If the user generic with ParamSpec parameter substituted with a parametrised list containing TypeVar, that TypeVar cannot be substituted. >>> from typing import * >>> T = TypeVar("T") >>> P = ParamSpec("P") >>> class X(Generic[P]): ... f: Callable[P, int] ... >>> Y = X[[int, T]] >>> Y __main__.X[(<class 'int'>, ~T)] >>> Y[str] Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/serhiy/py/cpython/Lib/typing.py", line 309, in inner return func(*args, **kwds) ^^^^^^^^^^^^^^^^^^^ File "/home/serhiy/py/cpython/Lib/typing.py", line 1028, in __getitem__ _check_generic(self, params, len(self.__parameters__)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/serhiy/py/cpython/Lib/typing.py", line 228, in _check_generic raise TypeError(f"{cls} is not a generic class") ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ TypeError: __main__.X[(<class 'int'>, ~T)] is not a generic class Expected result equal to X[[int, str]].