commit fb449ed038c8267c451d1682dbac0d9e1bef9b79
author Paul Berry <paulberry@google.com> Fri Jan 12 19:31:40 2024 +0000
committer dart-internal-monorepo <dart-internal-monorepo@dart-ci-internal.iam.gserviceaccount.com> Fri Jan 12 11:36:13 2024 -0800
tree e1374ea2157be51d5f81a0f57f4b0527eb8d5f91
parent 82b05be1120a1ede25988a63ba1c37cbe667ec5b

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: Id090f432500d75ba694383f1788d58353cd1fc72
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345860
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

https://dart.googlesource.com/sdk/+/973eca72f559493ee504bf694845d1389546ec39