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justin 938c1c5e12 [ty] diagnostic on overridden __setattr__ and __delattr__ in frozen dataclasses (#21430 )

## Summary
https://github.com/astral-sh/ty/issues/111

this pr adds an `invalid-dataclass-override` diagnostic when a custom
`__setattr__` or `__delattr__` is defined on a dataclass where
`frozen=True`
([docs](https://docs.python.org/3/library/dataclasses.html#frozen-instances))

### Runtime exception
```
Traceback (most recent call last):
  File "/Users/justinchapman/src/ty-playground/main.py", line 4, in <module>
    @dataclass(frozen=True)
     ~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/justinchapman/.local/share/uv/python/cpython-3.13.0-macos-aarch64-none/lib/python3.13/dataclasses.py", line 1295, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
                          frozen, match_args, kw_only, slots,
                          weakref_slot)
  File "/Users/justinchapman/.local/share/uv/python/cpython-3.13.0-macos-aarch64-none/lib/python3.13/dataclasses.py", line 1157, in _process_class
    func_builder.add_fns_to_class(cls)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/Users/justinchapman/.local/share/uv/python/cpython-3.13.0-macos-aarch64-none/lib/python3.13/dataclasses.py", line 516, in add_fns_to_class
    raise TypeError(error_msg)
TypeError: Cannot overwrite attribute __setattr__ in class A
```

### Diagnostic
```
error[invalid-dataclass-override]: Cannot overwrite attribute __setattr__ in class A
 --> /Users/justinchapman/src/ty-playground/main.py:6:5
  |
4 | @dataclass(frozen=True)
5 | class A:
6 |     def __setattr__(self, name: str, value: object) -> None: ...
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
info: __setattr__
info: rule `invalid-dataclass-override` is enabled by default

Found 1 diagnostic
```

## Test Plan
- new mdtests
- e2e
- the `attrs` mypy primer diff looks to be a [true
positive](https://github.com/python-attrs/attrs/blob/main/tests/test_setattr.py#L373)
- the other results have been unpredictable and have changed every time
i pushed new code, even if the diagnostic logic didn't change...

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

2026-01-17 02:24:39 +00:00