More specific type inference from structural pattern matching #16657

class Parent:
    pass


class A(Parent):
    def do_something(self) -> None:
        print("something A-ish")


class B(Parent):
    def do_something(self) -> None:
        print("something B-ish")


def my_function(val: Parent) -> None:
    match val:
        case A():
            pass
        case B():
            pass
        case Parent():
            return

    val.do_something()  # "Parent" has no attribute "do_something" Mypy(attr-defined)
    reveal_type(val)