On Wed, 22 May 2019 at 15:31, Guido van Rossum <guido@python.org> wrote:
If the implementation lands in typing_extensions *now*, maybe it's reasonable to review that and copy it into the stdlib as soon as the PEP has been accepted. We could make that work. Don't wait implementing until the PEP is accepted though!

This is a good idea! Having a reference implementation would simplify the process. Till, please go ahead and make a PR to typing_extensions (it still lives in python/typing repo).