Bug Report

NB: This may be more properly an issue in Typeshed, or require changes in that project to fix, but our team encountered it via MyPy.

The type annotations on builtins.filter cause correct and idiomatic usages to fail to typecheck:

To Reproduce

# filter-mypy.py
path_parts: List[Optional[str]] = ["foo", None, "bar"]

"/".join(filter(None, path_parts))
"/".join(filter(bool, path_parts))
"/".join(filter(lambda s: isinstance(s, str), path_parts))

Expected:

$> mypy filter-mypy.py
Success: no issues found in 1 source file

Actual result:

$> mypy filter-mypy.py
filter-mypy.py:6: error: Argument 1 to "filter" has incompatible type "Type[bool]"; expected "Callable[[Optional[str]], TypeGuard[str]]"
filter-mypy.py:7: error: Argument 1 to "filter" has incompatible type "Callable[[Any], bool]"; expected "Callable[[Optional[str]], TypeGuard[str]]"
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used:

$> mypy --version
mypy 0.950 (compiled: yes)

• Python version used:

$> python --version
Python 3.10.4

• Operating system and version: Ubuntu 20.04

The following is what my editor (VSCode) pulls up when I investigate the type hints for filter, though I doubt this is the same file that MyPy itself is consulting:

class filter(Iterator[_T], Generic[_T]):
    @overload
    def __init__(self, __function: None, __iterable: Iterable[_T | None]) -> None: ...
    @overload
    def __init__(self, __function: Callable[[_S], TypeGuard[_T]], __iterable: Iterable[_S]) -> None: ...
    @overload
    def __init__(self, __function: Callable[[_T], Any], __iterable: Iterable[_T]) -> None: ...

Note that both offending lines conform to the third type signature, but fail the second.