I wouldn’t consider my previous code as a workaround by the way…
Regarding your issue, this is the only solution I can provide:

class MyScreen:Screen {}
class OtherScreen:Screen {}final class NavigatorStub: Navigator {
 // MARK: - Stubs
 lazy var showMyScreenStub = stub(of: showMyScreen)
 lazy var showOtherScreenStub = stub(of: showOtherScreen)  // MARK: - API
 func showMyScreen(_ screen: MyScreen) {
  showMyScreenStub(screen)
 } func showOtherScreen(_ screen: OtherScreen) {   
  showOtherScreenStub(screen)
 } func show<ScreenType: Screen>(_ screen: ScreenType) {
  if let screen = screen as? MyScreen {
   showMyScreen(screen)
   return
  }
  if let screen = screen as? OtherScreen {
   showOtherScreen(screen)
   return
  }
  fatalError()
 }
}