pkg/nnbd_migration/test/api_test.dart

// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:analyzer_plugin/protocol/protocol_common.dart';
import 'package:nnbd_migration/nnbd_migration.dart';
import 'package:test/test.dart';
import 'package:test_reflective_loader/test_reflective_loader.dart';

import 'abstract_context.dart';
import 'api_test_base.dart';

main() {
  defineReflectiveSuite(() {
    defineReflectiveTests(_ProvisionalApiTest);
    defineReflectiveTests(_ProvisionalApiTestPermissive);
    defineReflectiveTests(_ProvisionalApiTestWithReset);
  });
}

/// Tests of the provisional API.
@reflectiveTest
class _ProvisionalApiTest extends _ProvisionalApiTestBase
    with _ProvisionalApiTestCases {
  @override
  bool get _usePermissiveMode => false;
}

/// Base class for provisional API tests.
abstract class _ProvisionalApiTestBase extends AbstractContextTest {
  String projectPath;

  bool get _usePermissiveMode;

  void setUp() {
    projectPath = convertPath(testsPath);
    super.setUp();
  }

  /// Hook invoked between stages of processing inputs.
  void _betweenStages() {}

  /// Verifies that migration of the files in [input] produces the output in
  /// [expectedOutput].
  ///
  /// Optional parameter [removeViaComments] indicates whether dead code should
  /// be removed in its entirety (the default) or removed by commenting it out.
  Future<void> _checkMultipleFileChanges(
      Map<String, String> input, Map<String, String> expectedOutput,
      {Map<String, String> migratedInput = const {},
      bool removeViaComments = false,
      bool warnOnWeakCode = false}) async {
    for (var path in migratedInput.keys) {
      newFile(path, content: migratedInput[path]);
    }
    for (var path in input.keys) {
      newFile(path, content: input[path]);
    }
    var listener = TestMigrationListener();
    var migration = NullabilityMigration(listener, getLineInfo,
        permissive: _usePermissiveMode,
        removeViaComments: removeViaComments,
        warnOnWeakCode: warnOnWeakCode);
    for (var path in input.keys) {
      if (!(session.getFile(path)).isPart) {
        for (var unit in (await session.getResolvedLibrary(path)).units) {
          migration.prepareInput(unit);
        }
      }
    }
    expect(migration.unmigratedDependencies, isEmpty);
    _betweenStages();
    for (var path in input.keys) {
      if (!(session.getFile(path)).isPart) {
        for (var unit in (await session.getResolvedLibrary(path)).units) {
          migration.processInput(unit);
        }
      }
    }
    _betweenStages();
    for (var path in input.keys) {
      if (!(session.getFile(path)).isPart) {
        for (var unit in (await session.getResolvedLibrary(path)).units) {
          migration.finalizeInput(unit);
        }
      }
    }
    migration.finish();
    var sourceEdits = <String, List<SourceEdit>>{};
    for (var entry in listener.edits.entries) {
      var path = entry.key.fullName;
      expect(expectedOutput.keys, contains(path));
      sourceEdits[path] = entry.value;
    }
    for (var path in expectedOutput.keys) {
      var sourceEditsForPath = sourceEdits[path] ?? [];
      sourceEditsForPath.sort((a, b) => b.offset.compareTo(a.offset));
      expect(SourceEdit.applySequence(input[path], sourceEditsForPath),
          expectedOutput[path]);
    }
  }

  /// Verifies that migraiton of the single file with the given [content]
  /// produces the [expected] output.
  ///
  /// Optional parameter [removeViaComments] indicates whether dead code should
  /// be removed in its entirety (the default) or removed by commenting it out.
  Future<void> _checkSingleFileChanges(String content, String expected,
      {Map<String, String> migratedInput = const {},
      bool removeViaComments = false,
      bool warnOnWeakCode = false}) async {
    var sourcePath = convertPath('$testsPath/lib/test.dart');
    await _checkMultipleFileChanges(
        {sourcePath: content}, {sourcePath: expected},
        migratedInput: migratedInput,
        removeViaComments: removeViaComments,
        warnOnWeakCode: warnOnWeakCode);
  }
}

/// Mixin containing test cases for the provisional API.
mixin _ProvisionalApiTestCases on _ProvisionalApiTestBase {
  Future<void> test_add_explicit_parameter_type() async {
    var content = '''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => value + 1);
}
''';
    // Under the new NNBD rules, `value` gets an inferred type of `Object?`.  We
    // need to change this to `dynamic` to avoid an "undefined operator +"
    // error.
    var expected = '''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((dynamic value) => value + 1);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/40476')
  Future<void> test_add_explicit_parameter_type_interpolation() async {
    var content = r'''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => '$value';
}
''';
    // Under the new NNBD rules, `value` gets an inferred type of `Object?`,
    // whereas it previously had a type of `dynamic`.  But we don't need to fix
    // it because `Object?` supports `toString`.
    var expected = r'''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => '$value';
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/40476')
  Future<void> test_add_explicit_parameter_type_object_method() async {
    var content = '''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => value.toString());
}
''';
    // Under the new NNBD rules, `value` gets an inferred type of `Object?`,
    // whereas it previously had a type of `dynamic`.  But we don't need to fix
    // it because `Object?` supports `toString`.
    var expected = '''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => value.toString());
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/40476')
  Future<void> test_add_explicit_parameter_type_unused() async {
    var content = '''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => 0);
}
''';
    // Under the new NNBD rules, `value` gets an inferred type of `Object?`,
    // whereas it previously had a type of `dynamic`.  But we don't need to fix
    // it because it's unused.
    var expected = '''
abstract class C {
  void m<T>(T Function(T) callback);
}
void test(C c) {
  c.m((value) => 0);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_add_required() async {
    var content = '''
int f({String s}) => s.length;
''';
    var expected = '''
int f({required String s}) => s.length;
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/39404')
  Future<void> test_add_type_parameter_bound() async {
    /// After a migration, a bound may be made nullable. Instantiate-to-bounds
    /// may need to be made explicit where the migration engine prefers a
    /// non-null type.
    var content = '''
class C<T extends num/*?*/> {}

void main() {
  C c = C();
}
''';
    var expected = '''
class C<T extends num?> {}

void main() {
  C<num> c = C();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_ambiguous_closure_parameter_in_local_variable() async {
    var content = '''
Object f<T>(Object Function(T) callback, Object obj) => 0;
g() {
  var y = f<Map<String, int>>(
      (x) => x.keys,
      f<List<bool>>(
          (x) => x.last, 0));
}
''';
    var expected = '''
Object f<T>(Object Function(T) callback, Object obj) => 0;
g() {
  var y = f<Map<String, int>>(
      (x) => x.keys,
      f<List<bool>>(
          (x) => x.last, 0));
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_argumentError_checkNotNull_implies_non_null_intent() async {
    var content = '''
void f(int i) {
  ArgumentError.checkNotNull(i);
}
void g(bool b, int i) {
  if (b) f(i);
}
main() {
  g(false, null);
}
''';
    var expected = '''
void f(int i) {
  ArgumentError.checkNotNull(i);
}
void g(bool b, int? i) {
  if (b) f(i!);
}
main() {
  g(false, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_as_allows_null() async {
    var content = '''
int f(Object o) => (o as int)?.gcd(1);
main() {
  f(null);
}
''';
    var expected = '''
int? f(Object? o) => (o as int?)?.gcd(1);
main() {
  f(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_assign_null_to_generic_type() async {
    var content = '''
main() {
  List<int> x = null;
}
''';
    var expected = '''
main() {
  List<int>? x = null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_assignment_to_promoted_var_can_undo_promotion() async {
    var content = '''
abstract class C {
  void test() {
    var x = f();
    while (x != null) {
      x = f();
    }
  }
  int/*?*/ f();
}
''';
    var expected = '''
abstract class C {
  void test() {
    var x = f();
    while (x != null) {
      x = f();
    }
  }
  int? f();
}
''';
    // Prior to the fix for https://github.com/dart-lang/sdk/issues/41411,
    // migration would consider the LHS of `x = f()` to have context type
    // non-nullable `int`, so it would add a null check to the value returned
    // from `f`.
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_avoid_redundant_future_or() async {
    // FutureOr<int?> and FutureOr<int?>? are equivalent types; we never insert
    // the redundant second `?`.
    var content = '''
import 'dart:async';
abstract class C {
  FutureOr<int/*?*/> f();
  FutureOr<int>/*?*/ g();
  FutureOr<int> h(bool b) => b ? f() : g();
}
''';
    var expected = '''
import 'dart:async';
abstract class C {
  FutureOr<int?> f();
  FutureOr<int>? g();
  FutureOr<int?> h(bool b) => b ? f() : g();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_back_propagation_stops_at_implicitly_typed_variables() async {
    var content = '''
class C {
  int v;
  C(this.v);
}
f(C c) {
  var x = c.v;
  print(x + 1);
}
main() {
  C(null);
}
''';
    var expected = '''
class C {
  int? v;
  C(this.v);
}
f(C c) {
  var x = c.v!;
  print(x + 1);
}
main() {
  C(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_call_already_migrated_extension() async {
    var content = '''
import 'already_migrated.dart';
void f() {
  <int>[].g();
}
''';
    var alreadyMigrated = '''
// @dart=2.12
extension Ext<T> on List<T> {
  g() {}
}
''';
    var expected = '''
import 'already_migrated.dart';
void f() {
  <int>[].g();
}
''';
    await _checkSingleFileChanges(content, expected, migratedInput: {
      '$projectPath/lib/already_migrated.dart': alreadyMigrated
    });
  }

