There is an (old) similar proposal https://github.com/python/typing/issues/402 btw.

Taking into account that this can be made only in 3.9, what is the benefit over ``from __future__ import annotations`` (that one can use already) do you see?

IMO there are only rare cases of type aliases and base classes, so I don't see much benefit. Also making it generic will require importing ``typing`` from ``os`` which may make it slower to import (``typing`` is a really heavy module, in particular because it imports many other modules).