Exhaustiveness checks (assert_never) don't work with 2-tuple of enums
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Description
Bug Report
The code below fails to type check, but should succeed because it's exhaustive.
There are some possibly related issues, but I don't know if they are all the same bug, the other tickets tend to depend on Union which this does not: #16650 #15426 #12364
To Reproduce
Playground link: https://mypy-play.net/?mypy=latest&python=3.12&gist=0d21d288fce8aa13d3cc60f4e418960a
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from enum import Enum
from typing import assert_never
class A(Enum):
A0 = 0
A1 = 1
class B(Enum):
B0 = 0
B1 = 1
def test(a: A, b: B) -> bool:
match (a, b):
case (A.A0, B.B0):
return True
case (A.A0, B.B1):
return True
case (A.A1, B.B0):
return True
case (A.A1, B.B1):
return True
case _ as never:
assert_never(never)
return False
Actual Behavior
typecheck_test.py:26: error: Argument 1 to "assert_never" has incompatible type "tuple[A, B]"; expected "NoReturn" [arg-type]
Your Environment
- Mypy version used: 1.8.0
- Mypy command-line flags: n/a
- Mypy configuration options from
mypy.ini(and other config files): n/a - Python version used: 3.11 (but also happens with 3.12 in playground link)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the supplied two-enum tuple example in the linked mypy playground or a local mypy checkout, then inspect the exhaustiveness-checking path for structural pattern matching and assert_never. Done means the example type-checks without the incompatible tuple[A, B] error while preserving the existing behavior for non-exhaustive matches.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100