Support type narrowing on __eq__ with TypeGuard/TypeIs

Feature

Handle type narrowing when overridden __eq__ method has a return type annotated with TypeIs or TypeGuard.

Pitch

When defining custom __eq__ methods, it might be useful in some contexts to leverage type narrowing. For example, consider an assertion helper[^1] like the following:

import typing

T = typing.TypeVar("T")


class IsInstance(typing.Generic[T]):
    def __init__(self, t: type[T]) -> None:
        self.t = t

    def __eq__(self, other: typing.Any) -> typing.TypeGuard[T]:
        return isinstance(other, self.t)


v: typing.Any = 1
assert v == IsInstance(int)
typing.reveal_type(v)  # Should ideally be "int"

Currently, mypy doesn't narrow the type. If we explicitly call __eq__, it works as intended:

v: typing.Any = 1
assert IsInstance(int).__eq__(v)
typing.reveal_type(v)  # int

[^1]: It's admittedly a bit naive for the sake of the example, but we could imagine more complex stuff.

Commencez par localiser la logique de réduction de type pour les comparaisons et son traitement des annotations de retour TypeGuard et TypeIs. Vérifiez le comportement avec l’exemple IsInstance de l’issue : après assert v == IsInstance(int), reveal_type(v) devrait indiquer int, comme lors de l’appel explicite à eq.