Mypy does not correctly understand MemberDescriptors for objects with slots.

Bug Report

When a Python object uses slots, MemberDescriptors are created for each attribute defined as a slot.

Mypy incorrectly interprets these fields as the type of the slotted value, rather than as a member descriptor.

To Reproduce

class TestObject(object):
   __slots__ = ["a", "b"]
   a: int
   b: str

   def __init__(self, a: int, b: str) -> None:
      self.a = a
      self.b = b

def test() -> None:
   x = TestObject(5, "hi")
   reveal_type(TestObject.a)
   reveal_type(TestObject.b)
   reveal_type(x.a)
   reveal_type(x.b)

Expected Behavior

Here is what the types actually are based on the python repl.

>>> TestObject
<class '__main__.TestObject'>
>>> x = TestObject(10, "hi")
>>> type(TestObject.a)
<class 'member_descriptor'>
>>> type(TestObject.b)
<class 'member_descriptor'>
>>> type(x.a)
<class 'int'>
>>> type(x.b)
<class 'str'>

Actual Behavior

Mypy output:
12: note: Revealed type is 'builtins.int'
13: note: Revealed type is 'builtins.str'
14: note: Revealed type is 'builtins.int'
15: note: Revealed type is 'builtins.str'

Your Environment

Python 3.8.1
mypy 0.782

Comienza con la reproducción proporcionada de slots y MemberDescriptor y, después, sigue cómo mypy determina los tipos revelados para los atributos de clase frente a los atributos de instancia. Confirma la corrección comprobando que TestObject.a y TestObject.b se reportan como descriptores de miembro, mientras que x.a y x.b siguen siendo int y str.