[CS] Penalize implicit pointer conversions to generic parameter types #84907
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hamishknight
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xedin
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We ought to consider outright banning these conversions if the destination is a generic parameter type, but for now let's penalize them such that we don't end up with ambiguities if you're doing an implicit pointer conversion through an identity generic function. rdar://161205293
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@swift-ci please test |
xedin
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Oct 16, 2025
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@swift-ci please test source compatibility |
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@swift-ci please smoke test |
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This works around rdar://122011759 which appears to have been exacerbated by swiftlang/swift#84907.
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…#1389) This works around rdar://122011759 which appears to have been exacerbated by swiftlang/swift#84907. For example: ```swift let file = try #require(fmemopen(nil, 1, "wb+")) ``` The third parameter of `fmemopen()` is of type `const char *` (AKA `UnsafePointer<CChar>`) and we're passing the string literal `"wb+"` which is implicitly cast to a C string by the compiler. However, overload resolution of the expansion function used by `#require()` is now preferring an overload that uses variadic generics, and that overload is impacted by a compiler issue (rdar://122011759) that causes it to pass a garbage pointer as the temporary C string. Previously, the type checker would pick an overload with a concrete, finite set of generic arguments (`<T, Arg0, Arg1, Arg2, R>` instead of `<T, each U, R>`.) ### Checklist: - [x] Code and documentation should follow the style of the [Style Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md). - [x] If public symbols are renamed or modified, DocC references should be updated.
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…#1389) This works around rdar://122011759 which appears to have been exacerbated by swiftlang/swift#84907. For example: ```swift let file = try #require(fmemopen(nil, 1, "wb+")) ``` The third parameter of `fmemopen()` is of type `const char *` (AKA `UnsafePointer<CChar>`) and we're passing the string literal `"wb+"` which is implicitly cast to a C string by the compiler. However, overload resolution of the expansion function used by `#require()` is now preferring an overload that uses variadic generics, and that overload is impacted by a compiler issue (rdar://122011759) that causes it to pass a garbage pointer as the temporary C string. Previously, the type checker would pick an overload with a concrete, finite set of generic arguments (`<T, Arg0, Arg1, Arg2, R>` instead of `<T, each U, R>`.) ### Checklist: - [x] Code and documentation should follow the style of the [Style Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md). - [x] If public symbols are renamed or modified, DocC references should be updated.
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We ought to consider outright banning these conversions if the destination is a generic parameter type, but for now let's penalize them such that we don't end up with ambiguities if you're doing an implicit pointer conversion through an identity generic function.
rdar://161205293