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

提供されている slots と MemberDescriptor の再現から始め、次に mypy がクラス属性とインスタンス属性について明らかにする型をどのように決定しているかを追跡します。TestObject.a と TestObject.b がメンバーディスクリプタとして報告され、x.a と x.b が int と str のままであることを確認して、修正を検証します。