Alias that is generic over a ParamSpec cannot be further specialized

[JelleZijlstra]

A type alias that is generic over a ParamSpec can be specialized with a list of types, and if this list includes a TypeVar, the resulting alias is still generic to type checkers, but it cannot be subscripted again at runtime.

>>> from collections.abc import Callable
>>> type X[**P] = Callable[P, int]
>>> T = TypeVar("T")
>>> X[[T]]
X[[~T]]
>>> X[[T]][str]
Traceback (most recent call last):
 File "<python-input-14>", line 1, in <module>
 X[[T]][str]
 ~~~~~~^^^^^
TypeError: X[[~T]] is not a generic class

Pyright accepts this code, and I think it's right: https://pyright-play.net/?strict=true&code=GYJw9gtgBAxmA28CmMAuBLMA7AzgOgEMAjGKdCABzBFSgGEDFjkAoUSKVATwvSwHMylarQAqPJADUCIFi1FQAvFHEUpMgBQAiUVoCUc7mqgANANoAqCwAUAukvqN4zJGesAaMllS2WJhcrmZqK2viwAJkjAUMAaAB4AXKaiZjioILZ6CSxQuVAgSABuSIwA%2BkZI8Xq5AMRQGmkg1QC0AHxeqCxAA

Reported by @Daraan in https://github.com/python/typing_extensions/pull/449/files#r1771594266.

I believe this could be fixed by making _Py_make_parameters in genericaliasobject.c recurse into lists.

Linked PRs

• gh-124445: Allow specializing generic ParamSpec aliases #124512

[tomasr8]

I'm working on a fix if that's ok :) After wrapping my head around the code a bit I think we'll also need to make _Py_subs_parameters recursive in order to get the correct substitution.