Covariant type variables should be allowed in __init__

[ilevkivskyi]

There are several issues about covariance, but I didn't find any discussion about __init__. Currently, mypy complains about this code:

from typing import Generic, TypeVar

T_co = TypeVar('T_co', covariant=True)

class C(Generic[T_co]):
    def __init__(self, x: T_co) -> None:
       # Error: Cannot use a covariant type variable as a parameter
       ...

Although this is normal in general, I believe this should be allowed for __init__. Otherwise it would be difficult (if possible at all) to put something into a covariant (immutable) container on instantiation.

[JukkaL]

Yeah, this should be allowed. Not sure if there are some additional edge cases related to __init__ we should handle differently once we allow this -- x.__init__(...) is already rejected, which is good.

[ilevkivskyi]

OK, here is a PR #2888 fixing this.