Detect attempts to override prohibited NamedTuple attributes #1684
Description
For example:
>>> from typing import NamedTuple
>>> class F(NamedTuple):
... _asdict = 42
...
Traceback (most recent call last):
File "<python-input-5>", line 1, in <module>
class F(NamedTuple):
_asdict = 42
File "/Users/alexw/Library/Application Support/uv/python/cpython-3.14.0-macos-aarch64-none/lib/python3.14/typing.py", line 3004, in __new__
raise AttributeError("Cannot overwrite NamedTuple attribute " + key)
AttributeError: Cannot overwrite NamedTuple attribute _asdictThis is separate to the check being added in astral-sh/ruff#21697 because _asdict here is not considered a NamedTuple field either by ty or at runtime (it doesn't have a type annotation). Most assignments in NamedTuple class bodies work fine, even if the assigned symbol has a leading underscore, as long as they don't have type annotations:
>>> from typing import NamedTuple
>>> class G(NamedTuple):
... _whatever = 42
...
>>> _asdict, however, is a synthesized method present on all NamedTuple classes, and users are not allowed to override it. The full list of prohibited overrides is at https://github.com/python/cpython/blob/77399436bfc87663f9058b749b6cb598bab273f9/Lib/typing.py#L2939-L2942.
Mypy detects this, though pyright and pyrefly do not at time of writing.