Sema: Relax availability checking for @MainActor

[tshortli]

The @MainActor global actor constraint on a declaration does not carry an inherent ABI impact and therefore use of this constraint should not be limited to OS versions where Swift concurrency is available.

This fixes a regression (introduced by #60672) in swiftinterface typechecking for modules that contain public types that implicitly inherit a @MainActor attribute.

Resolves rdar://105610970

[tshortli]

@swift-ci please test

[tshortli]

After discussing with Doug, I've updated the PR to relax availability checking on the @MainActor attribute in general, rather than restricting it to @MainActor(unsafe).

[Kyle-Ye]

Can we cherry-pick this PR to release/5.8? Otherwise Xcode 14.3 with Swift 5.8.0 will break many downstream xcframeworks consumers. If this is too late for Swift 5.8.0, will it be released with Swift 5.8.1?

[RamblinWreck77]

Upgraded to Xcode 14.3 RC1 today and ran into this issue, I agree with @Kyle-Ye this will likely cause a lot of commotion once 14.3 is fully released (next week?)

[tshortli]

Cherry-picked to release/5.8: #64538

[tshortli]

This is fixed in Xcode 14.3 RC 2: Xcode 14.3 RC 2 Release Notes | Apple Developer Documentation

[tiwoc]

This bug seems to be back in Xcode 15 beta 1 (15A5160n) with Swift 5.9.0.114.6. I'm seeing the "this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 5.7.2 (swiftlang-5.7.2.135.5 clang-1400.0.29.51)', while this compiler is 'Apple Swift version 5.9 (swiftlang-5.9.0.114.6 clang-1500.0.27.1)')" error in a project that compiled fine with Xcode 14.3.1 (Swift 5.8), and with an XCFramework that was definitely built with BUILD_LIBRARY_FOR_DISTRIBUTION turned on.

Has the fix not made it to the Swift 5.9 branch yet? 🤔

[tshortli]

@tiwoc Seeing this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 5.7.2 (swiftlang-5.7.2.135.5 clang-1400.0.29.51)', while this compiler is 'Apple Swift version 5.9 (swiftlang-5.9.0.114.6 clang-1500.0.27.1)') by itself doesn't indicate that you're seeing the issue that was reported and fixed here. That error message is a generic error message printed by the compiler whenever it can't build a module from its swiftinterface for any reason, and there are myriad reasons why a swiftinterface might be unbuildable.

I recommend either filing a GitHub issue or a Feedback with the full build log that includes the errors that were emitted before the this SDK is not supported message and also a reproducer project if possible.

[tiwoc]

Thanks for the clarification, @tshortli!