Command CompileSwift failed with a nonzero exit code in Xcode 10

I attempted

• Closing & Reopening Xcode
• Cleaning Build Folder
• Running pod install --repo-update

and all of these still did not fix the problem.

Restarting the Mac did the trick!

For me, just cleaning project works using ShiftCommandK & OptionShiftCommandK.

You're most likely not reading the entire error message. If you look above the "Command CompileSwift failed with a nonzero exit code" message you should find some specification like this:

In this example I had two files with the same name. Once I fixed it everything worked as it should.