[6.2] AST: Stop diagnosing potentially unavailable declarations in unavailable contexts #80707
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Apr 11, 2025
…ble contexts. Potential unavailability of a declaration has always been diagnosed in contexts that do not have a sufficient platform introduction constraint, even when those contexts are also unavailable on the target platform. This behavior is overly strict, since the potential unavailability will never matter, but it's a longstanding quirk of availability checking. As a result, some source code has been written to work around this quirk by marking declarations as simultaneously unavailable and introduced for a given platform: ``` @available(macOS, unavailable, introduced: 15) func unavailableAndIntroducedInMacOS15() { // ... allowed to call functions introduced in macOS 15. } ``` When availability checking was refactored to be based on a constraint engine in swiftlang#79260, the compiler started effectively treating `@available(macOS, unavailable, introduced: 15)` as just `@available(macOS, unavailable)` because the introduction constraint was treated as lower priority and therefore superseded by the unavailability constraint. This caused a regression for the code that was written to work around the availability checker's strictness. We could try to match the behavior from previous releases, but it's actually tricky to match the behavior well enough in the new availability checking architecture to fully fix source compatibility. Consequently, it seems like the best fix is actually to address this long standing issue and stop diagnosing potential unavailability in unavailable contexts. The main risk of this approach is source compatibility for regions of unavailable code. It's theoretically possible that restricting available declarations by introduction version in unavailable contexts is important to prevent ambiguities during overload resolution in some codebases. If we find that is a problem that is too prevalent, we may have to take a different approach. Resolves rdar://147945883.
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@available(macOS, unavailable, introduced: 15)as just@available(macOS, unavailable)because the introduction constraint was treated as lower priority and therefore superseded by the unavailability constraint. This caused a regression for the code that was written to work around the availability checker's strictness. The fix is to address this long standing issue and stop diagnosing potential unavailability in unavailable contexts.unavailablefor the target platform.