Union between a callable and a type not respected by functools.partial

[eli-schwartz]

Seems similar to #17646 but not fixed in https://mypy-play.net/?mypy=master&python=3.12
The linked #17659 is not applicable because I am not using Self.

Sample code:

import functools
import typing as T
from typing_extensions import TypeAlias

FooBarFunc: TypeAlias = T.Callable[..., 'FooBar']

class FooBar:
    def __init__(self, arg1: str) -> None:
        pass

def transform(t: T.Union[T.Type[FooBar], FooBarFunc]) -> None:
    val = functools.partial(t)

Error:

main.py:12: error: "type[FooBar] | Callable[..., FooBar]" not callable  [misc]

The codebase where this was discovered: https://github.com/mesonbuild/meson/blob/83f8de5357f31d6448ae033e1e8ed2b22c8c306a/mesonbuild/dependencies/factory.py#L75-L95

mesonbuild/dependencies/factory.py:95:38: error: "type[CMakeDependency] | Callable[..., CMakeDependency]" not callable  [misc]

Both sides of the union if cast and used independently, work.

[hauntsaninja]

This is sort of underlying behaviour in mypy that is exposed now that mypy checks functools.partial

E.g. on mypy 1.11 both of these will complain

def join(t1: T.Type[FooBar], t2: FooBarFunc):
    val1 = [t1, t2]
    val1[0]("asdf")
    
    val2 = t1 if random.random() < 0.5 else t2
    val2("asdf")

This could be fixed by improving the join logic (improving callable joins would fix many existing mypy issues) or potentially by changing the functools.partial logic to avoid using joins on unions (this could get messy)

[eli-schwartz]

Thanks! I've tested mypy from git and there is one less error report now when upgrading mypy (and looks like the others are all either our fault, unused "type: ignore", or a renamed error code). <3