[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

Comece em /usr/lib/python3.13/typing.py, em _generic_init_subclass, onde o TypeError relatado é gerado, e compare o comportamento com as seções sobre subclassing da PEP 696 e da especificação de typing. Considera-se concluído quando o exemplo mostrado de Bar/Baz segue o comportamento documentado de substituição padrão sem gerar o erro relatado.