  Future<void> test_call_generic_function_returns_generic_class() async {
    var content = '''
class B<E> implements List<E/*?*/> {
  final C c;
  B(this.c);
  B<T> cast<T>() => c._castFrom<E, T>(this);
}
abstract class C {
  B<T> _castFrom<S, T>(B<S> source);
}
''';
    var expected = '''
class B<E> implements List<E?> {
  final C c;
  B(this.c);
  B<T> cast<T>() => c._castFrom<E, T>(this);
}
abstract class C {
  B<T> _castFrom<S, T>(B<S> source);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_call_migrated_base_class_method_non_nullable() async {
    var content = '''
abstract class M<V> implements Map<String, V> {}
void f(bool b, M<int> m, int i) {
  if (b) {
    m['x'] = i;
  }
}
void g(bool b, M<int> m) {
  f(b, m, null);
}
''';
    var expected = '''
abstract class M<V> implements Map<String, V> {}
void f(bool b, M<int?> m, int? i) {
  if (b) {
    m['x'] = i;
  }
}
void g(bool b, M<int?> m) {
  f(b, m, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_call_migrated_base_class_method_nullable() async {
    var content = '''
abstract class M<V> implements Map<String, V> {}
int f(M<int> m) => m['x'];
''';
    var expected = '''
abstract class M<V> implements Map<String, V> {}
int? f(M<int> m) => m['x'];
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_catch_simple() async {
    var content = '''
void f() {
  try {} catch (ex, st) {}
}
''';
    var expected = '''
void f() {
  try {} catch (ex, st) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_catch_simple_with_modifications() async {
    var content = '''
void f(String x, StackTrace y) {
  try {} catch (ex, st) {
    ex = x;
    st = y;
  }
}
main() {
  f(null, null);
}
''';
    var expected = '''
void f(String? x, StackTrace? y) {
  try {} catch (ex, st) {
    ex = x;
    st = y!;
  }
}
main() {
  f(null, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_catch_with_on() async {
    var content = '''
void f() {
  try {} on String catch (ex, st) {}
}
''';
    var expected = '''
void f() {
  try {} on String catch (ex, st) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_catch_with_on_with_modifications() async {
    var content = '''
void f(String x, StackTrace y) {
  try {} on String catch (ex, st) {
    ex = x;
    st = y;
  }
}
main() {
  f(null, null);
}
''';
    var expected = '''
void f(String? x, StackTrace? y) {
  try {} on String? catch (ex, st) {
    ex = x;
    st = y!;
  }
}
main() {
  f(null, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_class_alias_synthetic_constructor_with_parameters() async {
    var content = '''
void main() {
  D d = D(null);
}
class C {
  C(int i);
}
mixin M {}
class D = C with M;
''';
    var expected = '''
void main() {
  D d = D(null);
}
class C {
  C(int? i);
}
mixin M {}
class D = C with M;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_class_alias_synthetic_constructor_with_parameters_and_subclass() async {
    var content = '''
void main() {
  E e = E(null);
}
class C {
  C(int i);
}
mixin M {}
class D = C with M;
class E extends D {
  E(int i) : super(i);
}
''';
    var expected = '''
void main() {
  E e = E(null);
}
class C {
  C(int? i);
}
mixin M {}
class D = C with M;
class E extends D {
  E(int? i) : super(i);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_class_type_param_bound_references_class() async {
    var content = '''
class Node<T extends Node<T>> {
  final List<T> nodes = <T>[];
}
class C extends Node<C> {}
main() {
  var x = C();
  x.nodes.add(x);
}
''';
    var expected = '''
class Node<T extends Node<T>> {
  final List<T> nodes = <T>[];
}
class C extends Node<C> {}
main() {
  var x = C();
  x.nodes.add(x);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_class_with_default_constructor() async {
    var content = '''
void main() => f(Foo());
f(Foo f) {}
class Foo {}
''';
    var expected = '''
void main() => f(Foo());
f(Foo f) {}
class Foo {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_code_inside_switch_does_not_imply_non_null_intent() async {
    var content = '''
int f(int i, int j) {
  switch (i) {
    case 0:
      return j + 1;
    default:
      return 0;
  }
}
int g(int i, int j) {
  if (i == 0) {
    return f(i, j);
  } else {
    return 0;
  }
}
main() {
  g(0, null);
}
''';
    var expected = '''
int f(int i, int? j) {
  switch (i) {
    case 0:
      return j! + 1;
    default:
      return 0;
  }
}
int g(int i, int? j) {
  if (i == 0) {
    return f(i, j);
  } else {
    return 0;
  }
}
main() {
  g(0, null);
}
''';
    // Note: prior to the fix for https://github.com/dart-lang/sdk/issues/41407,
    // we would consider the use of `j` in `f` to establish non-null intent, so
    // the null check would be erroneously placed in `g`'s call to `f`.
    await _checkSingleFileChanges(content, expected, warnOnWeakCode: true);
  }

  Future<void> test_comment_bang_implies_non_null_intent() async {
    var content = '''
void f(int/*!*/ i) {}
void g(bool b, int i) {
  if (b) f(i);
}
main() {
  g(false, null);
}
''';
    var expected = '''
void f(int i) {}
void g(bool b, int? i) {
  if (b) f(i!);
}
main() {
  g(false, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_comment_question_implies_nullable() async {
    var content = '''
void _f() {
  int/*?*/ i = 0;
}
''';
    var expected = '''
void _f() {
  int? i = 0;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_conditional_assert_statement_does_not_imply_non_null_intent() async {
    var content = '''
void f(bool b, int i) {
  if (b) return;
  assert(i != null);
}
void g(bool b, int i) {
  if (b) f(b, i);
}
main() {
  g(true, null);
}
''';
    var expected = '''
void f(bool b, int? i) {
  if (b) return;
  assert(i != null);
}
void g(bool b, int? i) {
  if (b) f(b, i);
}
main() {
  g(true, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_conditional_dereference_does_not_imply_non_null_intent() async {
    var content = '''
void f(bool b, int i) {
  if (b) i.abs();
}
void g(bool b, int i) {
  if (b) f(b, i);
}
main() {
  g(false, null);
}
''';
    var expected = '''
void f(bool b, int? i) {
  if (b) i!.abs();
}
void g(bool b, int? i) {
  if (b) f(b, i);
}
main() {
  g(false, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_conditional_expression_guard_subexpression() async {
    var content = '''
void f(String s, int x, int/*?*/ n) {
  s == null ? (x = n) : (x = s.length);
}
''';
    var expected = '''
void f(String s, int x, int? n) {
  s == null ? (x = n!) : (x = s.length);
}
''';
    await _checkSingleFileChanges(content, expected, warnOnWeakCode: true);
  }

  Future<void> test_conditional_expression_guard_value_ifFalse() async {
    var content = 'int f(String s, int/*?*/ n) => s != null ? s.length : n;';
    var expected = 'int f(String s, int? n) => s != null ? s.length : n!;';
    await _checkSingleFileChanges(content, expected, warnOnWeakCode: true);
  }

  Future<void> test_conditional_expression_guard_value_ifTrue() async {
    var content = 'int f(String s, int/*?*/ n) => s == null ? n : s.length;';
    var expected = 'int f(String s, int? n) => s == null ? n! : s.length;';
    await _checkSingleFileChanges(content, expected, warnOnWeakCode: true);
  }

