Technically, is there a reason func f(_: A) -> A couldn't be mapped to func f(_: Any) -> Any with a runtime trap if the argument isn't of the expected type? By itself this isn't a very satisfactory solution, but I think it's close to what people would expect to happen. And then maybe there's way to improve on that by making the runtime trap clearly visible in the code like in ex.f(a as! ex.A) or something like that.