regression in 1.12.0: un-suppressable error with no line number when subtyping dataclass when field has incompatible subtype

[DetachHead]

from dataclasses import dataclass


@dataclass
class Foo:
    a: object


@dataclass
class Bar(Foo):
    a: int

main.py: error: Argument 1 of "__replace__" is incompatible with supertype "Foo"; supertype defines the argument type as "object"  [override]
main.py: note: This violates the Liskov substitution principle
main.py: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
Found 1 error in 1 file (checked 1 source file)

playground

this error has no line number and therefore cannot be ignored. this is a big problem because not being able to type:ignore the error is preventing us from updating mypy

the error also occurs when the dataclass is frozen, in which case the error is a false positive here because the type of the field should be considered covariant:

from dataclasses import dataclass


@dataclass(frozen=True)
class Foo:
    a: object


@dataclass(frozen=True)
class Bar(Foo):
    a: int

(you could argue that the error is technically still correct because the generated __replace__ method takes the constructor arguments in a contravariant position, but dataclasses.replace is completely untyped anyway, so this additional safety has no benefit).

related issue on pyright: microsoft/pyright#9351

[brianschubert]

This error seems to only occur when running mypy with Python 3.13. Checking the same code with Python 3.12 produces no error for me (both locally and on the playground).

edit: this only occurring in Python 3.13 seems expected. The dataclass plugin only adds a __replace__ method to dataclass definitions when checking 3.13+:

mypy/mypy/plugins/dataclasses.py

Lines 388 to 389 in 78fb78b

	if self._api.options.python_version >= (3, 13):
	self._add_dunder_replace(attributes)

[JukkaL]

Fixed by #18122.