TypeError when omitting a Protocol type argument with default

This is very similar to https://github.com/python/cpython/issues/137191, but with a slightly different error message:

from typing_extensions import Generic, Protocol, TypeVar

T1 = TypeVar("T1")
T2 = TypeVar("T2", default=object)

class A(Protocol[T1]): ...
class B1(A[T2], Protocol, Generic[T1, T2]): ...  # the workaround
class B2(A[T2], Protocol[T1, T2]): ...  # the problem

B1[str]  # ok
B2[str]  # TypeError

on 3.12.11:

Traceback (most recent call last):
  File "/home/joren/huh.py", line 11, in <module>
    B2[str]  # TypeError
    ~~^^^^^
  File "/home/joren/.pyenv/versions/3.12.11/lib/python3.12/typing.py", line 398, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/home/joren/.pyenv/versions/3.12.11/lib/python3.12/typing.py", line 1110, in _generic_class_getitem
    _check_generic(cls, params, len(cls.__parameters__))
  File "/home/joren/.pyenv/versions/3.12.11/lib/python3.12/site-packages/typing_extensions.py", line 3085, in _check_generic
    raise TypeError(f"Too {'many' if alen > elen else 'few'} arguments"
TypeError: Too few arguments for <class '__main__.B2'>; actual 1, expected at least 1

on 3.9.23:

Traceback (most recent call last):
  File "/home/joren/huh.py", line 11, in <module>
    B2[str]  # TypeError
  File "/home/joren/.pyenv/versions/3.9.23/lib/python3.9/typing.py", line 277, in inner
    return func(*args, **kwds)
  File "/home/joren/.pyenv/versions/3.9.23/lib/python3.9/typing.py", line 1004, in __class_getitem__
    _check_generic(cls, params, len(cls.__parameters__))
  File "/home/joren/.pyenv/versions/3.9.23/lib/python3.9/site-packages/typing_extensions.py", line 3049, in _check_generic
    raise TypeError(f"Too {'many' if alen > elen else 'few'} {things}"
TypeError: Too few parameters for <class '__main__.B2'>; actual 1, expected at least 1

And since typing_extensions occurs in the traceback, I figured that this is not the same issue as https://github.com/python/cpython/issues/137191.

サポートされている Python バージョンで例を再現し、その後 traceback に表示される _check_generic の周辺を typing_extensions.py で調べてください。B2[str] と、動作する回避策である B1[str] を比較してください。報告された TypeError が解消され、ジェネリック引数の検証にリグレッションがないことを確認できれば完了です。