Mapping invariant on key?

It seems that Mapping[k, v] is invariant to k, but not when a dict literal is passed.

To Reproduce

main.py:25: error: Argument 1 to "test_map_key" has incompatible type "Mapping[str, int]"; expected "Mapping[str | int, Any]"  [arg-type]

Why is this an error? Mapping is immutable so it shouldn't matter that the passed key is more constrained that what is expected.
Why does passing a dict literal work?

See e.g. Make Mapping covariant in keys (too). typing#273 (comment) and Reconsider making Mapping covariant in the key type typing#445
Passing a dict literal works because mypy is able to infer the static type from type context. When you assign variables, mypy uses the explicit type hint or infers a type for the variable from the expression in the left hand side.

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