[6.2] AST: Fix existential erasure of long member types #81182
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slavapestov
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Suppose protocol P has a primary associated type A, and we have
a `any P<S>` value. We form the generalization signature <T>
with substitution map {T := S}, and the existential signature
<T, Self where T == Self.A>.
Now, if we call a protocol requirement that takes Self.A.A.A,
we see this is fixed concrete type, because the reduced type of
Self.A.A.A is T.A.A in the existential signature.
However, this type parameter is not formed from the
conformance requirements of the generalization signature
(there aren't any), so we cannot directly apply the outer
substitution map.
Instead, change the outer substitution conformance lookup
callback to check if the reduced type parameter is valid
in the generalization signature, and not just rooted in a
generic parameter of the generalization signature.
If it isn't, fall back to global conformance lookup.
A better fix would introduce new requirements into the
generalization signature to handle this, or store them
separately in the generic environment itself. But this is fine
for now.
- Fixes swiftlang#79763.
- Fixes rdar://problem/146111083.
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@swift-ci Please test |
DougGregor
approved these changes
Apr 30, 2025
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6.2 cherry-pick of #81181
Description: Fixes a compiler crash in some circumstances when erasing a type parameter that is rooted in a primary associated type of an existential value.
Origination: The regression shipped in Swift 6.1.
Reviewed by: @AnthonyLatsis
Risk: Low. This changes a frequently-called code path, but I believe it's well-tested. The isValidTypeParameter() check does not assert on invalid inputs, and it returns true in all cases that the old check returned true.