  Future<void>
      test_conditional_non_null_usage_does_not_imply_non_null_intent() async {
    var content = '''
void f(bool b, int i, int j) {
  if (b) i.gcd(j);
}
void g(bool b, int i, int j) {
  if (b) f(b, i, j);
}
main() {
  g(false, 0, null);
}
''';
    var expected = '''
void f(bool b, int i, int? j) {
  if (b) i.gcd(j!);
}
void g(bool b, int i, int? j) {
  if (b) f(b, i, j);
}
main() {
  g(false, 0, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_conditional_usage_does_not_propagate_non_null_intent() async {
    var content = '''
void f(int i) {
  assert(i != null);
}
void g(bool b, int i) {
  if (b) f(i);
}
void h(bool b1, bool b2, int i) {
  if (b1) g(b2, i);
}
main() {
  h(true, false, null);
}
''';
    var expected = '''
void f(int i) {
  assert(i != null);
}
void g(bool b, int? i) {
  if (b) f(i!);
}
void h(bool b1, bool b2, int? i) {
  if (b1) g(b2, i);
}
main() {
  h(true, false, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_conditionalExpression_typeParameter_bound() async {
    var content = '''
num f1<T extends num>(bool b, num x, T y) => b ? x : y;
num f2<T extends num>(bool b, num x, T y) => b ? x : y;
num f3<T extends num>(bool b, num x, T y) => b ? x : y;
num f4<T extends num>(bool b, num x, T y) => b ? x : y;

void main() {
  int x1 = f1<int/*?*/>(true, 0, null);
  int x2 = f2<int/*!*/>(true, 0, null);
  int x3 = f3<int>(true, null, 0);
  int x4 = f4<int>(true, 0, 0);
}
''';
    var expected = '''
num? f1<T extends num?>(bool b, num x, T y) => b ? x : y;
num? f2<T extends num>(bool b, num x, T? y) => b ? x : y;
num? f3<T extends num>(bool b, num? x, T y) => b ? x : y;
num f4<T extends num>(bool b, num x, T y) => b ? x : y;

void main() {
  int? x1 = f1<int?>(true, 0, null) as int?;
  int? x2 = f2<int>(true, 0, null) as int?;
  int? x3 = f3<int>(true, null, 0) as int?;
  int x4 = f4<int>(true, 0, 0) as int;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_constructorDeclaration_factory_non_null_return() async {
    var content = '''
class C {
  C._();
  factory C() {
    C c = f();
    return c;
  }
}
C f() => null;
''';
    var expected = '''
class C {
  C._();
  factory C() {
    C c = f()!;
    return c;
  }
}
C? f() => null;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_constructorDeclaration_factory_simple() async {
    var content = '''
class C {
  C._();
  factory C(int i) => C._();
}
main() {
  C(null);
}
''';
    var expected = '''
class C {
  C._();
  factory C(int? i) => C._();
}
main() {
  C(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_constructorDeclaration_named() async {
    var content = '''
class C {
  C.named(int i);
}
main() {
  C.named(null);
}
''';
    var expected = '''
class C {
  C.named(int? i);
}
main() {
  C.named(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_constructorDeclaration_namedParameter() async {
    var content = '''
class C {
  C({Key key});
}
class Key {}
''';
    var expected = '''
class C {
  C({Key? key});
}
class Key {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_convert_required() async {
    addMetaPackage();
    var content = '''
import 'package:meta/meta.dart';
void f({@required String s}) {}
''';
    var expected = '''
import 'package:meta/meta.dart';
void f({required String s}) {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_assignment_field() async {
    var content = '''
class C {
  int x = 0;
}
void f(C c) {
  c.x = null;
}
''';
    var expected = '''
class C {
  int? x = 0;
}
void f(C c) {
  c.x = null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_assignment_field_in_cascade() async {
    var content = '''
class C {
  int x = 0;
}
void f(C c) {
  c..x = null;
}
''';
    var expected = '''
class C {
  int? x = 0;
}
void f(C c) {
  c..x = null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_assignment_local() async {
    var content = '''
void main() {
  int i = 0;
  i = null;
}
''';
    var expected = '''
void main() {
  int? i = 0;
  i = null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_assignment_setter() async {
    var content = '''
class C {
  void set s(int value) {}
}
void f(C c) {
  c.s = null;
}
''';
    var expected = '''
class C {
  void set s(int? value) {}
}
void f(C c) {
  c.s = null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_field_read() async {
    var content = '''
class C {
  int/*?*/ f = 0;
}
int f(C c) => c.f;
''';
    var expected = '''
class C {
  int? f = 0;
}
int? f(C c) => c.f;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_function_return_type() async {
    var content = '''
int Function() f(int Function() x) => x;
int g() => null;
main() {
  f(g);
}
''';
    var expected = '''
int? Function() f(int? Function() x) => x;
int? g() => null;
main() {
  f(g);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_generic_contravariant_inward() async {
    var content = '''
class C<T> {
  void f(T t) {}
}
void g(C<int> c, int i) {
  c.f(i);
}
void test(C<int> c) {
  g(c, null);
}
''';

    // Default behavior is to add nullability at the call site.  Rationale: this
    // is correct in the common case where the generic parameter represents the
    // type of an item in a container.  Also, if there are many callers that are
    // largely independent, adding nullability to the callee would likely
    // propagate to a field in the class, and thence (via return values of other
    // methods) to most users of the class.  Whereas if we add nullability at
    // the call site it's possible that other call sites won't need it.
    var expected = '''
class C<T> {
  void f(T t) {}
}
void g(C<int?> c, int? i) {
  c.f(i);
}
void test(C<int?> c) {
  g(c, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_generic_contravariant_inward_function() async {
    var content = '''
T f<T>(T t) => t;
int g(int x) => f<int>(x);
void h() {
  g(null);
}
''';

    // As with the generic class case (see
    // [test_data_flow_generic_contravariant_inward_function]), we favor adding
    // nullability at the call site, so that other uses of `f` don't necessarily
    // see a nullable return value.
    var expected = '''
T f<T>(T t) => t;
int? g(int? x) => f<int?>(x);
void h() {
  g(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_data_flow_generic_contravariant_inward_using_core_class() async {
    var content = '''
void f(List<int> x, int i) {
  x.add(i);
}
void test(List<int> x) {
  f(x, null);
}
''';
    var expected = '''
void f(List<int?> x, int? i) {
  x.add(i);
}
void test(List<int?> x) {
  f(x, null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_generic_covariant_outward() async {
    var content = '''
class C<T> {
  T getValue() => null;
}
int f(C<int> x) => x.getValue();
''';
    var expected = '''
class C<T> {
  T? getValue() => null;
}
int? f(C<int> x) => x.getValue();
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_generic_covariant_substituted() async {
    var content = '''
abstract class C<T> {
  T getValue();
}
int f(C<int/*?*/> x) => x.getValue();
''';
    var expected = '''
abstract class C<T> {
  T getValue();
}
int? f(C<int?> x) => x.getValue();
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_indexed_get_index_value() async {
    var content = '''
class C {
  int operator[](int i) => 1;
}
int f(C c) => c[null];
''';
    var expected = '''
class C {
  int operator[](int? i) => 1;
}
int f(C c) => c[null];
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_indexed_get_value() async {
    var content = '''
class C {
  int operator[](int i) => null;
}
int f(C c) => c[0];
''';
    var expected = '''
class C {
  int? operator[](int i) => null;
}
int? f(C c) => c[0];
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_indexed_set_index_value() async {
    var content = '''
class C {
  void operator[]=(int i, int j) {}
}
void f(C c) {
  c[null] = 0;
}
''';
    var expected = '''
class C {
  void operator[]=(int? i, int j) {}
}
void f(C c) {
  c[null] = 0;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_indexed_set_index_value_in_cascade() async {
    var content = '''
class C {
  void operator[]=(int i, int j) {}
}
void f(C c) {
  c..[null] = 0;
}
''';
    var expected = '''
class C {
  void operator[]=(int? i, int j) {}
}
void f(C c) {
  c..[null] = 0;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_indexed_set_value() async {
    var content = '''
class C {
  void operator[]=(int i, int j) {}
}
void f(C c) {
  c[0] = null;
}
''';
    var expected = '''
class C {
  void operator[]=(int i, int? j) {}
}
void f(C c) {
  c[0] = null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_inward() async {
    var content = '''
int f(int i) => 0;
int g(int i) => f(i);
void test() {
  g(null);
}
''';

    var expected = '''
int f(int? i) => 0;
int g(int? i) => f(i);
void test() {
  g(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_inward_missing_type() async {
    var content = '''
int f(int i) => 0;
int g(i) => f(i); // TODO(danrubel): suggest type
void test() {
  g(null);
}
''';

