feature request: explicity metaclass().__getitem__ typing

[Levitanus]

Mypy does well with metaclass __getitem__ when the class is instatiated,
but in case of indexing just the child class, it fails:

class Other:
    ...

class GenMeta(type):
    def __getitem__(cls, ref: Type[int]) -> Type[Other]:
        return Other

class Gen(metaclass=GenMeta):
    ...

a = Gen[int]  # "Gen" expects no type arguments, but 1 given
reveal_type(a)  # Revealed type is 'Any'
b = Gen[int]()
reveal_type(b)  # Revealed type is 'pyksp.simple_test.Other'


def foo(arg: Gen[int]) -> None:  # "Gen" expects no type arguments, but 1 given
    reveal_type(arg)  # Revealed type is 'pyksp.simple_test.Gen'

I think it would be very useful to check, especially in case of function annotations

[ilevkivskyi]

It is expected that it works in runtime context but doesn't work in type context, because anything that appears in type context should be known statically. Consider a simpler example:

def f() -> Type[int]: ...

x = f()()  # OK, inferred type for x is `int`
y: f()  # Fails

The error message however is confusing, but then this is essentially a duplicate of #4030

[ilevkivskyi]

By the way, the workaround for your case is to use if TYPE_CHECKING: ....

[Levitanus]

Sorry, can you expand the last comment?
If TYPE_CHECKING where?

[ilevkivskyi]

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    <definition of Gen that would satisfy mypy>
else:
    <your definition that will be executed at runtime>

[A5rocks]

If I understand the reason this can't happen correctly, wouldn't this be solved by a type that has an invariant rather than covariant (?) type variable?