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Sema: Do not drop reference to type paramater if it reduces to ErrorType #79065

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Sema: Do not drop reference to type paramater if it reduces to ErrorType #79065

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3 changes: 2 additions & 1 deletion lib/Sema/OpenedExistentials.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -315,7 +315,8 @@ findGenericParameterReferencesRec(CanGenericSignature genericSig,
return GenericParameterReferenceInfo();
}

if (auto reducedTy = genericSig.getReducedType(type)) {
auto reducedTy = genericSig.getReducedType(type);
if (reducedTy && !reducedTy->is<ErrorType>()) {
if (!reducedTy->isEqual(type)) {
// Note: origParam becomes openedParam for the recursive call,
// because concreteTy is written in terms of genericSig and not
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27 changes: 27 additions & 0 deletions test/decl/protocol/existential_member_access/invalid_type_parameter.swift
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Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
// RUN: %target-typecheck-verify-swift -target %target-swift-5.9-abi-triple

// https://github.com/swiftlang/swift/issues/77840
do {
struct G<T> {}

protocol P {
associatedtype A where A == Undefined
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Why is the behavior here any different than the case where Undefined exists?

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@AnthonyLatsis AnthonyLatsis Jan 30, 2025 •
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The way I think about it is that if reduction returns an error type, then it makes sense to ignore the constraint that caused it to error out. That is, to not treat that error type as concrete when searching for type parameter references.

// expected-error@-1 {{cannot find type 'Undefined' in scope}}
associatedtype B where B == G<Undefined>
// expected-error@-1 {{cannot find type 'Undefined' in scope}}

func fooTakesA(_: A)
func fooTakesB(_: B)

func fooReturnsA() -> A
func fooReturnsB() -> B
}

let p: any P
let _ = p.fooTakesA
// expected-error@-1 {{member 'fooTakesA' cannot be used on value of type 'any P'; consider using a generic constraint instead}}
let _ = p.fooTakesB
// expected-error@-1 {{member 'fooTakesB' cannot be used on value of type 'any P'; consider using a generic constraint instead}}
let _ = p.fooReturnsA()
let _ = p.fooReturnsB()
}