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Existential Types Cannot Satisfy Superclass Bounds

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It is not the case that `any P` satisfies a class bound
even if the existential is a composition with an exact
match to the class bound. The resulting existential box
must be opened as the exact class type before this conversion
can succeed.

This appears to be a regression from Swift 5.1, which was the last
Swift compiler that banned this typing rule.

rdar://92358570
This commit is contained in:
Robert Widmann
2022-05-02 15:59:10 -07:00
parent 8b2ccd9ed5
commit 6d28831617
2 changed files with 27 additions and 2 deletions
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7
lib/Sema/CSSimplify.cpp
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@@ -3465,8 +3465,11 @@ ConstraintSystem::matchExistentialTypes(Type type1, Type type2,
if (!req)
return getTypeMatchFailure(locator);
if (type1->isPlaceholder() ||
req->getRequirementKind() == RequirementKind::Superclass)
// Superclass constraints are never satisfied by existentials,
// even those that contain the superclass a la `any C & P`.
if (!type1->isExistentialType() &&
(type1->isPlaceholder() ||
req->getRequirementKind() == RequirementKind::Superclass))
return getTypeMatchSuccess();
auto *fix = fixRequirementFailure(*this, type1, type2, locator);