) {
          case ProcedureKind.Operator:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            DartType returnType = functionType.returnType;
            if (functionType.typeParameters.isNotEmpty) {
              Substitution substitution = Substitution.fromPairs(
                  functionType.typeParameters,
                  target.inferredExtensionTypeArguments);
              returnType = substitution.substituteType(returnType);
            }
            if (!isNonNullableByDefault) {
              returnType = legacyErasure(returnType);
            }
            return returnType;
          default:
            throw unhandled('$target', 'getFunctionType', -1, null);
        }
      case ObjectAccessTargetKind.never:
        return const NeverType.nonNullable();
      case ObjectAccessTargetKind.invalid:
        return const InvalidType();
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        break;
    }
    return const DynamicType();
  }

  DartType getPositionalParameterTypeForTarget(
      ObjectAccessTarget target, DartType receiverType, int index) {
    switch (target.kind) {
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        FunctionType functionType = _getFunctionType(
            getGetterTypeForMemberTarget(target.member!, receiverType));
        if (functionType.positionalParameters.length > index) {
          return functionType.positionalParameters[index];
        }
        break;
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        FunctionType functionType =
            target.member!.function!.computeFunctionType(library.nonNullable);
        if (functionType.positionalParameters.length > index + 1) {
          DartType keyType = functionType.positionalParameters[index + 1];
          if (functionType.typeParameters.isNotEmpty) {
            Substitution substitution = Substitution.fromPairs(
                functionType.typeParameters,
                target.inferredExtensionTypeArguments);
            keyType = substitution.substituteType(keyType);
          }
          if (!isNonNullableByDefault) {
            keyType = legacyErasure(keyType);
          }
          return keyType;
        }
        break;
      case ObjectAccessTargetKind.invalid:
        return const InvalidType();
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.never:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        break;
    }
    return const DynamicType();
  }

  /// Returns the type of the 'key' parameter in an [] or []= implementation.
  ///
  /// For instance
  ///
  ///    class Class<K, V> {
  ///      V operator [](K key) => null;
  ///      void operator []=(K key, V value) {}
  ///    }
  ///
  ///    extension Extension<K, V> on Class<K, V> {
  ///      V operator [](K key) => null;
  ///      void operator []=(K key, V value) {}
  ///    }
  ///
  ///    new Class<int, String>()[0];             // The key type is `int`.
  ///    new Class<int, String>()[0] = 'foo';     // The key type is `int`.
  ///    Extension<int, String>(null)[0];         // The key type is `int`.
  ///    Extension<int, String>(null)[0] = 'foo'; // The key type is `int`.
  ///
  DartType getIndexKeyType(ObjectAccessTarget target, DartType receiverType) {
    switch (target.kind) {
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        FunctionType functionType = _getFunctionType(
            getGetterTypeForMemberTarget(target.member!, receiverType));
        if (functionType.positionalParameters.length >= 1) {
          return functionType.positionalParameters[0];
        }
        break;
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        switch (target.extensionMethodKind) {
          case ProcedureKind.Operator:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            if (functionType.positionalParameters.length >= 2) {
              DartType keyType = functionType.positionalParameters[1];
              if (functionType.typeParameters.isNotEmpty) {
                Substitution substitution = Substitution.fromPairs(
                    functionType.typeParameters,
                    target.inferredExtensionTypeArguments);
                keyType = substitution.substituteType(keyType);
              }
              if (!isNonNullableByDefault) {
                keyType = legacyErasure(keyType);
              }
              return keyType;
            }
            break;
          default:
            throw unhandled('$target', 'getFunctionType', -1, null);
        }
        break;
      case ObjectAccessTargetKind.invalid:
        return const InvalidType();
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.never:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        break;
    }
    return const DynamicType();
  }

  /// Returns the type of the 'value' parameter in an []= implementation.
  ///
  /// For instance
  ///
  ///    class Class<K, V> {
  ///      void operator []=(K key, V value) {}
  ///    }
  ///
  ///    extension Extension<K, V> on Class<K, V> {
  ///      void operator []=(K key, V value) {}
  ///    }
  ///
  ///    new Class<int, String>()[0] = 'foo';     // The value type is `String`.
  ///    Extension<int, String>(null)[0] = 'foo'; // The value type is `String`.
  ///
  DartType getIndexSetValueType(
      ObjectAccessTarget target, DartType? receiverType) {
    switch (target.kind) {
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        FunctionType functionType = _getFunctionType(
            getGetterTypeForMemberTarget(target.member!, receiverType!));
        if (functionType.positionalParameters.length >= 2) {
          return functionType.positionalParameters[1];
        }
        break;
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        switch (target.extensionMethodKind) {
          case ProcedureKind.Operator:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            if (functionType.positionalParameters.length >= 3) {
              DartType indexType = functionType.positionalParameters[2];
              if (functionType.typeParameters.isNotEmpty) {
                Substitution substitution = Substitution.fromPairs(
                    functionType.typeParameters,
                    target.inferredExtensionTypeArguments);
                indexType = substitution.substituteType(indexType);
              }
              if (!isNonNullableByDefault) {
                indexType = legacyErasure(indexType);
              }
              return indexType;
            }
            break;
          default:
            throw unhandled('$target', 'getFunctionType', -1, null);
        }
        break;
      case ObjectAccessTargetKind.invalid:
        return const InvalidType();
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.never:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        break;
    }
    return const DynamicType();
  }

  FunctionType _getFunctionType(DartType calleeType) {
    calleeType = resolveTypeParameter(calleeType);
    if (calleeType is FunctionType) {
      if (!isNonNullableByDefault) {
        calleeType = legacyErasure(calleeType);
      }
      return calleeType as FunctionType;
    }
    return unknownFunction;
  }

  FunctionType getFunctionTypeForImplicitCall(DartType calleeType) {
    calleeType = resolveTypeParameter(calleeType);
    if (calleeType is FunctionType) {
      if (!isNonNullableByDefault) {
        calleeType = legacyErasure(calleeType);
      }
      return calleeType as FunctionType;
    } else if (calleeType is InterfaceType) {
      Member? member =
          _getInterfaceMember(calleeType.classNode, callName, false, -1);
      if (member != null) {
        DartType callType = getGetterTypeForMemberTarget(member, calleeType);
        if (callType is FunctionType) {
          if (!isNonNullableByDefault) {
            callType = legacyErasure(callType);
          }
          return callType as FunctionType;
        }
      }
    }
    return unknownFunction;
  }

