**Bug Report** This is a follow-on from https://github.com/python/typeshed/issues/7739. See there for additional context provided by @srittau and @AlexWaygood. **To Reproduce** ```python from __future__ import annotations from functools import partialmethod class A: def my_add(self, other: A, reverse: bool) -> A: return self __add__ = partialmethod(my_add, reverse=False) a = A() aa = A() aaa = A() result = sum([aa, a], start=aaa) ``` **Expected Behavior** The above seems like a legitimate way to define `__add__`. **Actual Behavior** ``` $ mypy --version mypy 0.950 (compiled: yes) $ mypy --show-error-codes --strict mypy_bug_report.py mypy_bug_report.py:15: error: No overload variant of "sum" matches argument types "List[A]", "A" [call-overload] mypy_bug_report.py:15: note: Possible overload variants: mypy_bug_report.py:15: note: def [_SumT <: _SupportsSum] sum(Iterable[_SumT]) -> Union[_SumT, Literal[0]] mypy_bug_report.py:15: note: def [_SumT <: _SupportsSum, _SumS <: _SupportsSum] sum(Iterable[_SumT], start: _SumS) -> Union[_SumT, _SumS] Found 1 error in 1 file (checked 1 source file) ``` mypy 1.11.1's output is essentially unchanged. **Your Environment** <!-- Include as many relevant details about the environment you experienced the bug in --> - Mypy version used: 0.950, 1.11.1 - Mypy command-line flags: none, or as above - Mypy configuration options from `mypy.ini` (and other config files): none - Python version used: 3.12.5, from Debian cc @kaushikcfd