__match_args__ is not added to dataclasses with no fields

[hamdanal]

Bug Report

stubtest complains about missing __match_args__ if the dataclass has no fields

To Reproduce

Consider this python file:

from dataclasses import dataclass
from typing import ClassVar

@dataclass
class Good:
    x: ClassVar[bool]
    y: int = 0

@dataclass
class Bad:
    x: ClassVar[bool]

and the corresponding stub file

from dataclasses import dataclass
from typing import ClassVar

@dataclass
class Good:
    x: ClassVar[bool]
    y: int = ...

@dataclass
class Bad:
    x: ClassVar[bool]

Expected Behavior

Success: no issues found in 1 module

Actual Behavior

error: t.Bad.__match_args__ is not present in stub
Stub: in file /tmp/st-bug/t.pyi
MISSING
Runtime:
()

Found 1 error (checked 1 module)

Your Environment

• Mypy version used: mypy 1.4.1 (compiled: yes)
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.10.6

[AlexWaygood]

I think this manifests as a stubtest false positive, but the underlying issue is probably that mypy isn't synthesising __match_args__ attributes for dataclasses that have 0 fields

[hamdanal]

Could be from this condition

mypy/mypy/plugins/dataclasses.py

Line 368 in 14e7768

and attributes

I am not sure why there is a check for attributes here

[AlexWaygood]

Could be from this condition

Looks like it to me!

I am not sure why there is a check for attributes here

Nor me... want to make a PR? :)

[hamdanal]

Alright, I'll give it a try.

[hamdanal]

Filed #15749

Seems like the condition was added in the PR #10191 that introduced support for the match statement. This is a huge PR so I didn't follow to see if there is a reason for it or it is just an oversight :)

[AlexWaygood]

At a guess, I'd imagine it was probably just an oversight :)