Introduce error category [unsafe-overload]. #16060
Description
Feature
Specifically, consider this example from the documentation [mypy-play]:
from typing import overload, Union
@overload
def unsafe_func(x: int) -> int: ... # [misc] overlap with incompatible return types
@overload
def unsafe_func(x: object) -> str: ...
def unsafe_func(x: object) -> Union[int, str]:
if isinstance(x, int):
return 42
else:
return "some string"Pitch
misc is too generic and makes it difficult to specifically disable this kind of error. I propose to give it a specific name like unsafe-overload.
Due to the absence of Intersection and Not, this check raises false positives in certain code, for example:
@overload
def foo(x: Vector) -> Vector: ...
@overload
def foo(x: Iterable[Vector]) -> list[Vector]: ...
def foo(x):
if isinstance(x, Vector):
return x
return list(map(foo, x))Where Vector implements __iter__. Obviously, what we really mean here is:
@overload
def foo(x: Vector) -> Vector: ...
@overload
def foo(x: Iterable[Vector] & Not[Vector]) -> list[Vector]: ...
def foo(x):
if isinstance(x, Vector):
return x
return list(map(foo, x))Which is actually implicit by the ordering of overloads. The latter would not raise unsafe-overload, since Iterable[Vector] & Not[Vector] is not a subtype of Vector. But since it can't be expressed in the current type system, there needs to be a simple way to silence this check.