This worked for me:
Create an Empty Swift file using Xcode, such as named Void.swift, and every issue will be solved by Xcode automatically.
If you want to know what Xcode has done, submit the code before creating the file, and check the changes after creating the file.
Don't forget to check the correct Target subproject (if any) in the creation window and select "Create bridging header" if Xcode ask you about it.
The problem was probably a missing setting in the build settings of the project or missing/wrong bridging/swift header file (<project>-Swift.h and <project>-Bridging-Header.h)
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