  DartType? getDerivedTypeArgumentOf(DartType type, Class class_) {
    if (type is InterfaceType) {
      List<DartType>? typeArgumentsAsInstanceOfClass =
          classHierarchy.getTypeArgumentsAsInstanceOf(type, class_);
      if (typeArgumentsAsInstanceOfClass != null) {
        return typeArgumentsAsInstanceOfClass[0];
      }
    }
    return null;
  }

  /// If the [member] is a forwarding stub, return the target it forwards to.
  /// Otherwise return the given [member].
  Member getRealTarget(Member member) {
    if (member is Procedure && member.isForwardingStub) {
      return member.abstractForwardingStubTarget!;
    }
    return member;
  }

  DartType getSetterType(ObjectAccessTarget target, DartType receiverType) {
    switch (target.kind) {
      case ObjectAccessTargetKind.dynamic:
      case ObjectAccessTargetKind.never:
      case ObjectAccessTargetKind.missing:
      case ObjectAccessTargetKind.ambiguous:
        return const DynamicType();
      case ObjectAccessTargetKind.invalid:
        return const InvalidType();
      case ObjectAccessTargetKind.instanceMember:
      case ObjectAccessTargetKind.objectMember:
      case ObjectAccessTargetKind.nullableInstanceMember:
        Member interfaceMember = target.member!;
        Class memberClass = interfaceMember.enclosingClass!;
        DartType setterType;
        if (interfaceMember is Procedure) {
          assert(interfaceMember.kind == ProcedureKind.Setter);
          List<VariableDeclaration> setterParameters =
              interfaceMember.function.positionalParameters;
          setterType = setterParameters.length > 0
              ? setterParameters[0].type
              : const DynamicType();
        } else if (interfaceMember is Field) {
          setterType = interfaceMember.type;
        } else {
          throw unhandled(interfaceMember.runtimeType.toString(),
              'getSetterType', -1, null);
        }
        if (memberClass.typeParameters.isNotEmpty) {
          receiverType = resolveTypeParameter(receiverType);
          if (receiverType is InterfaceType) {
            setterType = Substitution.fromPairs(
                    memberClass.typeParameters,
                    classHierarchy.getTypeArgumentsAsInstanceOf(
                        receiverType, memberClass)!)
                .substituteType(setterType);
          }
        }
        if (!isNonNullableByDefault) {
          setterType = legacyErasure(setterType);
        }
        return setterType;
      case ObjectAccessTargetKind.extensionMember:
      case ObjectAccessTargetKind.nullableExtensionMember:
        switch (target.extensionMethodKind) {
          case ProcedureKind.Setter:
            FunctionType functionType = target.member!.function!
                .computeFunctionType(library.nonNullable);
            List<TypeParameter> extensionTypeParameters = functionType
                .typeParameters
                .take(target.inferredExtensionTypeArguments.length)
                .toList();
            Substitution substitution = Substitution.fromPairs(
                extensionTypeParameters, target.inferredExtensionTypeArguments);
            DartType setterType = substitution
                .substituteType(functionType.positionalParameters[1]);
            if (!isNonNullableByDefault) {
              setterType = legacyErasure(setterType);
            }
            return setterType;
          case ProcedureKind.Method:
          case ProcedureKind.Getter:
          case ProcedureKind.Factory:
          case ProcedureKind.Operator:
            break;
        }
        // TODO(johnniwinther): Compute the right setter type.
        return const DynamicType();
      case ObjectAccessTargetKind.callFunction:
      case ObjectAccessTargetKind.nullableCallFunction:
        break;
    }
    throw unhandled(target.runtimeType.toString(), 'getSetterType', -1, null);
  }

  DartType getTypeArgumentOf(DartType type, Class class_) {
    if (type is InterfaceType && identical(type.classNode, class_)) {
      return type.typeArguments[0];
    } else {
      return const UnknownType();
    }
  }

  /// Modifies a type as appropriate when inferring a declared variable's type.
  @override
  DartType inferDeclarationType(DartType initializerType,
      {bool forSyntheticVariable: false}) {
    if (initializerType is NullType) {
      // If the initializer type is Null or bottom, the inferred type is
      // dynamic.
      // TODO(paulberry): this rule is inherited from analyzer behavior but is
      // not spec'ed anywhere.
      return const DynamicType();
    }
    if (forSyntheticVariable) {
      return normalizeNullabilityInLibrary(initializerType, library.library);
    } else {
      return demoteTypeInLibrary(initializerType, library.library);
    }
  }

  void inferSyntheticVariable(VariableDeclarationImpl variable) {
    assert(variable.isImplicitlyTyped);
    assert(variable.initializer != null);
    ExpressionInferenceResult result = inferExpression(
        variable.initializer!, const UnknownType(), true,
        isVoidAllowed: true);
    variable.initializer = result.expression..parent = variable;
    DartType inferredType =
        inferDeclarationType(result.inferredType, forSyntheticVariable: true);
    instrumentation?.record(uriForInstrumentation, variable.fileOffset, 'type',
        new InstrumentationValueForType(inferredType));
    variable.type = inferredType;
  }

  Link<NullAwareGuard> inferSyntheticVariableNullAware(
      VariableDeclarationImpl variable) {
    assert(variable.isImplicitlyTyped);
    assert(variable.initializer != null);
    ExpressionInferenceResult result = inferNullAwareExpression(
        variable.initializer!, const UnknownType(), true,
        isVoidAllowed: true);

    Link<NullAwareGuard> nullAwareGuards = result.nullAwareGuards;
    variable.initializer = result.nullAwareAction..parent = variable;

    DartType inferredType =
        inferDeclarationType(result.inferredType, forSyntheticVariable: true);
    instrumentation?.record(uriForInstrumentation, variable.fileOffset, 'type',
        new InstrumentationValueForType(inferredType));
    variable.type = inferredType;
    return nullAwareGuards;
  }

  NullAwareGuard createNullAwareGuard(VariableDeclaration variable) {
    return new NullAwareGuard(variable, variable.fileOffset, this);
  }

  ExpressionInferenceResult wrapExpressionInferenceResultInProblem(
      ExpressionInferenceResult result,
      Message message,
      int fileOffset,
      int length,
      {List<LocatedMessage>? context}) {
    return createNullAwareExpressionInferenceResult(
        result.inferredType,
        helper!.wrapInProblem(
            result.nullAwareAction, message, fileOffset, length,
            context: context),
        result.nullAwareGuards);
  }

  ExpressionInferenceResult createNullAwareExpressionInferenceResult(
      DartType inferredType,
      Expression expression,
      Link<NullAwareGuard>? nullAwareGuards) {
    if (nullAwareGuards != null && nullAwareGuards.isNotEmpty) {
      return new NullAwareExpressionInferenceResult(
          computeNullable(inferredType),
          inferredType,
          nullAwareGuards,
          expression);
    } else {
      return new ExpressionInferenceResult(inferredType, expression);
    }
  }

