More specific type inference from structural pattern matching

[Doekeb]

Feature

Mypy should take a note from Pylance and infer more specific types as a result of structural pattern matching.

Pitch

The main example:

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)

The result of reveal_type(val) is Parent according to Mypy and A | B according to Pylance. While both of these correctly describe the type of val, the type of A | B is more specific which means the type checker would have no reason to complain about val.do_something().

[erictraut]

This is another join-v-union issue. It's not specific to pattern matching. There are already over 60 issues open on this topic in the mypy issue tracker.

[hauntsaninja]

Fixed by #18538