Support typing_extensions.Unpack

[picnixz]

The typing.Unpack construction is supported but not typing_extensions.Unpack because it is treated as a TypeVar instance (this is due to a hack in typing_extensions that sets __class__ to TypeVar).

This is built on top of #12256.

[picnixz]

Uh-oh. Good to know that Unpack is capricious actually.

[AA-Turner]

I've restricted the support for typing_extensions.Unpack to Python 3.9 & 3.10 -- I'm wary of opening the floodgates for typing_extensions, and there is already enough variance without two potential sources of truth for typing constructs.

Other than that, please have a look -- I cleaned up the branch history to work from the latest master, but I think I've kept every relevant change.

A

[picnixz]

I think it should be good. I may need to have a look at python/cpython#118168 though.

[picnixz]

AH ! I remember that test failure ! Let me check what happened

[picnixz]

Ah yes, actually, in typing_extensions, the name of the Unpack form is not under __name__ but under _name. So that's why we had the failure.

In addition, with Python 3.11, if you use typing_extensions, then it's not synchronized, so typing.get_origin(typing_extensions.Unpack) gives you typing_extensions.Unpack instead (hence the failure).

[picnixz]

I'm wary of opening the floodgates for typing_extensions, and there is already enough variance without two potential sources of truth for typing constructs.

The problem is that we 3.11 and 3.12 are not sync'ed for typing_extension, hence the sheningans. In general, they are synced but not in this case.