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 imported in Swift 3.2.1

Or:

Module compiled with Swift 4.0 cannot be imported in Swift 4.0.1

Answer 1

Swift 4.0 is compatible with Swift 3.2 only.
Swift 4.0.1 is compatible with Swift 3.2.1 only.

So:

• you can't mix 4.0 and 3.2.1
• you can't mix 4.0.1 and 3.2
• you can't mix 4.0 and 4.0.1
• you can't mix 3.2.1 and 3.2

To formulate it differently:

• you can mix Xcode 9.0 supported Swift versions together
• you can mix Xcode 9.1 supported Swift versions together
• you can't mix Xcode 9.0 and Xcode 9.1 Swift versions

If the module was built by you, simply rebuild it with the same Xcode.
If the module was built by a third party, request an Xcode 9.1 compatible build.

Answer 2

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.

Answer 3

Using the last Xcode version solved this issue