    var expected = '''
int f(int? i) => 0;
int g(i) => f(i); // TODO(danrubel): suggest type
void test() {
  g(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_local_declaration() async {
    var content = '''
void f(int i) {
  int j = i;
}
main() {
  f(null);
}
''';
    var expected = '''
void f(int? i) {
  int? j = i;
}
main() {
  f(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_local_reference() async {
    var content = '''
void f(int i) {}
void g(int i) {
  int j = i;
  f(i);
}
main() {
  g(null);
}
''';
    var expected = '''
void f(int? i) {}
void g(int? i) {
  int? j = i;
  f(i);
}
main() {
  g(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_method_call_in_cascade() async {
    var content = '''
class C {
  void m(int x) {}
}
void f(C c) {
  c..m(null);
}
''';
    var expected = '''
class C {
  void m(int? x) {}
}
void f(C c) {
  c..m(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_outward() async {
    var content = '''
int f(int i) => null;
int g(int i) => f(i);
''';

    var expected = '''
int? f(int i) => null;
int? g(int i) => f(i);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_data_flow_outward_missing_type() async {
    var content = '''
f(int i) => null; // TODO(danrubel): suggest type
int g(int i) => f(i);
''';

    var expected = '''
f(int i) => null; // TODO(danrubel): suggest type
int? g(int i) => f(i);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_definitely_assigned_value() async {
    var content = '''
String f(bool b) {
  String s;
  if (b) {
    s = 'true';
  } else {
    s = 'false';
  }
  return s;
}
''';
    var expected = '''
String f(bool b) {
  String s;
  if (b) {
    s = 'true';
  } else {
    s = 'false';
  }
  return s;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_discard_simple_condition_keep_else() async {
    var content = '''
int f(int i) {
  if (i == null) {
    return null;
  } else {
    return i + 1;
  }
}
''';

    var expected = '''
int f(int i) {
  /* if (i == null) {
    return null;
  } else {
    */ return i + 1; /*
  } */
}
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_discard_simple_condition_keep_then() async {
    var content = '''
int f(int i) {
  if (i != null) {
    return i + 1;
  } else {
    return null;
  }
}
''';

    var expected = '''
int f(int i) {
  /* if (i != null) {
    */ return i + 1; /*
  } else {
    return null;
  } */
}
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_do_not_add_question_to_null_type() async {
    var content = '''
Null f() => null;
''';
    var expected = '''
Null f() => null;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_do_not_propagate_non_null_intent_into_callback() async {
    var content = '''
void f(int/*!*/ Function(int) callback) {
  callback(null);
}
int g(int x) => x;
void test() {
  f(g);
}
''';
    // Even though `g` is passed to `f`'s `callback` parameter, non-null intent
    // is not allowed to propagate backward from the return type of `callback`
    // to the return type of `g`, because `g` might be used elsewhere in a
    // context where it's important for its return type to be nullable.  So no
    // null check is added to `g`, and instead a cast (which is guaranteed to
    // fail) is added at the site of the call to `f`.
    //
    // Note: https://github.com/dart-lang/sdk/issues/40471 tracks the fact that
    // we ought to alert the user to the presence of such casts.
    var expected = '''
void f(int Function(int?) callback) {
  callback(null);
}
int? g(int? x) => x;
void test() {
  f(g as int Function(int?));
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_downcast_dynamic_function_to_functionType() async {
    var content = '''
void f(Function a) {
  int Function<T>(String y) f1 = a;
  Function b = null;
  int Function<T>(String y) f2 = b;
}
''';
    // Don't assume any new nullabilities, but keep known nullabilities.
    var expected = '''
void f(Function a) {
  int Function<T>(String y) f1 = a as int Function<T>(String);
  Function? b = null;
  int Function<T>(String y)? f2 = b as int Function<T>(String)?;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_downcast_dynamic_to_functionType() async {
    var content = '''
void f(dynamic a) {
  int Function<T>(String y) f1 = a;
  dynamic b = null;
  int Function<T>(String y) f2 = b;
}
''';
    // Don't assume any new nullabilities, but keep known nullabilities.
    var expected = '''
void f(dynamic a) {
  int Function<T>(String y) f1 = a;
  dynamic b = null;
  int Function<T>(String y)? f2 = b;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_downcast_dynamic_type_argument() async {
    // This pattern is common and seems to have this as a best migration. It is
    // less clear, but plausible, that this holds for other types of type
    // parameter downcasts.
    var content = '''
List<int> f(List a) => a;
void main() {
  f(<int>[null]);
}
''';

    var expected = '''
List<int?> f(List a) => a as List<int?>;
void main() {
  f(<int?>[null]);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @failingTest
  Future<void> test_downcast_not_widest_type_type_parameters() async {
    // Fails because a hard assignment from List<int/*1*/> to List<int/*2*/>
    // doesn't create a hard edge from 1 to 2. Perhaps this is correct. In this
    // example it seems incorrect.
    var content = '''
void f(dynamic a) {
  List<int> hardToNonNullNonNull = a;
  List<int> hardToNullNonNull = a;
  List<int> hardToNonNullNull = a;
  List<int/*!*/>/*!*/ nonNullNonNull;
  List<int/*?*/>/*!*/ nullNonNull;
  List<int/*!*/>/*?*/ nonNullNull;
  nonNullNonNull = hardToNonNullNonNull
  nonNullNull = hardToNonNullNull
  nullNonNull = hardToNullNonNull
}
''';
    var expected = '''
void f(dynamic a) {
  List<int> hardToNonNullNonNull = a;
  List<int?> hardToNullNonNull = a;
  List<int>? hardToNonNullNull = a;
  List<int> nonNullNonNull;
  List<int?> nullNonNull;
  List<int>? nonNullNull;
  nonNullNonNull = hardToNonNullNonNull
  nonNullNull = hardToNonNullNull
  nullNonNull = hardToNullNonNull
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_downcast_to_null() async {
    // This probably doesn't arise too often for real-world code, since it is
    // most likely a mistake.  Still, we want to make sure we don't crash.
    var content = '''
test() {
  var x = List.filled(3, null);
  x[0] = 1;
}
''';
    var expected = '''
test() {
  var x = List.filled(3, null);
  x[0] = 1 as Null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_downcast_type_argument_preserve_nullability() async {
    // There are no examples in front of us yet where anyone downcasts a type
    // with a nullable type parameter. This is maybe correct, maybe not, and it
    // unblocks us to find out which at a later point in time.
    var content = '''
List<int> f(Iterable<num> a) => a;
void main() {
  f(<num>[null]);
}
''';

    var expected = '''
List<int?> f(Iterable<num?> a) => a as List<int?>;
void main() {
  f(<num?>[null]);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @failingTest
  Future<void> test_downcast_widest_type_from_related_type_parameters() async {
    var content = '''
List<int> f(Iterable<int/*?*/> a) => a;
''';
    var expected = '''
List<int?> f(Iterable<int?> a) => a;
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/39368')
  Future<void> test_downcast_widest_type_from_top_type_parameters() async {
    var content = '''
List<int> f1(dynamic a) => a;
List<int> f2(Object b) => b;
''';
    // Note: even though the type `dynamic` permits `null`, the migration engine
    // sees that there is no code path that could cause `f1` to be passed a null
    // value, so it leaves its return type as non-nullable.
    var expected = '''
List<int?> f1(dynamic a) => a;
List<int?> f2(Object b) => b;
''';
    await _checkSingleFileChanges(content, expected);
  }

  @failingTest
  Future<void>
      test_downcast_widest_type_from_unrelated_type_parameters() async {
    var content = '''
abstract class C<A, B> implements List<A> {}
C<int, num> f(List<int> a) => a;
''';
    var expected = '''
abstract class C<A, B> implements List<A> {}
C<int, num?> f(List<int> a) => a;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_dynamic_dispatch_to_object_method() async {
    var content = '''
String f(dynamic x) => x.toString();
''';
    var expected = '''
String f(dynamic x) => x.toString();
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_dynamic_method_call() async {
    var content = '''
class C {
  int g(int i) => i;
}
int f(bool b, dynamic d) {
  if (b) return 0;
  return d.g(null);
}
main() {
  f(true, null);
  f(false, C());
}
''';
    // `d.g(null)` is a dynamic call, so we can't tell that it will target `C.g`
    // at runtime.  So we can't figure out that we need to make g's argument and
    // return types nullable.
    //
    // We do, however, make f's return type nullable, since there is no way of
    // knowing whether a dynamic call will return `null`.
    var expected = '''
class C {
  int g(int i) => i;
}
int? f(bool b, dynamic d) {
  if (b) return 0;
  return d.g(null);
}
main() {
  f(true, null);
  f(false, C());
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_dynamic_property_access() async {
    var content = '''
class C {
  int get g => 0;
}
int f(bool b, dynamic d) {
  if (b) return 0;
  return d.g;
}
main() {
  f(true, null);
  f(false, C());
}
''';
    var expected = '''
class C {
  int get g => 0;
}
int? f(bool b, dynamic d) {
  if (b) return 0;
  return d.g;
}
main() {
  f(true, null);
  f(false, C());
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_dynamic_toString() async {
    var content = '''
String f(dynamic x) => x.toString();
''';
    var expected = '''
String f(dynamic x) => x.toString();
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/40174')
  Future<void> test_eliminate_dead_if_inside_for_element() async {
    var content = '''
List<int> _f(List<int/*!*/> xs) => [for(var x in xs) if (x == null) 1];
''';
    var expected = '''
List<int> _f(List<int> xs) => [];
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_enum() async {
    var content = '''
enum E {
  value
};