  /// Performs type inference on the given [expression].
  ///
  /// [typeContext] is the expected type of the expression, based on surrounding
  /// code.  [typeNeeded] indicates whether it is necessary to compute the
  /// actual type of the expression.  If [typeNeeded] is `true`,
  /// [ExpressionInferenceResult.inferredType] is the actual type of the
  /// expression; otherwise `null`.
  ///
  /// Derived classes should override this method with logic that dispatches on
  /// the expression type and calls the appropriate specialized "infer" method.
  ExpressionInferenceResult _inferExpression(
      Expression expression, DartType typeContext, bool typeNeeded,
      {bool isVoidAllowed: false, bool forEffect: false}) {
    registerIfUnreachableForTesting(expression);

    // `null` should never be used as the type context.  An instance of
    // `UnknownType` should be used instead.
    // ignore: unnecessary_null_comparison
    assert(typeContext != null);

    // For full (non-top level) inference, we need access to the
    // ExpressionGeneratorHelper so that we can perform error recovery.
    assert(isTopLevel || helper != null);

    // When doing top level inference, we skip subexpressions whose type isn't
    // needed so that we don't induce bogus dependencies on fields mentioned in
    // those subexpressions.
    if (!typeNeeded) {
      return new ExpressionInferenceResult(const UnknownType(), expression);
    }

    InferenceVisitor visitor = new InferenceVisitor(this);
    ExpressionInferenceResult result;
    if (expression is ExpressionJudgment) {
      result = expression.acceptInference(visitor, typeContext);
    } else if (expression is InternalExpression) {
      result = expression.acceptInference(visitor, typeContext);
    } else {
      result = expression.accept1(visitor, typeContext);
    }
    DartType inferredType = result.inferredType;
    // ignore: unnecessary_null_comparison
    assert(inferredType != null,
        "No type inferred for $expression (${expression.runtimeType}).");
    if (inferredType is VoidType && !isVoidAllowed) {
      if (expression.parent is! ArgumentsImpl) {
        helper?.addProblem(
            messageVoidExpression, expression.fileOffset, noLength);
      }
    }
    if (coreTypes.isBottom(result.inferredType)) {
      flowAnalysis.handleExit();
      if (shouldThrowUnsoundnessException &&
          // Don't throw on expressions that inherently return the bottom type.
          !(result.nullAwareAction is Throw ||
              result.nullAwareAction is Rethrow ||
              result.nullAwareAction is InvalidExpression)) {
        Expression replacement = createLet(
            createVariable(result.expression, result.inferredType),
            createReachabilityError(expression.fileOffset,
                messageNeverValueError, messageNeverValueWarning));
        flowAnalysis.forwardExpression(replacement, result.expression);
        result =
            new ExpressionInferenceResult(result.inferredType, replacement);
      }
    }
    return result;
  }

  @override
  ExpressionInferenceResult inferExpression(
      Expression expression, DartType typeContext, bool typeNeeded,
      {bool isVoidAllowed: false, bool forEffect: false}) {
    ExpressionInferenceResult result = _inferExpression(
        expression, typeContext, typeNeeded,
        isVoidAllowed: isVoidAllowed, forEffect: forEffect);
    return result.stopShorting();
  }

  ExpressionInferenceResult inferNullAwareExpression(
      Expression expression, DartType typeContext, bool typeNeeded,
      {bool isVoidAllowed: false, bool forEffect: false}) {
    ExpressionInferenceResult result = _inferExpression(
        expression, typeContext, typeNeeded,
        isVoidAllowed: isVoidAllowed, forEffect: forEffect);
    if (isNonNullableByDefault) {
      return result;
    } else {
      return result.stopShorting();
    }
  }

  @override
  Expression inferFieldInitializer(
    InferenceHelper helper,
    DartType context,
    Expression initializer,
  ) {
    assert(closureContext == null);
    assert(!isTopLevel);
    this.helper = helper;
    ExpressionInferenceResult initializerResult =
        inferExpression(initializer, context, true, isVoidAllowed: true);
    initializer = ensureAssignableResult(context, initializerResult,
        isVoidAllowed: context is VoidType);
    this.helper = null;
    return initializer;
  }

  @override
  InferredFunctionBody inferFunctionBody(InferenceHelper helper, int fileOffset,
      DartType returnType, AsyncMarker asyncMarker, Statement body) {
    // ignore: unnecessary_null_comparison
    assert(body != null);
    // ignore: unnecessary_null_comparison
    assert(closureContext == null);
    this.helper = helper;
    closureContext = new ClosureContext(this, asyncMarker, returnType, false);
    StatementInferenceResult result = inferStatement(body);
    if (dataForTesting != null) {
      if (!flowAnalysis.isReachable) {
        dataForTesting!.flowAnalysisResult.functionBodiesThatDontComplete
            .add(body);
      }
    }
    result =
        closureContext!.handleImplicitReturn(this, body, result, fileOffset);
    DartType? futureValueType = closureContext!.futureValueType;
    assert(
        !(isNonNullableByDefault &&
            asyncMarker == AsyncMarker.Async &&
            futureValueType == null),
        "No future value type computed.");
    closureContext = null;
    this.helper = null;
    flowAnalysis.finish();
    return new InferredFunctionBody(
        result.hasChanged ? result.statement : body, futureValueType);
  }

  InvocationInferenceResult inferInvocation(DartType typeContext, int offset,
      FunctionType calleeType, ArgumentsImpl arguments,
      {List<VariableDeclaration>? hoistedExpressions,
      bool isSpecialCasedBinaryOperator: false,
      bool isSpecialCasedTernaryOperator: false,
      DartType? receiverType,
      bool skipTypeArgumentInference: false,
      bool isConst: false,
      bool isImplicitExtensionMember: false,
      bool isImplicitCall: false,
      Member? staticTarget,
      bool isExtensionMemberInvocation = false}) {
    int extensionTypeParameterCount = getExtensionTypeParameterCount(arguments);
    if (extensionTypeParameterCount != 0) {
      return _inferGenericExtensionMethodInvocation(extensionTypeParameterCount,
          typeContext, offset, calleeType, arguments, hoistedExpressions,
          isSpecialCasedBinaryOperator: isSpecialCasedBinaryOperator,
          isSpecialCasedTernaryOperator: isSpecialCasedTernaryOperator,
          receiverType: receiverType,
          skipTypeArgumentInference: skipTypeArgumentInference,
          isConst: isConst,
          isImplicitExtensionMember: isImplicitExtensionMember);
    }
    return _inferInvocation(
        typeContext, offset, calleeType, arguments, hoistedExpressions,
        isSpecialCasedBinaryOperator: isSpecialCasedBinaryOperator,
        isSpecialCasedTernaryOperator: isSpecialCasedTernaryOperator,
        receiverType: receiverType,
        skipTypeArgumentInference: skipTypeArgumentInference,
        isConst: isConst,
        isImplicitExtensionMember: isImplicitExtensionMember,
        isImplicitCall: isImplicitCall,
        staticTarget: staticTarget,
        isExtensionMemberInvocation: isExtensionMemberInvocation);
  }

