I'm beginning implementation work for PEP 585. I've got a little pure Python class that I'll use to guide me. (See attached proxy.py.) Then I'll start the work, roughly like this:
At this point we're ready to start using the proxy class in typing.py, instead of the current implementation of _GenericAlias. We'll need to debate whether all the error checks we currently have in typing.py we *really* need to keep (usually these will also be detected by running a static type checker, so there's little harm in allowing nonsense like `list[1, 2, 3]`). Alternatively I'll need volunteers to implement those checks.
I expect the implementation work to result in proposed improvements to PEP 585. (I already have some -- I don't think `isinstance(x, list[int])` should work, and I don't think we should strive for `list[int] == list`.)