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 had the same problem and cleaning the build folder helped:

Command+Option+Shift+K

or

Product -> Option+Clean