`IntFloatTuple` from PEP 646 doesn't work as expected on Python<3.11

I was working on serialization / deserialization logic for the new PEP 646 in mashumaro, but unfortunately the following example from https://peps.python.org/pep-0646/#type-arguments-can-be-variadic doesn't work as expected on older python versions with typing-extensions 4.4.0:

from typing import Tuple, TypeVar
from typing_extensions import TypeVarTuple, Unpack, get_args


K = TypeVar("K")
T = TypeVar("T")
Ts = TypeVarTuple("Ts")

Ts1 = TypeVarTuple("Ts1")
Ts2 = TypeVarTuple("Ts2")
IntTuple = Tuple[int, Unpack[Ts1]]
IntFloatTuple = IntTuple[float, Unpack[Ts2]]

print(IntFloatTuple.__args__)
print(get_args(IntFloatTuple))

The PEP said:

Here, *Ts1 in the IntTuple alias is bound to Tuple[float, *Ts2], resulting in an alias IntFloatTuple equivalent to Tuple[int, float, *Ts2].

On python 3.11.0 it works seamlessly as expected:

print(IntFloatTuple.__args__)
# (<class 'int'>, <class 'float'>, *Ts2)

On python 3.10.9 it produces error on IntFloatTuple = IntTuple[float, Unpack[Ts2]]:

Traceback (most recent call last):
  File "/Users/alexander.tikhonov/projects/mashumaro/pep_646.py", line 12, in <module>
    IntFloatTuple = IntTuple[float, Unpack[Ts2]]
  File "/Users/alexander.tikhonov/.pyenv/versions/3.10.9/lib/python3.10/typing.py", line 309, in inner
    return cached(*args, **kwds)
  File "/Users/alexander.tikhonov/.pyenv/versions/3.10.9/lib/python3.10/typing.py", line 1070, in __getitem__
    arg = subst[arg]
KeyError: typing_extensions.Unpack[Ts1]

On python 3.9.16 it produces the same error on IntFloatTuple = IntTuple[float, Unpack[Ts2]]:

Traceback (most recent call last):
  File "/Users/alexander.tikhonov/projects/mashumaro/pep_646.py", line 12, in <module>
    IntFloatTuple = IntTuple[float, Unpack[Ts2]]
  File "/Users/alexander.tikhonov/.pyenv/versions/3.9.16/lib/python3.9/typing.py", line 274, in inner
    return cached(*args, **kwds)
  File "/Users/alexander.tikhonov/.pyenv/versions/3.9.16/lib/python3.9/typing.py", line 774, in __getitem__
    arg = subst[arg]
KeyError: typing_extensions.Unpack[Ts1]

On python 3.8.16 it produces wrong result:

print(IntFloatTuple.__args__)
(<class 'int'>, typing_extensions.Unpack[Ts1])
print(get_args(IntFloatTuple))
# (<class 'int'>, typing_extensions.Unpack[Ts1])

On python 3.7.16 it produces the same wrong result:

print(IntFloatTuple.__args__)
# (<class 'int'>, typing_extensions.Unpack[Ts1])
print(get_args(IntFloatTuple))
# (<class 'int'>, typing_extensions.Unpack[Ts1])

I couldn't figure out how to get type arguments the same on all python versions, and so it stops me from full support for variadic generics. At the moment I have to skip some tests like this one on python<3.11.

P.S. This problem also affects IntFloatsTuple = IntTuple[Unpack[Tuple[float, ...]]].

issue の PEP 646 の例から始め、報告されている Python バージョン間での動作を typing-extensions 4.4.0 と比較してください。次に、tests/test_pep_646.py の 392-408 行付近にあるスキップされたケースを調べてください。対応している古いバージョンで、ネストされた可変長エイリアスが一貫した型引数を生成すれば完了です。