Module compiled with Swift 4.0 cannot be imported in Swift 3.2.1

Question

Swift 4.0 modules were fine in Swift 3.2 apps with Xcode 9.0.

But now, with Xcode 9.1, I get:

Module compiled with Swift 4.0 cannot be importe

Answer 1

It's possible to build using Swift 3.2 in Xcode 9.1 beta 2 by installing the Xcode 9.0 toolchain and using that to build your project.

Then choose the Swift 4.0 release under the Xcode -> Toolchains menu.

You should now be able to build with either Swift 3.2 or 4.0 in Xcode 9.1 b2.