  InvocationInferenceResult _inferGenericExtensionMethodInvocation(
      int extensionTypeParameterCount,
      DartType typeContext,
      int offset,
      FunctionType calleeType,
      Arguments arguments,
      List<VariableDeclaration>? hoistedExpressions,
      {bool isSpecialCasedBinaryOperator: false,
      bool isSpecialCasedTernaryOperator: false,
      DartType? receiverType,
      bool skipTypeArgumentInference: false,
      bool isConst: false,
      bool isImplicitExtensionMember: false,
      bool isImplicitCall: false,
      Member? staticTarget}) {
    FunctionType extensionFunctionType = new FunctionType(
        [calleeType.positionalParameters.first],
        const DynamicType(),
        library.nonNullable,
        requiredParameterCount: 1,
        typeParameters: calleeType.typeParameters
            .take(extensionTypeParameterCount)
            .toList());
    ArgumentsImpl extensionArguments = engine.forest.createArguments(
        arguments.fileOffset, [arguments.positional.first],
        types: getExplicitExtensionTypeArguments(arguments));
    _inferInvocation(const UnknownType(), offset, extensionFunctionType,
        extensionArguments, hoistedExpressions,
        skipTypeArgumentInference: skipTypeArgumentInference,
        receiverType: receiverType,
        isImplicitExtensionMember: isImplicitExtensionMember,
        isImplicitCall: isImplicitCall,
        staticTarget: staticTarget,
        isExtensionMemberInvocation: true);
    Substitution extensionSubstitution = Substitution.fromPairs(
        extensionFunctionType.typeParameters, extensionArguments.types);

    List<TypeParameter> targetTypeParameters = const <TypeParameter>[];
    if (calleeType.typeParameters.length > extensionTypeParameterCount) {
      targetTypeParameters =
          calleeType.typeParameters.skip(extensionTypeParameterCount).toList();
    }
    FunctionType targetFunctionType = new FunctionType(
        calleeType.positionalParameters.skip(1).toList(),
        calleeType.returnType,
        library.nonNullable,
        requiredParameterCount: calleeType.requiredParameterCount - 1,
        namedParameters: calleeType.namedParameters,
        typeParameters: targetTypeParameters);
    targetFunctionType = extensionSubstitution
        .substituteType(targetFunctionType) as FunctionType;
    ArgumentsImpl targetArguments = engine.forest.createArguments(
        arguments.fileOffset, arguments.positional.skip(1).toList(),
        named: arguments.named, types: getExplicitTypeArguments(arguments));
    InvocationInferenceResult result = _inferInvocation(typeContext, offset,
        targetFunctionType, targetArguments, hoistedExpressions,
        isSpecialCasedBinaryOperator: isSpecialCasedBinaryOperator,
        isSpecialCasedTernaryOperator: isSpecialCasedTernaryOperator,
        skipTypeArgumentInference: skipTypeArgumentInference,
        isConst: isConst,
        isImplicitCall: isImplicitCall,
        staticTarget: staticTarget);
    arguments.positional.clear();
    arguments.positional.addAll(extensionArguments.positional);
    arguments.positional.addAll(targetArguments.positional);
    setParents(arguments.positional, arguments);
    // The `targetArguments.named` is the same list as `arguments.named` so
    // we just need to ensure that parent relations are realigned.
    setParents(arguments.named, arguments);
    arguments.types.clear();
    arguments.types.addAll(extensionArguments.types);
    arguments.types.addAll(targetArguments.types);
    return result;
  }

  /// Performs the type inference steps that are shared by all kinds of
  /// invocations (constructors, instance methods, and static methods).
  InvocationInferenceResult _inferInvocation(
      DartType typeContext,
      int offset,
      FunctionType calleeType,
      ArgumentsImpl arguments,
      List<VariableDeclaration>? hoistedExpressions,
      {bool isSpecialCasedBinaryOperator: false,
      bool isSpecialCasedTernaryOperator: false,
      DartType? receiverType,
      bool skipTypeArgumentInference: false,
      bool isConst: false,
      bool isImplicitExtensionMember: false,
      required bool isImplicitCall,
      Member? staticTarget,
      bool isExtensionMemberInvocation: false}) {
    // [receiverType] must be provided for special-cased operators.
    assert(!isSpecialCasedBinaryOperator && !isSpecialCasedTernaryOperator ||
        receiverType != null);

    List<TypeParameter> calleeTypeParameters = calleeType.typeParameters;
    if (calleeTypeParameters.isNotEmpty) {
      // It's possible that one of the callee type parameters might match a type
      // that already exists as part of inference (e.g. the type of an
      // argument).  This might happen, for instance, in the case where a
      // function or method makes a recursive call to itself.  To avoid the
      // callee type parameters accidentally matching a type that already
      // exists, and creating invalid inference results, we need to create fresh
      // type parameters for the callee (see dartbug.com/31759).
      // TODO(paulberry): is it possible to find a narrower set of circumstances
      // in which me must do this, to avoid a performance regression?
      FreshTypeParameters fresh = getFreshTypeParameters(calleeTypeParameters);
      calleeType = fresh.applyToFunctionType(calleeType);
      calleeTypeParameters = fresh.freshTypeParameters;
    }

    List<DartType>? explicitTypeArguments = getExplicitTypeArguments(arguments);

    bool inferenceNeeded = !skipTypeArgumentInference &&
        explicitTypeArguments == null &&
        calleeTypeParameters.isNotEmpty;
    bool typeChecksNeeded = !isTopLevel;

    List<DartType>? inferredTypes;
    Substitution? substitution;
    List<DartType>? formalTypes;
    List<DartType>? actualTypes;
    if (inferenceNeeded || typeChecksNeeded) {
      formalTypes = [];
      actualTypes = [];
    }

