Library evolution support is broken

[ugomarinelli]

Hello,

We are currently blocked when using XCode 13.2, with this message:

Failed to build module 'Mockingbird'; this SDK is not supported by the compiler (the SDK is built with 'Apple Swift version 5.5 (swiftlang-1300.0.31.1 clang-1300.0.29.1)', while this compiler is 'Apple Swift version 5.5.2 (swiftlang-1300.0.47.5 clang-1300.0.29.30)'). Please select a toolchain which matches the SDK.

Do you have any plans to support XCode 13.2 in the future ?
Thank you !

[andrewchang-bird]

Thanks for filing. Which package manager are you using? If you build the framework from source or via SPM does it still raise this error?

[andrewchang-bird]

So the issue is that library evolution / module stability isn’t working even though it’s enabled for the framework; BUILD_LIBRARY_FOR_DISTRIBUTION is explicitly set to YES and a Swift module interface is being generated with -enable-library-evolution listed in the module flags. Seems like it might be something with how Swift is doing library evolution as there are known problems (realm/realm-swift#7313).

In the meantime, you should build the framework from source. If you’re using Carthage, you’ll need to run carthage build whenever changing Xcode toolchains / Swift versions.

Edit: After additional investigation, it’s due to a known issue since Xcode 11.2:

If a module is built with BUILD_LIBRARIES_FOR_DISTRIBUTION and contains a public type with the same name as the module itself, clients will fail to import the module. (19481048) (FB5863238)

Workaround: Rename either the type or the module to remove the conflict.

Specifically, Mockingbird returns an XCTest.XCTestExpectation for asynchronous testing which is ambiguous since XCTest itself exports a class named XCTest... It might be worth a breaking API change to unblock module stability, but there doesn’t seem to be an easy path forward for now.