[1.16 regression] determining exhaustive match on an enum with type members

[delfick]

Hi,

I continued looking at how the codebase I work on responds to the upcoming mypy 1.16 release and I believe I found another bug

import datetime
import enum
from typing import assert_never


class ValueType(enum.Enum):
    BOOLEAN = bool
    DATE = datetime.date
    DATETIME = datetime.datetime


value_type: ValueType = ValueType.BOOLEAN

match value_type:
    case ValueType.BOOLEAN:
        pass
    case ValueType.DATE:
        pass
    case ValueType.DATETIME:
        pass
    case _:
        assert_never(value_type)

On mypy 1.16 I get this error

a.py:22: error: Argument 1 to "assert_never" has incompatible type "ValueType"; expected "Never"  [arg-type]

As far as I can tell this should be correct and I'm unsure why it doesn't think I have an exhaustive match.

Your Environment

• Mypy version used: release-1.16 (revision 96525a2, built locally with mypy_mypyc-wheels for cp312-macosx_arm64)
• Mypy command-line flags: mypy a.py
• Mypy configuration options from mypy.ini (and other config files): no config
• Python version used: python3.12

[hauntsaninja]

Thanks for testing pre-release. Bisects to #18675 cc @sobolevn

[vidhyavijayan3]

Hi @delfick @hauntsaninja

Can I work on this issue ?

[hauntsaninja]

Sure, please do. In general, don't need to ask, just put up PRs!

Obviously most principled thing to do is understand what enum actually does at runtime to differentiate the lambda case #18675 was thinking about. An ad hoc thing could be to special case type object function likes

[vidhyavijayan3]

Thanks for the go-ahead, @hauntsaninja! I’ll start digging into the issue and put up a PR as soon as I have a fix. I’ll also take a closer look at how enums behave at runtime to make sure the solution is solid. Appreciate the guidance!

[erictraut]

@vidhyavijayan3, there's one gotcha to keep in mind when you implement this. In general, enum types can be expanded into their individual literal member types. For example, the type ValueType is equivalent to Literal[ValueType.BOOLEAN, ValueType.DATE, ValueType.DATETIME] in the code sample above. This equivalency allows you to eliminate subtypes from the literal union based on pattern matches. However, this expansion doesn't work for classes that derive from enum.Flag, so this needs to be special-cased. For more details, refer to this section of the typing spec.

[vidhyavijayan3]

Thank you very much @erictraut for pointing that out. I understand the difference between enum and enum.Flag and the need to handle enum.Flag as a special case. I will review the relevant section of the typing specification to ensure I fully grasp the details before implementing the fix. I appreciate your helpful guidance!

[delfick]

I believe I found another example of this bug, and it seems to be more than just about enums specifically

So if we have two files

# a.py
from typing import assert_never

import b


def switch[T_Choice: b.One | b.Two](choice: type[T_Choice]) -> None:
    match choice:
        case b.One:
            print(1)
        case b.Two:
            print(2)
        case _:
            assert_never(choice)


switch(b.One)

and

# b.py
class One:
    pass


class Two:
    pass

Then I get no errors with mypy 1.15 and this error with 1.16

a.py:13: error: Argument 1 to "assert_never" has incompatible type "type[Two] | Never"; expected "Never"  [arg-type]

[hauntsaninja]

Hmm that one bisects to #18972 , opened a new issue at #19159 for that!