    List<VariableDeclaration>? localHoistedExpressions;
    if (library.enableNamedArgumentsAnywhereInLibrary &&
        arguments.argumentsOriginalOrder != null &&
        hoistedExpressions == null &&
        !isTopLevel) {
      hoistedExpressions = localHoistedExpressions = <VariableDeclaration>[];
    }

    if (inferenceNeeded) {
      // ignore: unnecessary_null_comparison
      if (isConst && typeContext != null) {
        typeContext = new TypeVariableEliminator(
                bottomType,
                isNonNullableByDefault
                    ? coreTypes.objectNullableRawType
                    : coreTypes.objectLegacyRawType)
            .substituteType(typeContext);
      }
      inferredTypes = new List<DartType>.filled(
          calleeTypeParameters.length, const UnknownType());
      typeSchemaEnvironment.inferGenericFunctionOrType(
          isNonNullableByDefault
              ? calleeType.returnType
              : legacyErasure(calleeType.returnType),
          calleeTypeParameters,
          null,
          null,
          typeContext,
          inferredTypes,
          library.library);
      substitution =
          Substitution.fromPairs(calleeTypeParameters, inferredTypes);
    } else if (explicitTypeArguments != null &&
        calleeTypeParameters.length == explicitTypeArguments.length) {
      substitution =
          Substitution.fromPairs(calleeTypeParameters, explicitTypeArguments);
    } else if (calleeTypeParameters.length != 0) {
      substitution = Substitution.fromPairs(
          calleeTypeParameters,
          new List<DartType>.filled(
              calleeTypeParameters.length, const DynamicType()));
    }
    bool isIdentical =
        staticTarget == typeSchemaEnvironment.coreTypes.identicalProcedure;
    // TODO(paulberry): if we are doing top level inference and type arguments
    // were omitted, report an error.
    List<Object?> argumentsEvaluationOrder;
    if (library.enableNamedArgumentsAnywhereInLibrary &&
        arguments.argumentsOriginalOrder != null) {
      if (staticTarget?.isExtensionMember ?? false) {
        // Add the receiver.
        argumentsEvaluationOrder = <Object?>[
          arguments.positional[0],
          ...arguments.argumentsOriginalOrder!
        ];
      } else {
        argumentsEvaluationOrder = arguments.argumentsOriginalOrder!;
      }
    } else {
      argumentsEvaluationOrder = <Object?>[
        ...arguments.positional,
        ...arguments.named
      ];
    }
    arguments.argumentsOriginalOrder = null;

    // The following loop determines how many argument expressions should be
    // hoisted to preserve the evaluation order. The computation is based on the
    // following observation: the largest suffix of the argument vector, such
    // that every positional argument in that suffix comes before any named
    // argument, retains the evaluation order after the rest of the arguments
    // are hoisted, and therefore doesn't need to be hoisted itself. The loop
    // below finds the starting position of such suffix and stores it in the
    // [hoistingEndIndex] variable. In case all positional arguments come
    // before all named arguments, the suffix coincides with the entire argument
    // vector, and none of the arguments is hoisted. That way the legacy
    // behavior is preserved.
    int hoistingEndIndex;
    if (library.enableNamedArgumentsAnywhereInLibrary) {
      hoistingEndIndex = argumentsEvaluationOrder.length - 1;
      for (int i = argumentsEvaluationOrder.length - 2;
          i >= 0 && hoistingEndIndex == i + 1;
          i--) {
        int previousWeight =
            argumentsEvaluationOrder[i + 1] is NamedExpression ? 1 : 0;
        int currentWeight =
            argumentsEvaluationOrder[i] is NamedExpression ? 1 : 0;
        if (currentWeight <= previousWeight) {
          --hoistingEndIndex;
        }
      }
    } else {
      hoistingEndIndex = 0;
    }

    int positionalIndex = 0;
    int namedIndex = 0;
    for (int evaluationOrderIndex = 0;
        evaluationOrderIndex < argumentsEvaluationOrder.length;
        evaluationOrderIndex++) {
      Object? argument = argumentsEvaluationOrder[evaluationOrderIndex];
      assert(
          argument is Expression || argument is NamedExpression,
          "Expected the argument to be either an Expression "
          "or a NamedExpression, got '${argument.runtimeType}'.");
      if (argument is Expression) {
        int index = positionalIndex++;
        DartType formalType = getPositionalParameterType(calleeType, index);
        DartType inferredFormalType = substitution != null
            ? substitution.substituteType(formalType)
            : formalType;
        DartType inferredType;
        if (isImplicitExtensionMember && index == 0) {
          assert(
              receiverType != null,
              "No receiver type provided for implicit extension member "
              "invocation.");
          continue;
        } else {
          if (isSpecialCasedBinaryOperator) {
            inferredFormalType = typeSchemaEnvironment
                .getContextTypeOfSpecialCasedBinaryOperator(
                    typeContext, receiverType!, inferredFormalType,
                    isNonNullableByDefault: isNonNullableByDefault);
          } else if (isSpecialCasedTernaryOperator) {
            inferredFormalType = typeSchemaEnvironment
                .getContextTypeOfSpecialCasedTernaryOperator(
                    typeContext, receiverType!, inferredFormalType,
                    isNonNullableByDefault: isNonNullableByDefault);
          }
          ExpressionInferenceResult result = inferExpression(
              arguments.positional[index],
              isNonNullableByDefault
                  ? inferredFormalType
                  : legacyErasure(inferredFormalType),
              inferenceNeeded ||
                  isSpecialCasedBinaryOperator ||
                  isSpecialCasedTernaryOperator ||
                  typeChecksNeeded);
          inferredType = identical(result.inferredType, noInferredType) ||
                  isNonNullableByDefault
              ? result.inferredType
              : legacyErasure(result.inferredType);
          if (localHoistedExpressions != null &&
              evaluationOrderIndex >= hoistingEndIndex) {
            hoistedExpressions = null;
          }
          Expression expression =
              _hoist(result.expression, inferredType, hoistedExpressions);
          if (isIdentical && arguments.positional.length == 2) {
            if (index == 0) {
              flowAnalysis.equalityOp_rightBegin(expression, inferredType);
            } else {
              flowAnalysis.equalityOp_end(
                  arguments.parent as Expression, expression, inferredType);
            }
          }
          arguments.positional[index] = expression..parent = arguments;
        }
        if (inferenceNeeded || typeChecksNeeded) {
          formalTypes!.add(formalType);
          actualTypes!.add(inferredType);
        }
      } else {
        assert(argument is NamedExpression);
        int index = namedIndex++;
        NamedExpression namedArgument = arguments.named[index];
        DartType formalType =
            getNamedParameterType(calleeType, namedArgument.name);
        DartType inferredFormalType = substitution != null
            ? substitution.substituteType(formalType)
            : formalType;
        ExpressionInferenceResult result = inferExpression(
            namedArgument.value,
            isNonNullableByDefault
                ? inferredFormalType
                : legacyErasure(inferredFormalType),
            inferenceNeeded ||
                isSpecialCasedBinaryOperator ||
                typeChecksNeeded);
        DartType inferredType =
            identical(result.inferredType, noInferredType) ||
                    isNonNullableByDefault
                ? result.inferredType
                : legacyErasure(result.inferredType);
        if (localHoistedExpressions != null &&
            evaluationOrderIndex >= hoistingEndIndex) {
          hoistedExpressions = null;
        }
        Expression expression =
            _hoist(result.expression, inferredType, hoistedExpressions);
        namedArgument.value = expression..parent = namedArgument;
        if (inferenceNeeded || typeChecksNeeded) {
          formalTypes!.add(formalType);
          actualTypes!.add(inferredType);
        }
      }
    }
    assert(
        positionalIndex == arguments.positional.length,
        "Expected 'positionalIndex' to be ${arguments.positional.length}, "
        "got ${positionalIndex}.");
    assert(
        namedIndex == arguments.named.length,
        "Expected 'namedIndex' to be ${arguments.named.length}, "
        "got ${namedIndex}.");
    if (isSpecialCasedBinaryOperator) {
      calleeType = replaceReturnType(
          calleeType,
          typeSchemaEnvironment.getTypeOfSpecialCasedBinaryOperator(
              receiverType!, actualTypes![0],
              isNonNullableByDefault: isNonNullableByDefault));
    } else if (isSpecialCasedTernaryOperator) {
      calleeType = replaceReturnType(
          calleeType,
          typeSchemaEnvironment.getTypeOfSpecialCasedTernaryOperator(
              receiverType!, actualTypes![0], actualTypes[1], library.library));
    }

