Module compiled with Swift 4.1.2 cannot be imported by the Swift 4.2 compiler

Question

Short story: Xcode 10 is set to Swift 4, but it is using Swift 4.2.

Long story: Our project is not ready for Swift 4.2, since our dependent frameworks are not Swift 4.2, yet. But, I would like to, at least, use Xcode 10.

• I have Xcode 9.4.1 and Xcode 10 installed.
• I have Command Line Tools set to use Xcode 9.4.1:

• In Xcode 10, I have the project setting, Swift Language Version, set to Swift 4.

• I have verified on the command line the Swift version the shell is using is 4.1.2:

"swift --version Apple Swift version 4.1.2 (swiftlang-902.0.54 clang-902.0.39.2)"

• I've run carthage update.
• I've quit Xcode 10 and relaunched.
• I have deep cleaned and deleted Derived Data.
• I build and I get this error:

"Module compiled with Swift 4.1.2 cannot be imported by the Swift 4.2 compiler: .../App/Carthage/Build/iOS/RealmSwift.framework/Modules/RealmSwift.swiftmodule/arm64.swiftmodule"

To me, this indicates that I have my intended Swift version (4.1.2) set correctly and Xcode is still trying to use the wrong version of Swift (4.2).

Does anyone have suggestions on how to resolve this? Am I missing something? Xcode bug?

Answer 1

I'm not positive, but my guess is that this is the reason:

I have Command Line Tools set to use Xcode 9.4.1

You have to recompile your Swift 4 dependencies using Xcode 10, because Swift is not ABI-stable, i.e. binaries compiled with different compiler versions are not compatible. And even Swift 4.0 binaries compiled with different versions of Xcode may not be compatible.

I don't have much experience with Carthage, but my guess is that you should use Xcode 10 as a command-line tool, but you have to set a different Swift language version to be used by Xcode 10. It should probably be specified in the project (in this case RealmSwift), or using environment variables, but you'll need someone else to answer with more details, I'm afraid.