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

Levi

I had this error message, too, and it turned out to be caused various syntax errors in my code. Once I fixed the syntax errors, the error went away. – Bridgettbridgette

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.

Levi

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.

Drosophila

Oh, correct... but then at the time of stable Xcode 9.1, custom toolchain won't be accepted by iTunes Connect for release. – Tincher

That's true. I suppose you could always continue to submit using Xcode 9.0 while testing on Xcode 9.1 beta. Sort of annoying that some of the iPhone X functionality is not working in the current Xcode though. – Drosophila

Answer 3

Using the last Xcode version solved this issue

Perth