Unexpected "need type type annotation" with contextmanager

[JukkaL]

This generates a false positive in g:

from typing import Generator, Callable, Iterator, TypeVar, ContextManager, Any
from typing_extensions import ParamSpec

from nonexistent import deco  # type: ignore

_T = TypeVar("_T")
_P = ParamSpec("_P")
_T_co = TypeVar("_T_co", covariant=True)
_F = TypeVar("_F", bound=Callable[..., Any])

class _GeneratorContextManager(ContextManager[_T_co]):
    def __call__(self, func: _F) -> _F: ...

def contextmanager(
    func: Callable[_P, Iterator[_T]]) -> Callable[_P, _GeneratorContextManager[_T]]: ...

@contextmanager
@deco
def f() -> Generator[str, None, None]:
    pass

def g() -> Generator[str, None, None]:
    with f() as x:  # <<<--------- Need type annotation for "x"
        yield x

This is a regression so it's high priority. It was introduced by me in #11594.