Shorter repro:
https://mypy-play.net/?mypy=master&python=3.12&gist=985cd92890a29ba55c4a37bf818e526a

The trouble is that override is created as a local reference. If you change

-override: Callable[[F], F]
+override: Callable[[F, int], F]

then you'd get

error: Incompatible import of "override" (imported name has type "Callable[[_F], _F]", local name has type "Callable[[F, int], F]")  [assignment]

so the right way to think about it is

-(1) remote reference
+(1) local declaration
+(2) assignment

and since we don't track the value throughout assignments, the override local ref cannot be later "blessed" with being a typing.override.

Does it make sense to either

• warn on the import?
• have the import mutate the node's fullname?

Doesn't it work as expected without the override: Callable[[F], F]?

I would write it like this:

from typing import Any, Callable, TypeVar, TYPE_CHECKING

F = TypeVar("F", bound=Callable[..., Any])

if TYPE_CHECKING:
    from typing_extensions import override
else:

    def override(method: F, /) -> F:
        return method

class ClassA:
    ...

class ClassB(ClassA):
    @override
    def this_should_error(self) -> None:
        pass
    
reveal_type(ClassB.this_should_error)

https://mypy-play.net/?mypy=master&python=3.11&gist=f8cef5ea618b97668d941510ad211550

...which seems to work?

Doesn't it work as expected without the override: Callable[[F], F]?

Yes, it does - that's why I wrote "accidental" in the title. The override: Callable[[F], F] is/was a leftover from my previous code using the overrides packages, which needs this kind of gymnastics for reasons that are not clear to me.

The point of this issue not to improve my code, but to make you aware that by typing override, one can silently render it useless.

Yeah, I think this is a won't fix. As ikonst says, this looks like something that could get rebound. pyright's behaviour is similar, except that it issues a no-redef kind of warning that you were looking to avoid in #13914

tmke8's suggestion of if TYPE_CHECKING is a good way to go for this kind of thing. Type checkers are special cased to always know about typing_extensions even if it's not installed, so everything should just work.