5f8c28e226
Judging by several test cases that have shown up in co19 tests and
informal discussions, it appears to be a common expectation that a
pattern like `int? x?` or `int? x!` should promote `x` to
non-nullable. Previous to this change, this didn't work in general.
Consider:
Object? o = ...;
switch (o) {
case int? x?:
print(x.isEven); // (1)
}
At (1), the null-check happens *before* the required type check of the
variable pattern; therefore it promotes the matched value type to
`Object`. This is not a subtype of the required type of the variable
pattern (which is `int?`), therefore, previous to this change, `x` was
not promoted.
With this change, since the matched value type of `Object` is
non-nullable, the required type of the variable pattern is
re-interpreted as its non-nullable counterpart, `int`.
In a fully null-safe program, this is sound, because if the matched
value type is non-nullable, that guarantees that the matched value is
not `null`, and therefore it is equivalent to type check against the
non-nullable counterpart of the required type.
In a program that is not fully null-safe, the matched value might have
originated in a non-null-safe library (and thus might be `null` in
violation of its static type). So, strictly speaking, it is not sound
to re-interpret the required type of the pattern as its non-nullable
counterpart. However, the only way this unsoundness can manifest is
for the matched value to be promoted to non-nullable when it is in
fact `null`, and that is precisely the sort of unsoundness thta we
permit in mixed-mode programs. So this change won't result in
unsoundness escalation.
Bug: https://github.com/dart-lang/sdk/issues/51644
Change-Id: I9479e3c29e12f2a62a9e165b32c3480d7e299c29
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287040
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
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| .github | ||
| benchmarks | ||
| build | ||
| docs | ||
| pkg | ||
| runtime | ||
| samples | ||
| sdk | ||
| tests | ||
| third_party | ||
| tools | ||
| utils | ||
| .clang-format | ||
| .gitattributes | ||
| .gitconfig | ||
| .gitignore | ||
| .gn | ||
| .mailmap | ||
| .style.yapf | ||
| .vpython | ||
| AUTHORS | ||
| BUILD.gn | ||
| CHANGELOG.md | ||
| codereview.settings | ||
| CONTRIBUTING.md | ||
| DEPS | ||
| LICENSE | ||
| OWNERS | ||
| PATENT_GRANT | ||
| PRESUBMIT.py | ||
| README.dart-sdk | ||
| README.md | ||
| sdk_args.gni | ||
| SECURITY.md | ||
| WATCHLISTS | ||
Dart
A client-optimized language for fast apps on any platform
Dart is:
-
Optimized for UI: Develop with a programming language specialized around the needs of user interface creation.
-
Productive: Make changes iteratively: use hot reload to see the result instantly in your running app.
-
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Dart's flexible compiler technology lets you run Dart code in different ways, depending on your target platform and goals:
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See LICENSE and PATENT_GRANT.
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