So I guess this short-hand form should be documented in this section: The Swift Programming Language: Redirect
(Not sure if request for doc improvements should be reported like this but, here you go: SR-7931.)
And more generally, a lot of the behavior of type aliases, associated types and extensions is both undocumented and inconsistent/broken.
Properly documenting the intended behavior would certainly help not only us (new and seasoned) users but also the compiler devs.