Summary

Consider the following snippet:

def b() -> bool:
    return False

class Foo:
    if b():
        x: int = 42

def f(y: Foo):
    if hasattr(y, "x"):
        reveal_type(y)  # revealed: Foo
        print(y.x)  # error[possibly-unbound-attribute]: Attribute `x` on type `Foo` is possibly unbound

https://play.ty.dev/4171e13d-f9e5-432d-baa9-fc25f414647e

In the hasattr() branch, we narrow the type of y to Foo & <protocol with members 'x'>, but then immediately simplify that intersection to Foo, since we know that all Foo members have an x member. But this simplification is incorrect, because it causes us to then emit a false positive when attempting to access the x member: we report the attribute as possibly unbound, even though the user just checked that it isn't!

We should only simplify these synthesized-protocol intersections if another element in the intersection has a definitely bound attribute of the relevant name.