[1.16 regression] Crashes on a combination of dataclass inheritance, InitVar, and properties.

[kkorotkov]

Crash Report

mypy started crashing on the code below starting with version 1.16.0 — it was still working fine in 1.15.0.

Traceback

# mypy mypy-internal-error.py --show-traceback
mypy-internal-error.py:27: error: Name "value" already defined on line 22 [no-redef]
mypy-internal-error.py:27: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.17.0
Traceback (most recent call last):
 File "mypy/checker.py", line 584, in accept
 File "mypy/nodes.py", line 610, in accept
 File "mypy/checker.py", line 657, in visit_overloaded_func_def
 File "mypy/checker.py", line 739, in _visit_overloaded_func_def
 File "mypy/checker.py", line 2081, in check_method_override
 File "mypy/checker.py", line 2119, in check_method_or_accessor_override_for_base
 File "mypy/checker.py", line 2172, in check_method_override_for_base_with_name
AssertionError:
mypy-internal-error.py:27: : note: use --pdb to drop into pdb

To Reproduce

from dataclasses import InitVar, dataclass, field
from typing import Callable


@dataclass
class ClassA:
 _value: int

 @property
 def value(self) -> int:
 return self._value

 @value.setter
 def value(self, value: int):
 self._value = value


@dataclass
class ClassB(ClassA):
 getter: Callable[[], int]
 setter: Callable[[int], None]
 value: InitVar[int]

 def __post_init__(self, value: int):
 self.value = value

 @property
 def value(self) -> int:
 return self.getter()

 @value.setter
 def value(self, value: int):
 self.setter(value)

Your Environment

• Mypy version used: >=1.16.0
• Mypy command-line flags: mypy mypy-internal-error.py --show-traceback
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10
• Operating system and version: macOS 15.5

[sterliakov]

Mypy probably won't support this use case (from the type checker perspective value is indeed redefined), but crashing is too bad.

The node is deleted here:

mypy/mypy/plugins/dataclasses.py

Lines 493 to 499 in a79e85e

	for attr in attributes:
	if attr.is_init_var:
	if attr.name in info.names:
	del info.names[attr.name]
	else:
	# Nodes of superclass InitVars not used in __init__ cannot be reached.
	assert attr.is_init_var

I think such redefinitions are too rare to support correctly, so just bailing out early with "not supported" and not crashing looks like a good compromise to me.

[kkorotkov]

@sterliakov , @hauntsaninja Thanks a ton for the quick fix!