filter does not narrow TypeVar used in TypeGuard signature

[Jasha10]

Bug Report

Using filter with a TypeGuard, the TypeVar used in the TypeGuard signature is not properly narrowed.

To Reproduce

# tmp.py
from typing import Iterator, Optional, TypeVar

from typing_extensions import TypeGuard

T = TypeVar("T")


def is_not_none(x: Optional[T]) -> TypeGuard[T]:
    return x is not None


maybe_ints: list[Optional[int]] = [1, None, 2]
ints: Iterator[int] = filter(is_not_none, maybe_ints)

$ mypy tmp.py
tmp.py:14: error: Argument 1 to "filter" has incompatible type "Callable[[Optional[T]], TypeGuard[T]]"; expected "Callable[[Optional[int]], TypeGuard[int]]"
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

I wouldn't expect any error. Since the second argument to filter is typed as list[Optional[int]], mypy should narrow the TypeVar T appropriately.

Your Environment

• Mypy version used: mypy 0.950
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

This may be related to #12682. I decided to open a new ticket instead of commenting on that issue because the repro above feels different from the one reported on that issue.

[PabloRuizCuevas]

I think this changed a bit already:

c:list[int|None] = [2,None, 4]
d = filter(lambda x: x is None, c)  # not working which is kind of annoying

e:list[int|None] = [2,None, 4]
f = filter(lambda x: isinstance(x,None), c)  # not workign

But this works:

k:list[int|None] = [2,None, 4]
g = filter(None, k)

def isNone[T](x: T | None) -> TypeGuard[T]:
    return x is None

a: list[int|None] = [2,None, 4]
b = filter(lambda x: isNone(x), a)

It would be nice if first ones are supported though, but still this issue can be closed already.