Limited inference with union of callables and assignment to a union

[Viicos]

Bug Report

Apologies if this was raised already. The only related issue I could find is #15887.

In Pydantic 2.10, we changed the definition of our field function to be:

from typing import Callable, TypeVar

_T = TypeVar('_T')

def Field(
    *,
    default_factory: Callable[[], _T] | Callable[[dict[str, Any]], _T],
) -> _T: ...

While this type checks correctly for some common use cases, we get errors when the Field function is assigned to a union of types, or when the default factory signature isn't trivial (e.g. contains overloads). Here are some examples failing:

(playground)

cvar = ContextVar[None]("session_id", default=None)


class Class:
    # error: Argument "default_factory" to "Field" has incompatible type "type[list[Any]]"; expected "Callable[[], Never] | Callable[[dict[str, Any]], Never]"  [arg-type]:
    a: list | None = Field(default_factory=list)

    # error: Argument "default_factory" to "Field" has incompatible type overloaded function; expected "Callable[[], Never] | Callable[[dict[str, Any]], Never]"  [arg-type]:
    b: None = Field(default_factory=cvar.get)  # note that `cvar.get` is overloaded. The first overload (`() -> None`) should match.


    # fine:
    fine: list = Field(default_factory=list)

Note that (at least for a), this worked as expected with the old type inference.

I first wanted to know if this can be considered a mypy limitation, or is the code really unsafe in some way? Or maybe is it because the typing spec does not provide any info regarding these use cases?

In comparison, pyright and pyre accepts the provided examples. We will currently recommend users to add a type: ignore comment and redirect to this issue. However, feel free to close if there's already an issue tracking this.

Thanks in advance

Your Environment

• Mypy version used: 1.13
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.13