Module not testable (ModuleNotTestable)

Overview

Modules imported using @testable import must have been compiled using the -enable-testing compiler flag.

@testable import allows clients of a module to access internal API when writing tests. A @testable import will report an error if the imported module was not compiled with the -enable-testing compiler flag. Use of -enable-testing is recommended only in debug builds or other builds not intended for production use.