Narrowing enums causes `.value` to be typed as `Any`

Bug Report

Apologies if this has already been reported, but I couldn't find an existing issue for it.

The issue I'm seeing is related to accessing the .value of an Enum after "narrowing" it with the is operator. It seems like the narrowing (to Union[Literal[...]]) works as expected, but then it fails to realize that the .value of all the variants have the same type (and so falls back to Any).

This only affects runs with --warn-return-any enabled:

$ mypy --config-file /dev/null --warn-return-any enum_narrowing.py

To Reproduce

It's probably easiest to see in this example:

import enum
from typing import Tuple

RGB = Tuple[int, int, int]

class Color(enum.Enum):
    WHITE = (255, 255, 255)
    BLACK = (0, 0, 0)

    def red(self) -> int:
        if self is Color.WHITE:
            return 255
        return self.value[0]  # error: Returning Any from function declared to return "int"

(playground URL: https://mypy-play.net/?mypy=latest&python=3.8&flags=warn-return-any&gist=b2a6df92fb37717e5c9d7542b6e88fec)

Expected Behavior

This should deduce that self.value has type RGB and succeed.

Actual Behavior

If I add some reveal_type(self) and reveal_type(self.value)s before/after the narrowing, I get this output:

enum_narrowing.py:11: note: Revealed type is "enum_narrowing.Color"
enum_narrowing.py:12: note: Revealed type is "Tuple[Literal[255]?, Literal[255]?, Literal[255]?]"
enum_narrowing.py:15: note: Revealed type is "Literal[enum_narrowing.Color.BLACK]"
enum_narrowing.py:16: note: Revealed type is "Any"

Your Environment

$ python --version
Python 3.8.0
$ mypy --version
mypy 0.991 (compiled: yes)

まず、提供されている mypy コマンドを enum_narrowing.py に対して実行し、is による絞り込みの前後で明示された型を比較します。Enum メンバーの絞り込みと .value の型処理を追跡します。self.value[0] が int として推論され、--warn-return-any を付けて例が通れば修正は完了です。