It looks like the code
i.operator+=(i.operator ++());
Works perfectly fine with regards to sequence points. Section 1.9.17 of the C++ ISO standard says this about sequence points and function evaluation:
When calling a function (whether or not the function is inline), there is a sequence point after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body. There is also a sequence point after the copying of a returned value and before the execution of any expressions outside the function.
This would indicate, for example, that the i.operator ++() as the parameter to operator += has a sequence point after its evaluation. In short, because overloaded operators are functions, the normal sequencing rules apply.
Great question, by the way! I really like how you’re forcing me to understand all the nuances of a language that I already thought I knew (and thought that I thought that I knew). 🙂