Covariant type variables should be allowed in class method

Currently, covariant type variables are allowed in __init__ so that you can put create an immutable containers with something inside of it. See https://github.com/python/mypy/issues/2850.

It would be useful if covariant types are also allowed in class method that serve as alternative constructors. Currently mypy errors on this example, which is an extension of the other issues example:

from typing import Generic, TypeVar

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


class C(Generic[T_co]):
    def __init__(self, x: T_co) -> None:
        ...

    @classmethod
    def custom_init(cls, x: T_co) -> "C[T_co]":
        # error: Cannot use a covariant type variable as a parameter
        return cls(x)

My use case is trying to define an immutable container protocol that has a create class method, which will initialize the container with some contents, like this:

T_cov = typing.TypeVar("T_cov", covariant=True)

class ListProtocol(typing_extensions.Protocol[T_cov]):
    @classmethod
    def create(cls, *items: T_cov) -> "ListProtocol[T_cov]":
        ...

    def getitem(self) -> T_cov:
        ...
    
    ...

まず、報告にある classmethod の例で問題を再現し、mypy がメソッドパラメーター内の共変な型変数をどのように検証しているかを調べます。代替コンストラクターに意図される動作を定義し、次に受け入れられるケースを示す回帰テストを追加して、関連する型チェッカーのテストを実行します。