Match exhaustion fails incorrectly when matching same option twice.

[superosku]

Mypy shows an incorrect error when the same option is twice in an match statement.

To Reproduce

from typing import assert_never
from enum import Enum, auto


class Medal(Enum):
 gold: auto
 silver: auto
 bronze: auto


def fail_1(medal: Medal) -> None:
 match medal:
 case Medal.gold:
 pass
 case Medal.silver | Medal.silver:
 pass
 case Medal.bronze:
 pass
 case _:
 assert_never(medal) # is "Literal[Medal.silver]" should be NoReturn


def fail_2(medal: Medal) -> None:
 match medal:
 case Medal.gold:
 pass
 case Medal.silver | Medal.silver:
 pass
 case _:
 assert_never(medal) # is "Literal[Medal.silver]" should be "Literal[Medal.bronze]"

Expected Behavior

See the comments in the example python code.

It should either fail on case Medal.silver | Medal.silver: or fail_1 should assert never properly and fail_2 should show Medal.bronze instead of Medal.silver

Actual Behavior

See the python example.

Your Environment

Mypy version used:

 $ mypy --version
mypy 1.10.0 (compiled: yes)

Python version used:

 $ python --version
Python 3.12.2

Mypy command-line flags: none

Mypy configuration options from mypy.ini (and other config files): none

[hauntsaninja]

The enum is mistakenly defined. If you use gold = auto() (instead of the incorrect gold: auto that doesn't work at runtime), mypy does the expected thing.

Future versions of mypy will highlight this error better if you use --warn-unreachable (I'm improving this in #20660)