[TypeChecker] SE-0352: Require coercion if result type contains existential(s) that would loose generic requirements #42559
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@DougGregor Please take a look to make sure that this is going in the right direction, I still need to implement this diagnostic for Self erasure cases but it shouldn't be too bad. |
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Apr 22, 2022
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@swift-ci please test |
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@swift-ci please test source compatibility |
hborla
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May 25, 2022
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@swift-ci please smoke test |
DougGregor
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May 25, 2022
…ments
Diagnose situations where inferring existential type for result of
a call would result in loss of generic requirements.
```swift
protocol P {
associatedtype A
}
protocol Q {
associatedtype B: P where B.A == Int
}
func getB<T: Q>(_: T) -> T.B { ... }
func test(v: any Q) {
let _ = getB(v) // <- produces `any P` which looses A == Int
}
```
…rcion If there is going to be any loss of generic requirements in the result, a result of a call has to use explicit coercion.
For types like `<Base>.B.C` we need to check that neither `B` nor `C` have any additional requirements because they cannot be expressed in the existential type.
…would loose information Accessing members on the protocol could result in existential opening and subsequence result erasure, which requires explicit coercion if there is any loss of generic requirements.
…d of declaration Since the inference is a call site specific it makes sense to attach a candidate note to the call itself instead of the declaration used.
Instead of checking for presence of `where` clause, let's use requirement signature and check whether any of the requires are anchored on a particular associated type.
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DougGregor
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May 26, 2022
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…ppressing opening at use site If erased result is passed as an argument to a call that requires implicit opening, the fix-it should use parens to avoid suppressing the opening at that argument position.
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@swift-ci please test macOS platform |
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Implements SE-0352 revision - require explicit
ascoercion when existentialerasure loses information.
For example: