Augmented assignment on asymmetric property uses setter type to resolve __iadd__ #22273

Bug Report

For an augmented assignment like obj.prop += x on a property whose setter type differs from its getter type, mypy picks and checks the in-place operator using the setter type instead of the getter type.

At runtime, obj.prop += x does:

tmp = obj.prop             # getter -> G
tmp = tmp.__iadd__(x)      # G.__iadd__ (or G.__add__ if there is no __iadd__)
obj.prop = tmp             # setter: result must be assignable to the setter type S

mypy instead looks up __iadd__ on S and calls S's __iadd__. This causes false positives when S has no __iadd__ but G does (for example, when S is a union). Descriptors whose __set__ value type differs from their __get__ return type have the same problem.

To Reproduce

from collections.abc import Iterable


class P:
    def __iadd__(self, x: int | Iterable[int]) -> "P":
        return self


class N:
    @property
    def a(self) -> P:
        return P()

    @a.setter
    def a(self, v: P) -> None: ...

    @property
    def b(self) -> P:
        return P()

    @b.setter
    def b(self, v: P | list[int]) -> None: ...


n = N()
n.a += 1  # OK
n.b += 1  # false positive

Expected Behavior

No errors. P.__iadd__(1) returns P, and P is assignable to P | list[int].

Actual Behavior

main.py:27: error: Unsupported operand types for + ("P" and "int")  [operator]
main.py:27: error: Incompatible types in assignment (expression has type "int", variable has type "P | list[int]")  [assignment]

Your Environment

• Mypy version used: 1.20.2; also reproduced on master (0cdcbfd)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.13.7