Here's what was happening in this example:

from typing import Callable, Iterable, Iterator, List, Optional, TypeVar

T = TypeVar("T")
R = TypeVar("R")

def filter(f: Callable[[T], TypeGuard[R]], it: Iterable[T]) -> Iterator[R]: ...

a: List[Optional[int]]
bb = filter(lambda x: x is not None, a)
reveal_type(bb)  # error, actually it is now: Revealed type is "typing.Iterator[R`2]"

Mypy uses special logic in infer_lambda_type_using_context to infer lambda type args and return type. Basically, it just returns the current callable context type. Which is Callable[[T], TypeGuard[R]] in our case.

But, now I've limited this function to ignore contexts with TypeGuard. Because I believe that TypeGuard must always be explicit.

After this change Mypy works correctly:

» mypy out/ex.py --show-traceback
out/ex.py:13: error: Argument 1 to "filter" has incompatible type "Callable[[Any], bool]"; expected "Callable[[Optional[int]], TypeGuard[bool]]"
out/ex.py:14: note: Revealed type is "typing.Iterator[builtins.bool*]"
Found 1 error in 1 file (checked 1 source file)

Because we don't allow bool return type where TypeGuard is expected after #11314

The test from @gvanrossum that used to be skipped now passes 🎉

Closes #9927