Module compiled with Swift 4.0 cannot be imported in Swift 4.0.1

Update:

For release versions of Xcode:

This error (and similar errors involving Swift 4.1, 4.2, etc.) will occur when opening a project with Xcode 9.1, 9.2, 9.3, 9.4, 10, etc. that uses frameworks that were built with earlier Xcode tools.

To fix the issue, update and rebuild your frameworks using Carthage ( carthage update --platform iOS), Cocoapods (pod update or pod install), or manually, with the new updated Xcode tools. The tools should be updated automatically when you update Xcode, but if not, you can follow the steps outlined below in the original answer.

You may also need to clean your project cmd + shift + k and possibly your build folder cmd + option + shift + k to get Xcode to not use cached framework builds.

In some cases you may also need to delete your derived data folder (Easily found by going to Xcode Preferences -> Locations -> Derived Data Folder (Thanks Stunner)

For beta versions of Xcode:

See original answer below and then follow steps above.

Original Answer:

You probably still have your xcodebuild tools set to Xcode 9.0 which builds with Swift 4.0 and is incompatible with Xcode 9.1 beta's Swift 4.0.1.

Check in the terminal using the command:

xcodebuild -version

Or just go into Xcode preferences -> Locations and check/change the command line tools to Xcode 9.1. You should be set then.

My issue was due to an external framework not being compiled for Swift 4.0.2. I had added this file via Carthage, so running:

carthage update --platform iOS

And then cleaning the project cmd + shift + k and build folder cmd + option + shift + k and deleting the derived data folder (merely moving it to trash works as well) worked for me.

In my case, I downloaded xCode 11 Beta version to test SwiftUI. When i run my project, i got "Module compiled with Swift 5.1 cannot be imported by the Swift 5.0.1 compiler:..." error.

To do fix this issue. i changed xCode command line tool to new xCode 11 comment line tool. After that, you can create new xcode project with SwiftUI.

Xcode -> Preferences -> Location -> XCode command Line Tool

i hope, this will help someone who want to test with new freamework :)