commit e5244c88c91e03f4e311d0ddf5b21eb70f20c093
author Paul Berry <paulberry@google.com> Wed Jan 17 23:57:13 2024 +0000
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Wed Jan 17 23:57:13 2024 +0000
tree e15485edd4825efbffd47ac86cac9941aa5fb03e
parent 909f02415016b796e3367bbe96fe2a25b8cd18b2

[beta] Fix handling of extension types in relational patterns.

In https://dart-review.googlesource.com/c/sdk/+/345082, type erasure
was added to the handling of relational patterns, to address some co19
failures, e.g.:

    extension type const BoolET1(bool _) {}
    const True1 = BoolET1(true);
    String testStatement1(bool b) {
      switch (b) {
        case == True1:
	  ...
      }
    }

This was failing because the type of `True1` (`BoolET1`) is not
assignable to the argument type of `operator==`, which is
`Object`. (This is because extension types do not, by default, extend
`Object`; they extend `Object?`).

Adding type erasure elimited the co19 failure, but it caused other
code to be allowed that shouldn't be allowed, such as:

    extension type const E(int representation) implements Object {}
    class A {
      bool operator <(int other) => ...;
    }
    const E0 = E(0);
    test(A a) {
      if (a case < E0) ...;
    }

This shouldn't be allowed because the type expected by `A.<` is `int`;
allowing `E0` to be passed to this operator breaks extension type
encapsulation.

The correct fix is for assignability checks for `operator==` to use
`S?` rather than `S`, where `S` is the argument type of
`operator==`. This is consistent with the patterns specification, and
it ensures that `== null` and `!= null` are allowed, while continuing
to prohibit relational patterns that break extension type
encapsulation.

Fixes https://github.com/dart-lang/sdk/issues/54594.

Bug: https://github.com/dart-lang/sdk/issues/54594.
Change-Id: Id1f7a52f75e91c0b3b3d535aab68d38da68d37a5
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/345860
Cherry-pick-request: https://github.com/dart-lang/sdk/issues/54612
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345826
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>