ios - Module compiled with Swift 5.1 cannot be imported by the Swift 5.1.2 compiler

Question

I have a framework (in this instance it's RxSwift) which I've compiled using Xcode 11.0 into the traditional RxSwift.framework style package

This imported fine into Xcode 11.0 and also 11.1 never had any problems with it

Today, upon Apple's release of Xcode 11.2, I upgraded, and I am presented with the error:

Module compiled with Swift 5.1 cannot be imported by the Swift 5.1.2 compiler

I'm used to swift compiler mismatches, and I'm aware I can just recompile RxSwift using Xcode 11.2 and carry on, but the headline feature of Swift 5.1 was module stability.

I was under the impression that now that we have module stability, frameworks won't need to keep getting recompiled with every new Xcode release, yet this is clearly not the case.

If anyone can explain what is going on here I would much appreciate it. Is Xcode 11.2 exhibiting a bug? or did I somehow need to tell it I wanted module stability when I originally compiled with Xcode 11.0?

Answer 1

OK, Turns out if you watch the WWDC video, they explain it: https://developer.apple.com/videos/play/wwdc2019/416/

You need to set the Build Settings > Build Options > Build Libraries for Distribution option to Yes in your framework's build settings, otherwise the swift compiler doesn't generate the neccessary .swiftinterface files which are the key to future compilers being able to load your old library.

This ends up in your project.pbxproj file as:

BUILD_LIBRARY_FOR_DISTRIBUTION = YES;

After setting this flag, a framework I compiled using Xcode 11.0 (swift 5.1) was able to be used by Xcode 11.2 (swift 5.1.2) and everything appears to be working correctly.

Hopefully this question/answer will serve as a useful reference for everyone who hasn't watched all the WWDC videos

If the error still persists go to Product > Clean Build Folder and Build again.