E f() => E.value;
int g() => f().index;

void h() {
  for(var value in E.values) {}
  E.values.forEach((value) {});

  f().toString();
  f().runtimeType;
  f().hashCode;
  f().noSuchMethod(throw '');
  f() == f();
}
''';
    var expected = '''
enum E {
  value
};

E f() => E.value;
int g() => f().index;

void h() {
  for(var value in E.values) {}
  E.values.forEach((value) {});

  f().toString();
  f().runtimeType;
  f().hashCode;
  f().noSuchMethod(throw '');
  f() == f();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_exact_nullability_counterexample() async {
    var content = '''
void f(List<int> x) {
  x.add(1);
}
void g() {
  f([null]);
}
void h(List<int> x) {
  f(x);
}
''';
    // The `null` in `g` causes `f`'s `x` argument to have type `List<int?>`.
    // Even though `f` calls a method that uses `List`'s type parameter
    // contravariantly (the `add` method), that is not sufficient to cause exact
    // nullability propagation, since value passed to `add` has a
    // non-nullable type.  So nullability is *not* propagated back to `h`.
    var expected = '''
void f(List<int?> x) {
  x.add(1);
}
void g() {
  f([null]);
}
void h(List<int> x) {
  f(x);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_exact_nullability_doesnt_affect_function_args() async {
    // Test attempting to create a bug from #40625. Currently passes, but if it
    // breaks, that bug may need to be reopened.
    var content = '''
class C<T> {
  int Function(T) f;
}
void main() {
  C<String> c;
  int Function(String) f1 = c.f; // should not have a nullable arg
  c.f(null); // exact nullability induced here
}
''';
    var expected = '''
class C<T> {
  int Function(T)? f;
}
void main() {
  C<String?> c;
  int Function(String)? f1 = c.f; // should not have a nullable arg
  c.f!(null); // exact nullability induced here
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_exact_nullability_doesnt_affect_function_returns() async {
    // Test attempting to create a bug from #40625. Currently passes, but if it
    // breaks, that bug may need to be reopened.
    var content = '''
class C<T> {
  T Function(String) f;
}
int Function(String) f1; // should not have a nullable return
void main() {
  C<int> c;
  c.f = f1;
  c.f = (_) => null; // exact nullability induced here
}
''';
    var expected = '''
class C<T> {
  T Function(String)? f;
}
int Function(String)? f1; // should not have a nullable return
void main() {
  C<int?> c;
  c.f = f1;
  c.f = (_) => null; // exact nullability induced here
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_exact_nullability_doesnt_affect_typedef_args() async {
    // Test attempting to create a bug from #40625. Currently passes, but if it
    // breaks, that bug may need to be reopened.
    var content = '''
typedef F<T> = int Function(T);
F<String> f1;

void main() {
  f1(null); // induce exact nullability
  int Function(String) f2 = f1; // shouldn't have a nullable arg
}
''';
    var expected = '''
typedef F<T> = int Function(T);
F<String?>? f1;

void main() {
  f1!(null); // induce exact nullability
  int Function(String)? f2 = f1; // shouldn't have a nullable arg
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_exact_nullability_doesnt_affect_typedef_returns() async {
    // Test attempting to create a bug from #40625. Currently passes, but if it
    // breaks, that bug may need to be reopened.
    var content = '''
typedef F<T> = T Function(String);
int Function(String) f1; // should not have a nullable return
void main() {
  F<int> f2 = f1;
  f2 = (_) => null; // exact nullability induced here
}
''';
    var expected = '''
typedef F<T> = T Function(String);
int Function(String)? f1; // should not have a nullable return
void main() {
  F<int?>? f2 = f1;
  f2 = (_) => null; // exact nullability induced here
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/41409')
  Future<void> test_exact_nullability_in_nested_list() async {
    var content = '''
f(List<int/*?*/> y) {
  var x = <List<int>>[];
  x.add(y);
}
''';
    var expected = '''
f(List<int?> y) {
  var x = <List<int?>>[];
  x.add(y);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_explicit_nullable_overrides_hard_edge() async {
    var content = '''
int f(int/*?*/ i) => i + 1;
''';
    var expected = '''
int f(int? i) => i! + 1;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_expression_bang_hint() async {
    var content = '''
int f(int/*?*/ i) => i/*!*/;
''';
    var expected = '''
int f(int? i) => i!;
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_expression_bang_hint_in_as() async {
    var content = '''
int f(num/*?*/ i) => i as int/*!*/;
''';
    var expected = '''
int f(num? i) => i as int;
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/41788')
  Future<void> test_expression_bang_hint_in_as_wrapped() async {
    var content = '''
int f(num/*?*/ i) => (i as int)/*!*/;
''';
    var expected = '''
int f(num? i) => (i as int?)!;
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_expression_bang_hint_unnecessary() async {
    var content = '''
int/*?*/ f(int/*?*/ i) => i/*!*/;
''';
    // The user requested a null check so we should add it even if it's not
    // required to avoid compile errors.
    var expected = '''
int? f(int? i) => i!;
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_expression_bang_hint_unnecessary_literal() async {
    var content = 'int/*?*/ f() => 1/*!*/;';
    // The user requested a null check so we should add it even if it's not
    // required to avoid compile errors.
    var expected = 'int? f() => 1!;';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_expression_bang_hint_with_cast() async {
    var content = 'int f(Object/*?*/ o) => o/*!*/;';
    var expected = 'int f(Object? o) => o! as int;';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_expression_nullable_cast_then_checked() async {
    var content = '''
int/*!*/ f(num/*?*/ i) => (i as int);
''';
    var expected = '''
int f(num? i) => (i as int);
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/41788')
  Future<void> test_expression_wrapped_with_null_check_and_null_intent() async {
    var content = '''
int/*!*/ f(int/*?*/ i) => (i)/*!*/;
''';
    var expected = '''
int f(int? i) => i!;
''';
    await _checkSingleFileChanges(content, expected, removeViaComments: true);
  }

