(🎁) Warn about `TypeAlias` to bare `TypeVar`s

[KotlinIsland]

from typing import TypeVar, TypeAlias

T = TypeVar("T")
TAlias: TypeAlias = T

def foo(
    t1: TAlias[int],  # no error
    t2: TAlias,  # error: Missing type parameters for generic type "TAlias"
) -> None: ...

Treating it as a generic type and not as TypeVar is incorrect as at runtime it is not subscriptable:

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    t1: TAlias[int],  # no error
TypeError: 'TypeVar' object is not subscriptable

playground

[hauntsaninja]

I think this is "as designed". This is just a very dumb generic type alias

[KotlinIsland]

@hauntsaninja I know, the issue is that it's a runtime error:

TypeError: 'TypeVar' object is not subscriptable

(OP has now been updated)

[sobolevn]

Also related https://github.com/python/mypy/issues/13449#issuecomment-1221507260

[KotlinIsland]

Strangely, it correctly reports the error when the TypeAlias is ommitted.

from typing import TypeVar, TypeAlias

T = TypeVar("T")
TAlias = T  # error: Type variable "__main__.T" is invalid as target for type alias  [misc]

Can we have https://github.com/python/mypy/labels/topic-runtime-semantics?