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 inhttps://github.com/python/typing_extensions/pull/449/files#r1771594266.

I believe this could be fixed by making_Py_make_parameters ingenericaliasobject.c recurse into lists.

Linked PRs

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