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.

Beginne damit, die Logik zur Typverengung für Vergleiche und deren Umgang mit TypeGuard- und TypeIs-Rückgabeannotationen zu lokalisieren. Überprüfe das Verhalten anhand des IsInstance-Beispiels aus dem Issue: Nach assert v == IsInstance(int) sollte reveal_type(v) int melden, entsprechend dem expliziten Aufruf von eq.