[PEP 696] `TypeError` when subclassing and omitting `TypeVar` with default.

Bug report

Bug description:

The following example raises a TypeError: Some type variables (T) are not listed in Generic[U]:

class Bar[T, S=T]: ...

class Baz[U](Bar[U]): ...

Traceback (most recent call last):
  File "/home/rscholz/Downloads/tmp.py", line 3, in <module>
    class Baz[U](Bar[U]): ...
  File "/home/rscholz/Downloads/tmp.py", line 3, in <generic parameters of Baz>
    class Baz[U](Bar[U]): ...
  File "/usr/lib/python3.13/typing.py", line 1288, in _generic_init_subclass
    raise TypeError(f"Some type variables ({s_vars}) are"
                    f" not listed in Generic[{s_args}]")
TypeError: Some type variables (T) are not listed in Generic[U]

The PEP states that

non-overridden defaults should be substituted in, and type parameters with such defaults can be further specialised down the line.

And the typing spec uses similar phrasing, which makes me believe this is a bug. In the example above, I'd expect that Baz[U] is a Bar[U, U] by substitution + default.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-155646

/usr/lib/python3.13/typing.py の _generic_init_subclass から始めます。ここで報告された TypeError が発生するため、PEP 696 と typing 仕様のサブクラス化に関するセクションと動作を比較してください。完了の条件は、示されている Bar/Baz の例が、報告されたエラーを発生させずに、文書化されたデフォルト置換の動作に従うことです。