    // Check for and remove duplicated named arguments.
    List<NamedExpression> named = arguments.named;
    if (named.length == 2) {
      if (named[0].name == named[1].name) {
        String name = named[1].name;
        Expression error = helper!.wrapInProblem(
            _createDuplicateExpression(
                named[0].fileOffset, named[0].value, named[1].value),
            templateDuplicatedNamedArgument.withArguments(name),
            named[1].fileOffset,
            name.length);
        arguments.named = [new NamedExpression(named[1].name, error)];
        formalTypes!.removeLast();
        actualTypes!.removeLast();
      }
    } else if (named.length > 2) {
      Map<String, NamedExpression> seenNames = <String, NamedExpression>{};
      bool hasProblem = false;
      int namedTypeIndex = arguments.positional.length;
      List<NamedExpression> uniqueNamed = <NamedExpression>[];
      for (NamedExpression expression in named) {
        String name = expression.name;
        if (seenNames.containsKey(name)) {
          hasProblem = true;
          NamedExpression prevNamedExpression = seenNames[name]!;
          prevNamedExpression.value = helper!.wrapInProblem(
              _createDuplicateExpression(prevNamedExpression.fileOffset,
                  prevNamedExpression.value, expression.value),
              templateDuplicatedNamedArgument.withArguments(name),
              expression.fileOffset,
              name.length)
            ..parent = prevNamedExpression;
          formalTypes!.removeAt(namedTypeIndex);
          actualTypes!.removeAt(namedTypeIndex);
        } else {
          seenNames[name] = expression;
          uniqueNamed.add(expression);
          namedTypeIndex++;
        }
      }
      if (hasProblem) {
        arguments.named = uniqueNamed;
      }
    }

    if (inferenceNeeded) {
      typeSchemaEnvironment.inferGenericFunctionOrType(
          calleeType.returnType,
          calleeTypeParameters,
          formalTypes,
          actualTypes,
          typeContext,
          inferredTypes!,
          library.library);
      assert(inferredTypes.every((type) => isKnown(type)),
          "Unknown type(s) in inferred types: $inferredTypes.");
      assert(inferredTypes.every((type) => !hasPromotedTypeVariable(type)),
          "Promoted type variable(s) in inferred types: $inferredTypes.");
      substitution =
          Substitution.fromPairs(calleeTypeParameters, inferredTypes);
      instrumentation?.record(uriForInstrumentation, offset, 'typeArgs',
          new InstrumentationValueForTypeArgs(inferredTypes));
      arguments.types.clear();
      arguments.types.addAll(inferredTypes);
      if (dataForTesting != null) {
        assert(arguments.fileOffset != TreeNode.noOffset);
        dataForTesting!.typeInferenceResult.inferredTypeArguments[arguments] =
            inferredTypes;
      }
    }
    List<DartType> positionalArgumentTypes = [];
    List<NamedType> namedArgumentTypes = [];
    if (typeChecksNeeded && !identical(calleeType, unknownFunction)) {
      LocatedMessage? argMessage = helper!.checkArgumentsForType(
          calleeType, arguments, offset,
          isExtensionMemberInvocation: isExtensionMemberInvocation);
      if (argMessage != null) {
        return new WrapInProblemInferenceResult(
            const InvalidType(),
            const InvalidType(),
            argMessage.messageObject,
            argMessage.charOffset,
            argMessage.length,
            helper!,
            isInapplicable: true);
      } else {
        // Argument counts and names match. Compare types.
        int positionalShift = isImplicitExtensionMember ? 1 : 0;
        int numPositionalArgs = arguments.positional.length - positionalShift;
        for (int i = 0; i < formalTypes!.length; i++) {
          DartType formalType = formalTypes[i];
          DartType expectedType = substitution != null
              ? substitution.substituteType(formalType)
              : formalType;
          DartType actualType = actualTypes![i];
          Expression expression;
          NamedExpression? namedExpression;
          if (i < numPositionalArgs) {
            expression = arguments.positional[positionalShift + i];
            positionalArgumentTypes.add(actualType);
          } else {
            namedExpression = arguments.named[i - numPositionalArgs];
            expression = namedExpression.value;
            namedArgumentTypes
                .add(new NamedType(namedExpression.name, actualType));
          }
          expression = ensureAssignable(expectedType, actualType, expression,
              isVoidAllowed: expectedType is VoidType,
              // TODO(johnniwinther): Specialize message for operator
              // invocations.
              errorTemplate: templateArgumentTypeNotAssignable,
              nullabilityErrorTemplate:
                  templateArgumentTypeNotAssignableNullability,
              nullabilityPartErrorTemplate:
                  templateArgumentTypeNotAssignablePartNullability,
              nullabilityNullErrorTemplate:
                  templateArgumentTypeNotAssignableNullabilityNull,
              nullabilityNullTypeErrorTemplate:
                  templateArgumentTypeNotAssignableNullabilityNullType);
          if (namedExpression == null) {
            arguments.positional[positionalShift + i] = expression
              ..parent = arguments;
          } else {
            namedExpression.value = expression..parent = namedExpression;
          }
        }
      }
    }
    DartType inferredType;
    if (substitution != null) {
      calleeType = substitution.substituteType(calleeType.withoutTypeParameters)
          as FunctionType;
    }
    inferredType = calleeType.returnType;
    assert(
        !containsFreeFunctionTypeVariables(inferredType),
        "Inferred return type $inferredType contains free variables."
        "Inferred function type: $calleeType.");

    if (!isNonNullableByDefault) {
      inferredType = legacyErasure(inferredType);
      calleeType = legacyErasure(calleeType) as FunctionType;
    }

    return new SuccessfulInferenceResult(inferredType, calleeType,
        hoistedArguments: localHoistedExpressions,
        inferredReceiverType: receiverType);
  }

