[quetzal-china]

Fixes #22273

obj.prop += x evaluates as tmp = obj.prop (getter type), tmp = tmp.__iadd__(x) (or tmp = tmp + x), then obj.prop = tmp (checked against the setter type). Mypy resolved the operator from the lvalue type, which for a member lvalue is the setter type:

• a setter type that does not implement the in-place method silently fell back to plain + on the getter type — Unsupported operand types for + even though the getter defines __iadd__
• a union setter skipped the in-place path entirely (the in-place probe only handles Instance/TypedDictType), which is the exact repro from the issue
• descriptors whose __set__ value type differs from their __get__ return type hit the same problem

This resolves the operator on the getter type and keeps the setter type for the read-only/final diagnostics and for the final Result type of + incompatible in assignment check.

Test cases in check-statements.test:

• testInplaceOperatorAsymmetricProperty — the issue repro, now clean
• testInplaceOperatorAsymmetricPropertyBadResult — a __iadd__ result not assignable to the setter still errors
• testInplaceOperatorReadOnlyProperty — read-only property diagnostics preserved

The affected check suites pass on this Windows machine with the same six pre-existing stub-related failures as an unmodified checkout; self-check on mypy/checker.py is clean.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