Answer by v6ak for Play framework tutorial: Cannot resolve symbol 'index'?

1. Open "Project" sidebar
2. Go to target/scala-<version>/src_managed
3. Right-click the src_managed, "Mark Directory As"/"Generated Source Root"
4. Subdirectories of src_managed are from a strange reason marked as source roots. You can disable it.
5. Enjoy.

Note that javascript routes seem to be still ignored from a strange reason, so this is not a perfect solution. Nevertheless, it is better than nothing.