@property returning descriptor instance triggers false positive

🐛 Bug Report

Accessing a field implementing the descriptor protocol via a property method causes MyPy to complain that there is no matching overload for __get__().

To Reproduce

Run MyPy with all default settings on the following code:

from typing import Optional
from typing import overload
from typing import Type

class A:
    pass

class Field:
    def __init__(self, attr: Optional["Field"] = None):
        self.attr = attr

    @property
    def crashing_property(self) -> Optional["Field"]:
        return self.attr

    @overload
    def __get__(self, instance: None, owner: Type["A"]) -> "Field":
        ...

    @overload
    def __get__(self, instance: "A", owner: Type["A"]) -> Optional["Field"]:
        ...

    def __get__(self, instance, owner):
        if not instance:
            return self
        return self.attr

f = Field()  # type: Field
f.attr  # Works fine
f.crashing_property  # Triggers "no overload variant"

n.b. adding __set__() makes no difference

Expected Behavior

Accessing crashing_property shouldn't cause an error.

Actual Behavior

The last line in the example throws the following error:

test.py:39: error: No overload variant of "__get__" of "Field" matches argument types "Field", "Type[Field]"
test.py:39: note: Possible overload variants:
test.py:39: note:     def __get__(self, instance: None, owner: Type[A]) -> Field
test.py:39: note:     def __get__(self, instance: A, owner: Type[A]) -> Optional[Field]

If you add this the code validates fine:

@overload
def __get__(self, instance: "Field", owner: Type["Field"]):
    ...

Your Environment

• Mypy version used: 0.782, 0.800, 0.812, 0.910
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None, using all defaults
• Python version used: CPython 3.7.7, 3.8.6, 3.9.1
• Operating system and version: Ubuntu 20.04, MacOS 11.1 (Big Sur)

Inizia eseguendo il riproduttore Python fornito con le impostazioni predefinite di MyPy e conferma l’errore di overload falso positivo nell’accesso alla proprietà. Il lavoro è completato quando tale accesso viene validato senza richiedere un overload get aggiuntivo per le istanze di Field, mantenendo valido il comportamento esistente del descrittore.