Fix cross-variable type-narrowing example

[InSyncWithFoo]

From Type narrowing § Limitations:

def f(a: str | None, b: str | None) -> str:
    if a is not None or b is not None:
        return a or b  # Incompatible return value type (got "str | None", expected "str")
    return 'spam'

A trivial counter-example is f('', None), which returns None. Ironically, this somewhat makes Mypy's diagnostic "correct".

I propose that str be replaced with a custom class C whose __bool__() is not defined (does it have to be @final too?):

class C:
    pass

def f(a: C | None, b: C | None) -> C:
    if a is not None or b is not None:
        return a or b  # Incompatible return value type (got "C | None", expected "C")
    return C()

[hauntsaninja]

Thank you for fixing!