Interaction between an `enum` alias and `assert_never()` seems to fail

[jhbuhrman]

Bug Report

If you have defined an enum with some values and an extra value which is an alias of an already defined one, the completeness-check using assert_never(...) does not work as (I) expected.

To Reproduce

# file mypy_enum_alias.py
import enum

from typing import assert_never

class Version(enum.Enum):
    V1 = enum.auto()
    V2 = enum.auto()
    DEFAULT = V2

def print_version(version: Version) -> None:
    match version:
        case Version.V1:
            print("Version 1")
        case Version.V2:
            print("Version 2")
        case _:
            assert_never(version)

print(*(repr(v) for v in Version))

$ mypy mypy_enum_alias.py

Expected Behavior

Success: no issues found in 1 source file

Actual Behavior

mypy_enum_alias.py:18: error: Argument 1 to "assert_never" has incompatible type "Literal[Version.DEFAULT]"; expected "Never"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.19.1 (compiled: yes)
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files):

  # pyproject.toml
  # ...
  [tool.mypy]
  plugins = [
      "pydantic.mypy"
  ]
  # ...

• Python version used: Python 3.13.9

If you run the script, Python bravely emits only the (two) non-aliased values:

$ python mypy_enum_alias.py
<Version.V1: 1> <Version.V2: 2>

If mypy considers all "names" of an enum, it renders the completeness check useless IMHO.

I could add the following code snippet just before case _: ... as a work-around, but that would just be pathetic:

        case Version.DEFAULT:
            pass

But for the rest I'm quite happy with mypy. A great helper in early bug detection!

[jhbuhrman]

I'm not sure whether it relates to #8657, I guessed not...

[A5rocks]

Yeah I think this should work:

>>> import enum
>>> class Version(enum.Enum):
...     V1 = enum.auto()
...     V2 = enum.auto()
...     DEFAULT = V2
...
>>> Version.V2 == Version.DEFAULT
True

[jhbuhrman]

Yeah I think this should work:

>>> import enum
>>> class Version(enum.Enum):
...     V1 = enum.auto()
...     V2 = enum.auto()
...     DEFAULT = V2
...
>>> Version.V2 == Version.DEFAULT
True

Sure! Even more so, these are aliases to the same object:

>>> ...
>>> print(f"{(Version.V2 is Version.DEFAULT)=}")
(Version.V2 is Version.DEFAULT)=True