Prefer witnesses with no difference effects to ones that have fewer effects #40088
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…ffects. One can overload on async vs. non-async, and the constraint solver has a preference rule based on context. Extend that preference rule to witness matching, so we prefer a witness that exactly matches the effects of the requirement to one that has fewer effects. Fixes rdar://84034057.
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kavon
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You can overload a function based on its `async`-ness, and resolution is carried out based on async-ness of calling context. But during protocol conformance checking, for an `async` requirement we were accidentally choosing the non-`async overload instead of the `async` one. The `async` one is the choice people would expect, since the `async` requirement is in essence the "context" that forwards to the underlying witness. This intended behavior is also inferred from: swiftlang#40088 The problem boiled down to a bad check when categorizing the witness matches prior to ranking them. Resolves rdar://109135488 / swiftlang#60318
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You can overload a function based on its `async`-ness, and resolution is carried out based on async-ness of calling context. But during protocol conformance checking, for an `async` requirement we were accidentally choosing the non-`async overload instead of the `async` one. The `async` one is the choice people would expect, since the `async` requirement is in essence the "context" that forwards to the underlying witness. This intended behavior is also inferred from: swiftlang#40088 The problem boiled down to a bad check when categorizing the witness matches prior to ranking them. Resolves rdar://109135488 / swiftlang#60318
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One can overload on async vs. non-async, and the constraint solver has a
preference rule based on context. Extend that preference rule to
witness matching, so we prefer a witness that exactly matches the
effects of the requirement to one that has fewer effects.
Fixes rdar://84034057.