no error when using covariant or contravariant TypeVar on class with writable attribute

[DetachHead]

from typing import Generic, TypeVar

T = TypeVar("T", covariant=True)


class Foo(Generic[T]):
    def __init__(self, value: T) -> None:
        self.foo = value

here, T is used in both an in and out position as the value can be modified as well.

jailbreak:

objects = Foo[object](None)

ints = Foo[int](1)


def do_thing(value: Foo[object]) -> None:
    value.foo = "asdf"


do_thing(ints)

if TYPE_CHECKING:
    reveal_type(ints.foo) # int
print(ints.foo) # "asdf" (hacked!!!!!!)

playground

[hauntsaninja]

Thanks, I think this is basically just a special case of #3208

[hauntsaninja]

Posted a comment in that issue, so if we ever do something about that we will also do something about this