Erroneous arg-type error with Mapping and reversed Mapping

[kohlrabi]

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.

[A5rocks]

This is an inference issue. Note the revealed type:

from collections.abc import Mapping


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

fun(reveal_type({1: "a", 2: "b"}))  # N: Revealed type is "builtins.dict[builtins.int, builtins.str]"