[red-knot] Descriptor protocol

[sharkdp]

We need to understand the descriptor protocol in order to infer proper types for attribute accesses like in this simple example:

from typing import Literal

class Ten:
    def __get__(self, instance: object, owner: type | None = None) -> Literal[10]:
        return 10

    def __set__(self, instance: object, value: Literal[10]) -> None:
        pass

class C:
    ten = Ten()

c = C()

# TODO: this should be `Literal[10]`
reveal_type(c.ten)  # revealed: Unknown | Ten

# TODO: This should `Literal[10]`
reveal_type(C.ten)  # revealed: Unknown | Ten

# These are fine:
c.ten = 10
C.ten = 10

# TODO: Both of these should be errors
c.ten = 11
C.ten = 11

References: Python Documentation: Descriptor Guide

part of: #14164

[carljm]

An important case to consider here that isn't shown in the example in summary is where the value argument to __set__ does not have the same type as the return type of __get__, meaning we need to use a different type for type-checking assignments to the attribute than we use for inferring the type of an attribute access.

[sharkdp]

where the value argument to __set__ does not have the same type as the return type of __get__

Yes, thanks. This is already covered in the provisional tests (#15972).