  FunctionType inferLocalFunction(FunctionNode function, DartType? typeContext,
      int fileOffset, DartType? returnContext) {
    bool hasImplicitReturnType = false;
    if (returnContext == null) {
      hasImplicitReturnType = true;
      returnContext =
          isNonNullableByDefault ? const UnknownType() : const DynamicType();
    }
    if (!isTopLevel) {
      List<VariableDeclaration> positionalParameters =
          function.positionalParameters;
      for (int i = 0; i < positionalParameters.length; i++) {
        VariableDeclaration parameter = positionalParameters[i];
        flowAnalysis.declare(parameter, true);
        inferMetadataKeepingHelper(parameter, parameter.annotations);
        if (parameter.initializer != null) {
          ExpressionInferenceResult initializerResult = inferExpression(
              parameter.initializer!, parameter.type, !isTopLevel);
          parameter.initializer = initializerResult.expression
            ..parent = parameter;
        }
      }
      for (VariableDeclaration parameter in function.namedParameters) {
        flowAnalysis.declare(parameter, true);
        inferMetadataKeepingHelper(parameter, parameter.annotations);
        ExpressionInferenceResult initializerResult = inferExpression(
            parameter.initializer!, parameter.type, !isTopLevel);
        parameter.initializer = initializerResult.expression
          ..parent = parameter;
      }
    }

    // Let `<T0, ..., Tn>` be the set of type parameters of the closure (with
    // `n`=0 if there are no type parameters).
    List<TypeParameter> typeParameters = function.typeParameters;

    // Let `(P0 x0, ..., Pm xm)` be the set of formal parameters of the closure
    // (including required, positional optional, and named optional parameters).
    // If any type `Pi` is missing, denote it as `_`.
    List<VariableDeclaration> formals = function.positionalParameters.toList()
      ..addAll(function.namedParameters);

    // Let `B` denote the closure body.  If `B` is an expression function body
    // (`=> e`), treat it as equivalent to a block function body containing a
    // single `return` statement (`{ return e; }`).

    // Attempt to match `K` as a function type compatible with the closure (that
    // is, one having n type parameters and a compatible set of formal
    // parameters).  If there is a successful match, let `<S0, ..., Sn>` be the
    // set of matched type parameters and `(Q0, ..., Qm)` be the set of matched
    // formal parameter types, and let `N` be the return type.
    Substitution substitution;
    List<DartType?> formalTypesFromContext =
        new List<DartType?>.filled(formals.length, null);
    if (typeContext is FunctionType &&
        typeContext.typeParameters.length == typeParameters.length) {
      for (int i = 0; i < formals.length; i++) {
        if (i < function.positionalParameters.length) {
          formalTypesFromContext[i] =
              getPositionalParameterType(typeContext, i);
        } else {
          formalTypesFromContext[i] =
              getNamedParameterType(typeContext, formals[i].name!);
        }
      }
      returnContext = typeContext.returnType;

      // Let `[T/S]` denote the type substitution where each `Si` is replaced
      // with the corresponding `Ti`.
      Map<TypeParameter, DartType> substitutionMap =
          <TypeParameter, DartType>{};
      for (int i = 0; i < typeContext.typeParameters.length; i++) {
        substitutionMap[typeContext.typeParameters[i]] =
            i < typeParameters.length
                ? new TypeParameterType.forAlphaRenaming(
                    typeContext.typeParameters[i], typeParameters[i])
                : const DynamicType();
      }
      substitution = Substitution.fromMap(substitutionMap);
    } else {
      // If the match is not successful because  `K` is `_`, let all `Si`, all
      // `Qi`, and `N` all be `_`.

      // If the match is not successful for any other reason, this will result
      // in a type error, so the implementation is free to choose the best
      // error recovery path.
      substitution = Substitution.empty;
    }

    // Define `Ri` as follows: if `Pi` is not `_`, let `Ri` be `Pi`.
    // Otherwise, if `Qi` is not `_`, let `Ri` be the greatest closure of
    // `Qi[T/S]` with respect to `?`.  Otherwise, let `Ri` be `dynamic`.
    for (int i = 0; i < formals.length; i++) {
      VariableDeclarationImpl formal = formals[i] as VariableDeclarationImpl;
      if (formal.isImplicitlyTyped) {
        DartType inferredType;
        if (formalTypesFromContext[i] != null) {
          inferredType = computeGreatestClosure2(
              substitution.substituteType(formalTypesFromContext[i]!));
          if (typeSchemaEnvironment.isSubtypeOf(
              inferredType,
              const NullType(),
              isNonNullableByDefault
                  ? SubtypeCheckMode.withNullabilities
                  : SubtypeCheckMode.ignoringNullabilities)) {
            inferredType = coreTypes.objectRawType(library.nullable);
          }
        } else {
          inferredType = const DynamicType();
        }
        instrumentation?.record(uriForInstrumentation, formal.fileOffset,
            'type', new InstrumentationValueForType(inferredType));
        formal.type = demoteTypeInLibrary(inferredType, library.library);
        if (dataForTesting != null) {
          dataForTesting!.typeInferenceResult.inferredVariableTypes[formal] =
              formal.type;
        }
      }

      if (isNonNullableByDefault) {
        // If a parameter is a positional or named optional parameter and its
        // type is potentially non-nullable, it should have an initializer.
        bool isOptionalPositional = function.requiredParameterCount <= i &&
            i < function.positionalParameters.length;
        bool isOptionalNamed =
            i >= function.positionalParameters.length && !formal.isRequired;
        if ((isOptionalPositional || isOptionalNamed) &&
            formal.type.isPotentiallyNonNullable &&
            !formal.hasDeclaredInitializer) {
          library.addProblem(
              templateOptionalNonNullableWithoutInitializerError.withArguments(
                  formal.name!, formal.type, isNonNullableByDefault),
              formal.fileOffset,
              formal.name!.length,
              library.importUri);
        }
      }
    }

    if (isNonNullableByDefault) {
      for (VariableDeclaration parameter in function.namedParameters) {
        VariableDeclarationImpl formal = parameter as VariableDeclarationImpl;
        // Required named parameters shouldn't have initializers.
        if (formal.isRequired && formal.hasDeclaredInitializer) {
          library.addProblem(
              templateRequiredNamedParameterHasDefaultValueError
                  .withArguments(formal.name!),
              formal.fileOffset,
              formal.name!.length,
              library.importUri);
        }
      }
    }

