Erroneous arg-type error with Mapping and reversed Mapping

I get an arg-type error in MyPy 1.14.1 on the following code using a Union of inverted collections.abc.Mapping:

from collections.abc import Mapping


def fun(x: Mapping[str, int | None] | Mapping[int | None, str]):
    pass


fun({"a": 1, "b": 2})

fun({"a": 1, "b": 2, "c": None})

fun({1: "a", 2: "b"})

fun({1: "a", 2: "b", None: "c"})

The error is:

mypy_type_error.py:12: error: Argument 1 to "fun" has incompatible type "dict[int, str]"; expected "Mapping[str, int | None] | Mapping[int | None, str]" [arg-type]

That is, MyPy takes issue with the parameters in the third function call. The last call including the None key does not produce an error, though. I'd expect the third call to not show an error, either.

首先使用 MyPy 1.14.1 运行报告中的代码片段，并比较四次调用。然后跟踪参数类型检查和 union-of-Mapping 的处理，以找到相关的回归测试区域；完成的标准是第三次调用与第四次调用一样能够稳定地被接受，同时不使其他示例出现回归。