  Future<void> test_extension_null_check_target() async {
    var content = '''
extension E on int/*!*/ {
  int get plusOne => this + 1;
}
int f(int/*?*/ x) => x.plusOne;
''';
    var expected = '''
extension E on int {
  int get plusOne => this + 1;
}
int f(int? x) => x!.plusOne;
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/42529')
  Future<void> test_extension_nullable_target() async {
    var content = '''
extension E on int {
  int get one => 1;
}
int f(int/*?*/ x) => x.one;
''';
    var expected = '''
extension E on int? {
  int get one => 1;
}
int f(int? x) => x.one;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_nullableOnType_addsNullCheckToThis() async {
    var content = '''
extension E on String /*?*/ {
  void m() => this.length;
}
''';
    var expected = '''
extension E on String? {
  void m() => this!.length;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_nullableOnType_typeArgument() async {
    var content = '''
extension E on List<String> {
  void m() {}
}
void f(List<String> list) => list.m();
void g() => f([null]);
''';
    var expected = '''
extension E on List<String?> {
  void m() {}
}
void f(List<String?> list) => list.m();
void g() => f([null]);
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/40023')
  Future<void> test_extension_nullableOnType_typeVariable() async {
    var content = '''
extension E<T> on List<T> {
  void m() {}
}
void f<U>(List<U> list) => list.m();
void g() => f([null]);
''';
    var expected = '''
extension E<T> on List<T?> {
  void m() {}
}
void f<U>(List<U?> list) => list.m();
void g() => f([null]);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_nullableOnType_viaExplicitInvocation() async {
    var content = '''
class C {}
extension E on C {
  void m() {}
}
void f() => E(null).m();
''';
    var expected = '''
class C {}
extension E on C? {
  void m() {}
}
void f() => E(null).m();
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/40023')
  Future<void> test_extension_nullableOnType_viaImplicitInvocation() async {
    var content = '''
class C {}
extension E on C {
  void m() {}
}
void f(C c) => c.m();
void g() => f(null);
''';
    var expected = '''
class C {}
extension E on C? {
  void m() {}
}
void f(C? c) => c.m();
void g() => f(null);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_on_generic_type() async {
    var content = '''
class C<T> {
  final T value;
  C(this.value);
}
extension E<T> on Future<C<T/*?*/>> {
  Future<T> get asyncValue async => (await this).value;
}
''';
    var expected = '''
class C<T> {
  final T value;
  C(this.value);
}
extension E<T> on Future<C<T?>> {
  Future<T?> get asyncValue async => (await this).value;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_on_type_param_implementation() async {
    var content = '''
abstract class C {
  C _clone();
}
extension Cloner<T extends C> on T {
  T clone() => _clone() as T;
}
''';
    var expected = '''
abstract class C {
  C _clone();
}
extension Cloner<T extends C> on T {
  T clone() => _clone() as T;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_on_type_param_usage() async {
    var content = '''
abstract class C {
  C _clone();
}
extension Cloner<T extends C> on T {
  T clone() => throw Exception();
}
C f(C c) => c.clone();
''';
    var expected = '''
abstract class C {
  C _clone();
}
extension Cloner<T extends C> on T {
  T clone() => throw Exception();
}
C f(C c) => c.clone();
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_override() async {
    var content = '''
extension E on int {
  int get plusOne => this + 1;
}
int f(int x) => E(x).plusOne;
''';
    var expected = '''
extension E on int {
  int get plusOne => this + 1;
}
int f(int x) => E(x).plusOne;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_override_null_check_target() async {
    var content = '''
extension E on int/*!*/ {
  int get plusOne => this + 1;
}
int f(int/*?*/ x) => E(x).plusOne;
''';
    var expected = '''
extension E on int {
  int get plusOne => this + 1;
}
int f(int? x) => E(x!).plusOne;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_extension_override_nullable_result_type() async {
    var content = '''
extension E on int {
  int get nullValue => null;
}
int f(int x) => E(x).nullValue;
''';
    var expected = '''
extension E on int {
  int? get nullValue => null;
}
int? f(int x) => E(x).nullValue;
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/42529')
  Future<void> test_extension_override_nullable_target() async {
    var content = '''
extension E on int {
  int get one => 1;
}
int f(int/*?*/ x) => E(x).one;
''';
    var expected = '''
extension E on int? {
  int get one => 1;
}
int f(int? x) => E(x).one;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_final_uninitalized_used() async {
    var content = '''
class C {
  final String s;

  f() {
    g(s);
  }
}
g(String /*!*/ s) {}
''';
    var expected = '''
class C {
  late final String s;

  f() {
    g(s);
  }
}
g(String s) {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_formal_param_typed() async {
    var content = '''
class C {
  int i;
  C(int this.i);
}
main() {
  C(null);
}
''';
    var expected = '''
class C {
  int? i;
  C(int? this.i);
}
main() {
  C(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_formal_param_typed_non_nullable() async {
    var content = '''
class C {
  int/*!*/ i;
  C(int this.i);
}
void f(int i, bool b) {
  if (b) {
    C(i);
  }
}
main() {
  f(null, false);
}
''';
    var expected = '''
class C {
  int i;
  C(int this.i);
}
void f(int? i, bool b) {
  if (b) {
    C(i!);
  }
}
main() {
  f(null, false);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_formal_param_untyped() async {
    var content = '''
class C {
  int i;
  C(this.i);
}
main() {
  C(null);
}
''';
    var expected = '''
class C {
  int? i;
  C(this.i);
}
main() {
  C(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_formal_parameters_do_not_promote() async {
    var content = '''
class A {}

class B extends A {}

class C extends A {}

abstract class D {
  final A x;
  D(this.x) {
    if (x is B) {
      visitB(x);
    } else {
      visitC(x as C);
    }
  }

  void visitB(B b);

  void visitC(C c);
}
''';
    var expected = '''
class A {}

class B extends A {}

class C extends A {}

abstract class D {
  final A x;
  D(this.x) {
    if (x is B) {
      visitB(x as B);
    } else {
      visitC(x as C);
    }
  }

  void visitB(B b);

  void visitC(C c);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initialized_at_declaration_site() async {
    var content = '''
class C {
  int i = 0;
  C();
}
''';
    var expected = '''
class C {
  int i = 0;
  C();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_field_initialized_at_declaration_site_no_constructor() async {
    var content = '''
class C {
  int i = 0;
}
''';
    var expected = '''
class C {
  int i = 0;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initialized_in_constructor() async {
    var content = '''
class C {
  int i;
  C() : i = 0;
}
''';
    var expected = '''
class C {
  int i;
  C() : i = 0;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_field_initialized_in_constructor_with_factories_and_redirects() async {
    var content = '''
class C {
  int i;
  C() : i = 0;
  factory C.factoryConstructor => C();
  factory C.factoryRedirect = D;
  C.redirect : this();
}
class D extends C {}
''';
    var expected = '''
class C {
  int i;
  C() : i = 0;
  factory C.factoryConstructor => C();
  factory C.factoryRedirect = D;
  C.redirect : this();
}
class D extends C {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initializer_simple() async {
    var content = '''
class C {
  int f;
  C(int i) : f = i;
}
main() {
  C(null);
}
''';
    var expected = '''
class C {
  int? f;
  C(int? i) : f = i;
}
main() {
  C(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initializer_typed_list_literal() async {
    var content = '''
class C {
  List<int> f;
  C() : f = <int>[null];
}
''';
    var expected = '''
class C {
  List<int?> f;
  C() : f = <int?>[null];
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initializer_untyped_list_literal() async {
    var content = '''
class C {
  List<int> f;
  C() : f = [null];
}
''';
    var expected = '''
class C {
  List<int?> f;
  C() : f = [null];
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initializer_untyped_map_literal() async {
    var content = '''
class C {
  Map<String, int> f;
  C() : f = {"foo": null};
}
''';
    var expected = '''
class C {
  Map<String, int?> f;
  C() : f = {"foo": null};
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_initializer_untyped_set_literal() async {
    var content = '''
class C {
  Set<int> f;
  C() : f = {null};
}
''';
    var expected = '''
class C {
  Set<int?> f;
  C() : f = {null};
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_not_initialized() async {
    var content = '''
class C {
  int i;
  C();
}
''';
    var expected = '''
class C {
  int? i;
  C();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_not_initialized_no_constructor() async {
    var content = '''
class C {
  int i;
}
''';
    var expected = '''
class C {
  int? i;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_overrides_getter() async {
    var content = '''
abstract class C {
  int get i;
}
class D implements C {
  @override
  final int i;
  D._() : i = computeI();
}
int computeI() => null;
''';
    var expected = '''
abstract class C {
  int? get i;
}
class D implements C {
  @override
  final int? i;
  D._() : i = computeI();
}
int? computeI() => null;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_type_inferred() async {
    var content = '''
int f() => null;
class C {
  var x = 1;
  void g() {
    x = f();
  }
}
''';
    // The type of x is inferred as non-nullable from its initializer, but we
    // try to assign a nullable value to it.  So an explicit type must be added.
    var expected = '''
int? f() => null;
class C {
  int? x = 1;
  void g() {
    x = f();
  }
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_uninitalized_used() async {
    var content = '''
class C {
  String s;

  f() {
    g(s);
  }
}
g(String /*!*/ s) {}
''';
    var expected = '''
class C {
  late String s;

  f() {
    g(s);
  }
}
g(String s) {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_field_uninitalized_used_hint() async {
    var content = '''
class C {
  String /*?*/ s;

  f() {
    g(s);
  }
}
g(String /*!*/ s) {}
''';
    var expected = '''
class C {
  String? s;

  f() {
    g(s!);
  }
}
g(String s) {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_firstWhere_complex_target() async {
    // See https://github.com/dart-lang/sdk/issues/43956
    var content = '''
Iterable<Match> allMatches(String str) => 'x'.allMatches(str);

Match matchAsPrefix(String str, [int start = 0]) {
  return allMatches(str)
      .firstWhere((match) => match.start == start, orElse: () => null);
}
''';
    var expected = '''
import 'package:collection/collection.dart' show IterableExtension;

Iterable<Match> allMatches(String str) => 'x'.allMatches(str);

