Mapping overloads cause incorrect variance inference

https://mypy-play.net/?gist=42b235155079dd44a79e66cbcffb0060

from collections.abc import Mapping

class A: ...
class B(A): ...

def test_mapping() -> None:
    class MyMapping[K, V](Mapping[K, V]): ...

    def _0[K](arg: Mapping[K, B]) -> Mapping[K, A]: return arg
    def _1[K](arg: MyMapping[K, B]) -> MyMapping[K, A]: return arg  # ❌️

mypy doesn't seem to reuse known variance of parent classes, but instead rechecks everything. For mapping, this breaks and mypy incorrectly infers invariance in the value type because of:

https://github.com/python/typeshed/blob/a9fbf47e676865bd5bcd94e121624d9e4b53c882/stdlib/typing.pyi#L787-L788

While I do think mypy could improve this behavior on their side, we really, really should avoid using covariant parameters in contravariant positions, especially with the knowledge that it can screw up variance inference when using 3.12 style annotations.

まず、提供された例を mypy playground で再現し、次にリンク先の typeshed typing.pyi の 787-788 行にある Mapping の定義を調べます。それらの variance annotation が 3.12-style MyMapping の例とどのように相互作用するかを確認します。例が誤った variance error を生成しなくなり、同等の annotation の問題も導入していなければ完了です。