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Match exhaustion fails incorrectly when matching same option twice. #17372
Description
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
Activity
hauntsaninja commentedon Jan 28, 2026
The enum is mistakenly defined. If you use
gold = auto()(instead of the incorrectgold: autothat 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)Improve reachability in narrowing logic (#20660)