Match? matchAsPrefix(String str, [int start = 0]) {
  return allMatches(str)
      .firstWhereOrNull((match) => match.start == start);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_firstWhere_non_nullable() async {
    var content = '''
int firstEven(Iterable<int> x)
    => x.firstWhere((x) => x.isEven, orElse: () => null);
''';
    var expected = '''
import 'package:collection/collection.dart' show IterableExtension;

int? firstEven(Iterable<int> x)
    => x.firstWhereOrNull((x) => x.isEven);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_firstWhere_non_nullable_with_cast() async {
    var content = '''
int firstNonZero(Iterable<num> x)
    => x.firstWhere((x) => x != 0, orElse: () => null);
''';
    var expected = '''
import 'package:collection/collection.dart' show IterableExtension;

int? firstNonZero(Iterable<num> x)
    => x.firstWhereOrNull((x) => x != 0) as int?;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_firstWhere_non_nullable_with_non_null_assertion() async {
    var content = '''
int/*!*/ firstEven(Iterable<int> x)
    => x.firstWhere((x) => x.isEven, orElse: () => null);
''';
    var expected = '''
import 'package:collection/collection.dart' show IterableExtension;

int firstEven(Iterable<int> x)
    => x.firstWhereOrNull((x) => x.isEven)!;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_firstWhere_nullable() async {
    var content = '''
int firstEven(Iterable<int> x)
    => x.firstWhere((x) => x.isEven, orElse: () => null);
f() => firstEven([null]);
''';
    var expected = '''
int? firstEven(Iterable<int?> x)
    => x.firstWhere((x) => x!.isEven, orElse: () => null);
f() => firstEven([null]);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_firstWhere_nullable_with_cast() async {
    var content = '''
int firstNonZero(Iterable<num> x)
    => x.firstWhere((x) => x != 0, orElse: () => null);
f() => firstNonZero([null]);
''';
    var expected = '''
int? firstNonZero(Iterable<num?> x)
    => x.firstWhere((x) => x != 0, orElse: () => null) as int?;
f() => firstNonZero([null]);
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_flow_analysis_complex() async {
    var content = '''
int f(int x) {
  while (x == null) {
    x = g(x);
  }
  return x;
}
int g(int x) => x == null ? 1 : null;
main() {
  f(null);
}
''';
    // Flow analysis can tell that the loop only exits if x is non-null, so the
    // return type of `f` can remain `int`, and no null check is needed.
    var expected = '''
int f(int? x) {
  while (x == null) {
    x = g(x);
  }
  return x;
}
int? g(int? x) => x == null ? 1 : null;
main() {
  f(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_flow_analysis_simple() async {
    var content = '''
int f(int x) {
  if (x == null) {
    return 0;
  } else {
    return x;
  }
}
main() {
  f(null);
}
''';
    var expected = '''
int f(int? x) {
  if (x == null) {
    return 0;
  } else {
    return x;
  }
}
main() {
  f(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_for_each_basic() async {
    var content = '''
void f(List<int> l) {
  for (var x in l) {
    g(x);
  }
}
void g(int x) {}
main() {
  f([null]);
}
''';
    var expected = '''
void f(List<int?> l) {
  for (var x in l) {
    g(x);
  }
}
void g(int? x) {}
main() {
  f([null]);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_for_each_variable_initialized() async {
    var content = '''
int sum(List<int> list) {
  int total = 0;
  for (var i in list) {
    total = total + i;
  }
  return total;
}
''';
    var expected = '''
int sum(List<int> list) {
  int total = 0;
  for (var i in list) {
    total = total + i;
  }
  return total;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_expression() async {
    var content = '''
int f(int i) {
  var g = (int j) => i;
  return g(i);
}
main() {
  f(null);
}
''';
    var expected = '''
int? f(int? i) {
  var g = (int? j) => i;
  return g(i);
}
main() {
  f(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_expression_invocation() async {
    var content = '''
abstract class C {
  void Function(int) f();
  int/*?*/ Function() g();
}
int test(C c) {
  c.f()(null);
  return c.g()();
}
''';
    var expected = '''
abstract class C {
  void Function(int?) f();
  int? Function() g();
}
int? test(C c) {
  c.f()(null);
  return c.g()();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_expression_invocation_via_getter() async {
    var content = '''
abstract class C {
  void Function(int) get f;
  int/*?*/ Function() get g;
}
int test(C c) {
  c.f(null);
  return c.g();
}
''';
    var expected = '''
abstract class C {
  void Function(int?) get f;
  int? Function() get g;
}
int? test(C c) {
  c.f(null);
  return c.g();
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_typed_field_formal_param() async {
    var content = '''
class C {
  void Function(int) f;
  C(void this.f(int i));
}
main() {
  C(null);
}
''';
    var expected = '''
class C {
  void Function(int)? f;
  C(void this.f(int i)?);
}
main() {
  C(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_typed_field_formal_param_accepts_hint() async {
    var content = '''
class C {
  void Function(int) f;
  C(void this.f(int i) /*?*/);
}
''';
    var expected = '''
class C {
  void Function(int)? f;
  C(void this.f(int i)?);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_typed_field_formal_param_inner_types() async {
    var content = '''
class C {
  int Function(int) f;
  C(int this.f(int i));
}
int g(int i) => i;
int test(int i) => C(g).f(i);
main() {
  test(null);
}
''';
    var expected = '''
class C {
  int? Function(int?) f;
  C(int? this.f(int? i));
}
int? g(int? i) => i;
int? test(int? i) => C(g).f(i);
main() {
  test(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_typed_formal_param() async {
    var content = '''
void f(g()) {}
void main() {
  f(null);
}
''';
    var expected = '''
void f(g()?) {}
void main() {
  f(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_typed_formal_param_accepts_hint() async {
    var content = '''
void f(g() /*?*/) {}
''';
    var expected = '''
void f(g()?) {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_function_typed_formal_param_inner_types() async {
    var content = '''
int f(int callback(int i), int j) => callback(j);
int g(int i) => i;
int test(int i) => f(g, i);
main() {
  test(null);
}
''';
    var expected = '''
int? f(int? callback(int? i), int? j) => callback(j);
int? g(int? i) => i;
int? test(int? i) => f(g, i);
main() {
  test(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_future_or_t_downcast_to_t() async {
    var content = '''
import 'dart:async';
void f(
    FutureOr<int> foi1,
    FutureOr<int/*?*/> foi2,
    FutureOr<int>/*?*/ foi3,
    FutureOr<int/*?*/>/*?*/ foi4
) {
  int i1 = foi1;
  int i2 = foi2;
  int i3 = foi3;
  int i4 = foi4;
}
''';
    var expected = '''
import 'dart:async';
void f(
    FutureOr<int> foi1,
    FutureOr<int?> foi2,
    FutureOr<int>? foi3,
    FutureOr<int?> foi4
) {
  int i1 = foi1 as int;
  int? i2 = foi2 as int?;
  int? i3 = foi3 as int?;
  int? i4 = foi4 as int?;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_generic_exact_propagation() async {
    var content = '''
class C<T> {
  List<T> values;
  C() : values = <T>[];
  void add(T t) => values.add(t);
  T operator[](int i) => values[i];
}
void f() {
  C<int> x = new C<int>();
  g(x);
}
void g(C<int> y) {
  y.add(null);
}
''';
    var expected = '''
class C<T> {
  List<T> values;
  C() : values = <T>[];
  void add(T t) => values.add(t);
  T operator[](int i) => values[i];
}
void f() {
  C<int?> x = new C<int?>();
  g(x);
}
void g(C<int?> y) {
  y.add(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_generic_exact_propagation_premigratedListClass() async {
    var content = '''
void f() {
  List<int> x = new List<int>();
  g(x);
}
void g(List<int> y) {
  y.add(null);
}
''';
    var expected = '''
void f() {
  List<int?> x = new List<int?>();
  g(x);
}
void g(List<int?> y) {
  y.add(null);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_generic_function_type_syntax_inferred_dynamic_return() async {
    var content = '''
abstract class C {
  Function() f();
}
Object g(C c) => c.f()();
''';
    // Note: even though the type `dynamic` permits `null`, the migration engine
    // sees that there is no code path that could cause `g` to return a null
    // value, so it leaves its return type as `Object`, and there is an implicit
    // downcast.
    var expected = '''
abstract class C {
  Function() f();
}
Object g(C c) => c.f()();
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_generic_typedef_respects_explicit_nullability_of_type_arg() async {
    var content = '''
class C {
  final Comparator<int/*!*/> comparison;
  C(int Function(int, int) comparison) : comparison = comparison;
  void test() {
    comparison(f(), f());
  }
}
int f() => null;
''';
    var expected = '''
class C {
  final Comparator<int> comparison;
  C(int Function(int, int) comparison) : comparison = comparison;
  void test() {
    comparison(f()!, f()!);
  }
}
int? f() => null;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_genericType_noTypeArguments() async {
    var content = '''
void f(C c) {}
class C<E> {}
''';
    var expected = '''
void f(C c) {}
class C<E> {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/39404')
  Future<void> test_genericType_noTypeArguments_use_bound() async {
    var content = '''
abstract class C<T extends Object> { // (1)
  void put(T t);
  T get();
}
