Mypy thinks `dict[Never, Never]` is a subtype of `Mapping[str, object]`

[randolf-scholz]

This behaves as expected: https://mypy-play.net/?mypy=latest&python=3.12&gist=8718e96550599dc3ae48a13879005762

from typing import Mapping, Never

def upcast(d: dict[Never, Never]) -> Mapping[str, object]:
    return d  # E: [return-value] (✅ expected)

But it breaks here (due to caching?) https://mypy-play.net/?mypy=latest&python=3.12&gist=6199ba50b6b02e2350e70cb3a8ff4bba

from typing import Mapping, Never

def get[M: Mapping[str, object]](arg: M, /) -> M:
    return arg

reveal_type(get({}))  # dict[Never, Never] (❌ incorrect)

def upcast(d: dict[Never, Never]) -> Mapping[str, object]:
    return d  # <-- No error anymore 😱

[randolf-scholz]

@sterliakov @ilevkivskyi This was only partially fixed; the caching part is adressed but mypy still incorrectly reveals dict[Never, Never] on master branch.

https://mypy-play.net/?mypy=master&python=3.12&gist=79b0378f3b9997bc7bc1ffea94a7dc15

from typing import Mapping, Never

def get[M: Mapping[str, object]](arg: M, /) -> M:
    return arg

reveal_type(get({}))  

main.py:6: note: Revealed type is "builtins.dict[Never, Never]"

But dict[Never, Never] is not a subtype of Mapping[str, object]...