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.

首先在 mypy playground 中复现所提供的示例，然后检查链接的 typeshed typing.pyi 第 787-788 行中的 Mapping 定义。确定它们的方差注解如何与 3.12-style MyMapping 示例交互；完成的标准是该示例不再产生错误的方差错误，同时不会引入等效的注解问题。