Object f(C c) => c.get();            // (2)
void g(C<int> c) {                   // (3)
  c.put(null);                       // (4)
}
''';
    // (4) forces `...C<int?>...` at (3), which means (1) must be
    // `...extends Object?`.  Therefore (2) is equivalent to
    // `...f(C<Object?> c)...`, so the return type of `f` is `Object?`.
    var expected = '''
abstract class C<T extends Object?> { // (1)
  void put(T t);
  T get();
}
Object? f(C c) => c.get();            // (2)
void g(C<int?> c) {                   // (3)
  c.put(null);                       // (4)
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void>
      test_getter_implicit_returnType_overrides_implicit_getter() async {
    var content = '''
class A {
  final String s = "x";
}
class C implements A {
  get s => false ? "y" : null;
}
''';
    var expected = '''
class A {
  final String? s = "x";
}
class C implements A {
  get s => false ? "y" : null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_overrides_implicit_getter() async {
    var content = '''
class A {
  final String s = "x";
}
class C implements A {
  String get s => false ? "y" : null;
}
''';
    var expected = '''
class A {
  final String? s = "x";
}
class C implements A {
  String? get s => false ? "y" : null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_overrides_implicit_getter_with_generics() async {
    var content = '''
class A<T> {
  final T value;
  A(this.value);
}
class C implements A<String/*!*/> {
  String get value => false ? "y" : null;
}
''';
    var expected = '''
class A<T> {
  final T? value;
  A(this.value);
}
class C implements A<String> {
  String? get value => false ? "y" : null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_getter_in_interface() async {
    var content = '''
class B {
  int get x => null;
}
abstract class C implements B {
  void set x(int value) {}
}
''';
    var expected = '''
class B {
  int? get x => null;
}
abstract class C implements B {
  void set x(int? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_getter_in_interface_field() async {
    var content = '''
class B {
  final int x = null;
}
abstract class C implements B {
  void set x(int value) {}
}
''';
    var expected = '''
class B {
  final int? x = null;
}
abstract class C implements B {
  void set x(int? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_getter_in_interfaces() async {
    var content = '''
class B1 {
  int get x => null;
}
class B2 {
  int get x => null;
}
abstract class C implements B1, B2 {
  void set x(int value) {}
}
''';
    var expected = '''
class B1 {
  int? get x => null;
}
class B2 {
  int? get x => null;
}
abstract class C implements B1, B2 {
  void set x(int? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_getter_in_superclass() async {
    var content = '''
class B {
  int get x => null;
}
class C extends B {
  void set x(int value) {}
}
''';
    var expected = '''
class B {
  int? get x => null;
}
class C extends B {
  void set x(int? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_getter_in_superclass_substituted() async {
    var content = '''
class B<T> {
  T get x => throw '';
}
class C extends B<List<int/*?*/>> {
  void set x(List<int> value) {}
}
''';
    var expected = '''
class B<T> {
  T get x => throw '';
}
class C extends B<List<int?>> {
  void set x(List<int?> value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_setter_in_interface() async {
    var content = '''
class B {
  void set x(int value) {}
}
abstract class C implements B {
  int get x => null;
}
''';
    var expected = '''
class B {
  void set x(int? value) {}
}
abstract class C implements B {
  int? get x => null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_setter_in_interface_field() async {
    var content = '''
class B {
  int x;
  B(this.x);
}
abstract class C implements B {
  int get x => null;
}
''';
    var expected = '''
class B {
  int? x;
  B(this.x);
}
abstract class C implements B {
  int? get x => null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_setter_in_interfaces() async {
    var content = '''
class B1 {
  void set x(int value) {}
}
class B2 {
  void set x(int value) {}
}
abstract class C implements B1, B2 {
  int get x => null;
}
''';
    var expected = '''
class B1 {
  void set x(int? value) {}
}
class B2 {
  void set x(int? value) {}
}
abstract class C implements B1, B2 {
  int? get x => null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_setter_in_superclass() async {
    var content = '''
class B {
  void set x(int value) {}
}
class C extends B {
  int get x => null;
}
''';
    var expected = '''
class B {
  void set x(int? value) {}
}
class C extends B {
  int? get x => null;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_setter_in_superclass_substituted() async {
    var content = '''
class B<T> {
  void set x(T value) {}
}
class C extends B<List<int>> {
  List<int> get x => [null];
}
''';
    var expected = '''
class B<T> {
  void set x(T value) {}
}
class C extends B<List<int?>> {
  List<int?> get x => [null];
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_single_class() async {
    var content = '''
class C {
  int get x => null;
  void set x(int value) {}
}
''';
    var expected = '''
class C {
  int? get x => null;
  void set x(int? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_single_class_generic() async {
    var content = '''
class C<T extends Object/*!*/> {
  T get x => null;
  void set x(T value) {}
}
''';
    var expected = '''
class C<T extends Object> {
  T? get x => null;
  void set x(T? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_static() async {
    var content = '''
class C {
  static int get x => null;
  static void set x(int value) {}
}
''';
    var expected = '''
class C {
  static int? get x => null;
  static void set x(int? value) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_setter_top_level() async {
    var content = '''
int get x => null;
void set x(int value) {}
''';
    var expected = '''
int? get x => null;
void set x(int? value) {}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_getter_topLevel() async {
    var content = '''
int get g => 0;
''';
    var expected = '''
int get g => 0;
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_hint_contradicts_exact_nullability() async {
    var content = '''
void f(List<int> x) {
  x.add(null);
}
void g() {
  f(<int/*!*/>[]);
}
''';
    // `f.x` needs to change to List<int?> to allow `null` to be added to the
    // list.  Ordinarily this would be propagated back to the explicit list in
    // `g`, but we don't override the hint `/*!*/`.
    //
    // TODO(paulberry): we should probably issue some sort of warning to the
    // user instead.
    var expected = '''
void f(List<int?> x) {
  x.add(null);
}
void g() {
  f(<int>[]);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_ifStatement_nullCheck_noElse() async {
    var content = '''
int f(int x) {
  if (x == null) return 0;
  return x;
}
''';
    var expected = '''
int f(int x) {
  return x;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_implicit_parameter_type_override_does_not_union() async {
    var content = '''
abstract class A {
  void f(int/*?*/ i);
}
abstract class B {
  void f(int i);
}
class C implements A, B {
  void f(i) {}
}
''';
    // Even though the parameter type of C.f is implicit, its nullability
    // shouldn't be unioned with that of A and B, because that would
    // unnecessarily force B.f's parameter type to be nullable.
    var expected = '''
abstract class A {
  void f(int? i);
}
abstract class B {
  void f(int i);
}
class C implements A, B {
  void f(i) {}
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_implicit_return_type_override_does_not_union() async {
    var content = '''
abstract class A {
  int/*?*/ f();
}
abstract class B {
  int f();
}
class C implements A, B {
  f() => 0;
}
''';
    // Even though the return type of C.f is implicit, its nullability shouldn't
    // be unioned with that of A and B, because that would unnecessarily force
    // B.f's return type to be nullable.
    var expected = '''
abstract class A {
  int? f();
}
abstract class B {
  int f();
}
class C implements A, B {
  f() => 0;
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_implicit_type_parameter_bound_nullable() async {
    var content = '''
class C<T> {
  f(T t) {
    Object o = t;
  }
}
''';
    var expected = '''
class C<T> {
  f(T t) {
    Object? o = t;
  }
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  @FailingTest(issue: 'https://github.com/dart-lang/sdk/issues/39376')
  Future<void> test_infer_required() async {
    var content = '''
void _f(bool b, {int x}) {
  if (b) {
    print(x + 1);
  }
}
main() {
  _f(true, x: 1);
}
''';
    var expected = '''
void _f(bool b, {required int x}) {
  if (b) {
    print(x + 1);
  }
}
main() {
  _f(true, x: 1);
}
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_inferred_method_return_type_non_nullable() async {
    var content = '''
class B {
  int f() => 1;
}
class C extends B {
  f() => 1;
}
int g(C c) => c.f();
''';
    // B.f's return type is `int`.  Since C.f's return type is inferred from
    // B.f's, it has a return type of `int` too.  Therefore g's return type
    // must be `int`.
    var expected = '''
class B {
  int f() => 1;
}
class C extends B {
  f() => 1;
}
int g(C c) => c.f();
''';
    await _checkSingleFileChanges(content, expected);
  }

  Future<void> test_insert_as_prefixed_type() async {