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Treating createSentinel as returning a value of type Never caused issues whenever its result flowed into a union, since the union would just simplify to its other component. This meant that if a late field started out uninitialized and was later initialized to a constant, we would see it as only ever having the constant value. This caused isSentinel to always return false even if the field hadn't been initialized yet. Instead, we support a late sentinel value in the abstract value domain and use this value as the return type of createSentinel during inference. Additionally, inference treats _lateReadCheck as a simple narrowing to exclude the late sentinel value. Change-Id: I086bfd77576930e3ca2a4cfc9bd63476b6636685 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210646 Reviewed-by: Stephen Adams <sra@google.com>
24 lines
787 B
Dart
24 lines
787 B
Dart
// Copyright (c) 2021, the Dart project authors. Please see the AUTHORS file
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// for details. All rights reserved. Use of this source code is governed by a
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// BSD-style license that can be found in the LICENSE file.
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import 'package:expect/expect.dart';
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// Tests to ensure that narrowing type information does not discard late
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// sentinel values unintentionally.
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class Foo<T> {
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// Since `List<T>` contains a free type variable, any access of [x] will be
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// immediately followed by a narrowing to the appropriate instantiation of
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// `List<T>`. This narrowing should not exclude the late sentinel value from
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// the abstract value.
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late final List<T> x;
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}
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void main() {
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Foo<int> foo = Foo();
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Expect.throws(() => foo.x);
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foo.x = const [];
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Expect.isTrue(foo.x.isEmpty);
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}
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