gh-118168: Fix Unpack interaction with builtin aliases

[JelleZijlstra]

• Issue: Incorrect argument substitution on Unpack[tuple[...]] #118168

[AlexWaygood]

It feels weirdly inconsistent that we allow nonsensical things with the new syntax, but not with the "backwards-compatible syntax":

>>> from typing import TypeVarTuple, Unpack
>>> class Foo[*Ts]: ...
...
>>> Ts = TypeVarTuple("Ts")
>>> Bar = Foo[int, *Ts]
>>> Bar[*list[str]]
__main__.Foo[int, *list[str]]
>>> Bar[Unpack[list[str]]]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    Bar[Unpack[list[str]]]
    ~~~^^^^^^^^^^^^^^^^^^^
  File "/Users/alexw/dev/cpython/Lib/typing.py", line 394, in inner
    return func(*args, **kwds)
           ~~~~^^^^^^^^^^^^^^^
  File "/Users/alexw/dev/cpython/Lib/typing.py", line 1394, in __getitem__
    args = _unpack_args(args)
           ~~~~~~~~~~~~^^^^^^
  File "/Users/alexw/dev/cpython/Lib/typing.py", line 306, in _unpack_args
    subargs = getattr(arg, '__typing_unpacked_tuple_args__', None)
              ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexw/dev/cpython/Lib/typing.py", line 1791, in __typing_unpacked_tuple_args__
    raise TypeError("Unpack[...] must be used with a tuple type")
TypeError: Unpack[...] must be used with a tuple type

But I guess that's a preexisting problem, not new to this PR. We previously discussed the issue of "all generic aliases being unpackable" in #103450.

[JelleZijlstra]

@AlexWaygood thanks for the review! Do you think this is safe to backport? (I'm leaning yes.)

[AlexWaygood]

@AlexWaygood thanks for the review! Do you think this is safe to backport? (I'm leaning yes.)

yeah, I think should be fine

[miss-islington-app]

Thanks @JelleZijlstra for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-118178 is a backport of this pull request to the 3.12 branch.