Old-style generic `ParamSpec(default=...)` fails if not provided as a type argument and not in the last position of a type list when passing type arguments

[bzoracler]

Bug report

Bug description:

In accordance to the typing specifications, default= of ParamSpec can take an ellipsis .... However, this is not handled properly with old-style generics in the following example and causes a runtime error:

from typing import Generic, ParamSpec, TypeVar

P_default = ParamSpec("P_default", default=...)
T = TypeVar("T")
T_default = TypeVar("T_default", default=object)

class A(Generic[T, P_default, T_default]): pass

>>> A[int]
Traceback (most recent call last):
  ...
TypeError: can only concatenate list (not "tuple") to list

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-138859: Fix TypeError when there is a ParamSpec substitution with a default #138867
• gh-138859: Account for ParamSpec defaults that are not lists … #138868
• [3.14] gh-138859: Account for ParamSpec defaults that are not lists … (GH-138868) #140207
• [3.13] gh-138859: Account for ParamSpec defaults that are not lists … (GH-138868) #140208

[bzoracler]

@sobolevn I'm running the tests locally now with a fix, will make a PR in the next few minutes.

[sobolevn]

Oh, sorry! I've missed it. I will close my to let you contribute 👍