Is there a way to have something like this work?

Some module declares a protocol:

public protocol SomeModuleProtocol {
    static func test() -> String
}

final class SomeModuleClass: SomeModuleProtocol {}

Then in the main project, have some extension to give the default implementation:

import SomeModule

extension SomeModuleProtocol {
    static func test() -> String {
        return "whatever"
    }
}

So you can use it in your module like this:

let test = SomeModuleClass.test()    // "whatever"

Right now I can’t make it work though I don’t understand why.