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Mypy thinks dict[Never, Never] is a subtype of Mapping[str, object] #19641
Description
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 😱
Activity
[-][caching?] Mypy thinks `dict[Never, Never]` is a subtype of `Mapping[str, object]`[/-][+]Mypy thinks `dict[Never, Never]` is a subtype of `Mapping[str, object]`[/+]Include `ambiguous` into `UninhabitedType` identity (#19648)
randolf-scholz commentedon Sep 19, 2025
@sterliakov @ilevkivskyi This was only partially fixed; the caching part is adressed but
mypystill incorrectly revealsdict[Never, Never]on master branch.https://mypy-play.net/?mypy=master&python=3.12&gist=79b0378f3b9997bc7bc1ffea94a7dc15
But
dict[Never, Never]is not a subtype ofMapping[str, object]...