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

[randolf-scholz]

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]): ...

Full Traceback

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

[randolf-scholz]

I think the root cause is that runtime does not correctly perform the substitution:

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

print(Bar[int])  # __main__.Bar[int, T], but should be Bar[int, int]

Opened #140665 with this more precise bug report, I would suspect when #140665 is fixed, the OP bug report evaporates automatically.