Module compiled with swift 4.0 cannot be imported in swift 3.1

Question

Apparently I have managed to build my project in Xcode 9 beta and now I only get the error

Module compiled with swift 4.0 cannot be imported in swift 3.1<

Answer 1

I met this problem in a project where dependency is managed by Carthage. In my case, I didn't set command line tool in xcode (Type in xcodebuild -version, you will know whether you set it up or not), so first step is to go to XCode --> Preference --> Locations then select the xcode you want to serve as command line tool. Then you can follow the steps that @Domsware mentioned above to rebuild all frameworks you are gonna use.

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Same problem here but using Carthage. And here is the answer:

rm -rf ~/Library/Caches/org.carthage.CarthageKit/DerivedData
delete the Carthage folder for the project
Update Carthage: carthage update --platform iOS

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Then don't forget to delete old links under 'Linked frameworks and libraries' and drag all frameworks from /Carthage folder under you project to 'Linked frameworks and libraries'.

Then voilà!

For those who are using CocoaPods, I suspect (Disclaimer: I didn't encounter this problem in project where CocoaPods is the dependency manager) the solution would be run the following command in terminal:

$ pod deintegrate
$ pod clean
$ pod install

where you might need to install 'deintegrate' and 'clean' tool for CocoaPod

$ sudo gem install cocoapods-deintegrate cocoapods-clean

more details see post How to remove CocoaPods from a project?