`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, ...]]].

从 issue 中的 PEP 646 示例开始，使用 typing-extensions 4.4.0 比较其在所报告的各个 Python 版本中的行为。然后检查 tests/test_pep_646.py 中 392-408 行附近被跳过的测试用例；当嵌套的可变参数别名在受支持的较旧版本上产生一致的类型参数时，即表示完成。