Infer descriptor type

[iwanb]

Feature

For the common case of a descriptor which behaves like a normal attribute, infer:

class X:
    y: int = descriptor()

to:

class X:
    y: descriptor[int] = descriptor()

if descriptor is a Generic with one parameter. Extra checks should also be done to make sure that the descriptor is properly typed to behave like an attribute of that type (__get__ and __set__ expect that type on an instance).

Pitch

Type annotations apply to object instances by default, so it's strange to see y: descriptor[int] as type annotation while x.y returns an int.
You could also see a normal attribute as an invisible descriptor whose behavior is to set __dict__[name] (and raise AttributeError on the class), so this could be a way to unify these things.

It's also related to:
#7724
#5481

I implemented it partially as a plugin but only for my custom descriptors.

[anis-campos]

Is there any way to do this without a plugin ? I'm so frustrated, seeing what descriptor is able to do, it's so disappointing to not be able to properly type the attribute.

Also my implementation of a descriptor is used as a convenient factory that gets the concrete type from the __annotations__ of the owner object.

    def __set_name__(self, owner: Any, name: str) -> None:
        self.public_name = name
        self.private_name = "_" + name
        self.type_ = owner.__annotations__[self.public_name]

So it would be nice to also have a way to just ignore the type of __get__ in that case, the annotation is just the only typing necessary.