    // Let `N'` be `N[T/S]`.  The [ClosureContext] constructor will adjust
    // accordingly if the closure is declared with `async`, `async*`, or
    // `sync*`.
    if (returnContext is! UnknownType) {
      returnContext = substitution.substituteType(returnContext);
    }

    // Apply type inference to `B` in return context `N’`, with any references
    // to `xi` in `B` having type `Pi`.  This produces `B’`.
    bool needToSetReturnType = hasImplicitReturnType;
    ClosureContext? oldClosureContext = this.closureContext;
    ClosureContext closureContext = new ClosureContext(
        this, function.asyncMarker, returnContext, needToSetReturnType);
    this.closureContext = closureContext;
    StatementInferenceResult bodyResult = inferStatement(function.body!);

    // If the closure is declared with `async*` or `sync*`, let `M` be the
    // least upper bound of the types of the `yield` expressions in `B’`, or
    // `void` if `B’` contains no `yield` expressions.  Otherwise, let `M` be
    // the least upper bound of the types of the `return` expressions in `B’`,
    // or `void` if `B’` contains no `return` expressions.
    if (needToSetReturnType) {
      DartType inferredReturnType = closureContext.inferReturnType(this,
          hasImplicitReturn: flowAnalysis.isReachable);

      // Then the result of inference is `<T0, ..., Tn>(R0 x0, ..., Rn xn) B`
      // with type `<T0, ..., Tn>(R0, ..., Rn) -> M’` (with some of the `Ri` and
      // `xi` denoted as optional or named parameters, if appropriate).
      instrumentation?.record(uriForInstrumentation, fileOffset, 'returnType',
          new InstrumentationValueForType(inferredReturnType));
      function.returnType = inferredReturnType;
    }
    bodyResult = closureContext.handleImplicitReturn(
        this, function.body!, bodyResult, fileOffset);
    function.futureValueType = closureContext.futureValueType;
    assert(
        !(isNonNullableByDefault &&
            function.asyncMarker == AsyncMarker.Async &&
            function.futureValueType == null),
        "No future value type computed.");

    if (bodyResult.hasChanged) {
      function.body = bodyResult.statement..parent = function;
    }
    this.closureContext = oldClosureContext;
    return function.computeFunctionType(library.nonNullable);
  }

  @override
  void inferMetadata(
      InferenceHelper helper, TreeNode? parent, List<Expression>? annotations) {
    if (annotations != null) {
      this.helper = helper;
      inferMetadataKeepingHelper(parent, annotations);
      this.helper = null;
    }
  }

  @override
  void inferMetadataKeepingHelper(
      TreeNode? parent, List<Expression>? annotations) {
    if (annotations != null) {
      for (int index = 0; index < annotations.length; index++) {
        ExpressionInferenceResult result = inferExpression(
            annotations[index], const UnknownType(), !isTopLevel);
        annotations[index] = result.expression..parent = parent;
      }
    }
  }

  StaticInvocation transformExtensionMethodInvocation(int fileOffset,
      ObjectAccessTarget target, Expression receiver, Arguments arguments) {
    assert(target.isExtensionMember || target.isNullableExtensionMember);
    Procedure procedure = target.member as Procedure;
    return engine.forest.createStaticInvocation(
        fileOffset,
        procedure,
        engine.forest.createArgumentsForExtensionMethod(
            arguments.fileOffset,
            target.inferredExtensionTypeArguments.length,
            procedure.function.typeParameters.length -
                target.inferredExtensionTypeArguments.length,
            receiver,
            extensionTypeArguments: target.inferredExtensionTypeArguments,
            positionalArguments: arguments.positional,
            namedArguments: arguments.named,
            typeArguments: arguments.types));
  }

  ExpressionInferenceResult _inferDynamicInvocation(
      int fileOffset,
      Link<NullAwareGuard> nullAwareGuards,
      Expression receiver,
      Name name,
      ArgumentsImpl arguments,
      DartType typeContext,
      List<VariableDeclaration>? hoistedExpressions,
      {required bool isImplicitCall}) {
    // ignore: unnecessary_null_comparison
    assert(isImplicitCall != null);
    InvocationInferenceResult result = inferInvocation(
        typeContext, fileOffset, unknownFunction, arguments,
        hoistedExpressions: hoistedExpressions,
        receiverType: const DynamicType(),
        isImplicitCall: isImplicitCall);
    assert(name != equalsName);
    Expression expression = new DynamicInvocation(
        DynamicAccessKind.Dynamic, receiver, name, arguments)
      ..fileOffset = fileOffset;
    return createNullAwareExpressionInferenceResult(
        result.inferredType, result.applyResult(expression), nullAwareGuards);
  }

  ExpressionInferenceResult _inferNeverInvocation(
      int fileOffset,
      Link<NullAwareGuard> nullAwareGuards,
      Expression receiver,
      NeverType receiverType,
      Name name,
      ArgumentsImpl arguments,
      DartType typeContext,
      List<VariableDeclaration>? hoistedExpressions,
      {required bool isImplicitCall}) {
    // ignore: unnecessary_null_comparison
    assert(isImplicitCall != null);
    InvocationInferenceResult result = inferInvocation(
        typeContext, fileOffset, unknownFunction, arguments,
        hoistedExpressions: hoistedExpressions,
        receiverType: receiverType,
        isImplicitCall: isImplicitCall);
    assert(name != equalsName);
    Expression expression = new DynamicInvocation(
        DynamicAccessKind.Never, receiver, name, arguments)
      ..fileOffset = fileOffset;
    return createNullAwareExpressionInferenceResult(
        const NeverType.nonNullable(),
        result.applyResult(expression),
        nullAwareGuards);
  }

  ExpressionInferenceResult _inferMissingInvocation(
      int fileOffset,
      Link<NullAwareGuard> nullAwareGuards,
      Expression receiver,
      DartType receiverType,
      ObjectAccessTarget target,
      Name name,
      ArgumentsImpl arguments,
      DartType typeContext,
      List<VariableDeclaration>? hoistedExpressions,
      {required bool isExpressionInvocation,