Support exhaustiveness checking of single enumerable properties

Feature

Support exhaustiveness checking of single boolean and enum properties and attributes of objects.

Pitch

The proposal is to improve exhaustiveness checking by supporting properties/attributes on a matched object. While it would be amazing to have infinitely nested support for this, I think we can recognize that there is value in supporting just single-level enumerable attributes.

In this example mypy isn't currently recognizing that all possible variations of Foo are handled, and gives an error on the assert_never().

from typing import assert_never

class Sentinel: ...

class Foo:
    prop: bool

val: Foo | Sentinel

match val:
    case Sentinel():
        ...
    case Foo(prop=True):
        ...
    case Foo(prop=False):
        ...
    case no_match:
        assert_never(no_match)  # Currently a false positive here, no_match should be <nothing>.

https://mypy-play.net/?mypy=latest&python=3.11&gist=1edcb8f3ea56334662ed2036404563a1

Likewise, if the feature is accepted, we should expect an unreachable error for the third case in this example.

from typing import assert_never

class Foo:
    prop: bool

val: Foo

match val:
    case Foo(prop=True):
        ...
    case Foo(prop=False):
        ...
    case Foo():
        ...  # Currently a false negative here, should be unreachable.

https://mypy-play.net/?mypy=latest&python=3.11&gist=74b23b6868c0c52056be1ade612b3052

Inizia con i due esempi di match di Python e le relative riproduzioni collegate in mypy-play. Definisci il comportamento in termini di esaustività e di casi irraggiungibili per le proprietà booleane e le proprietà di tipo enum a un solo livello, quindi verifica che vengano corretti i risultati false-positive e false-negative descritti nell’issue.