I was wrong.

I was looking at this:

https://en.cppreference.com/cpp/language/eval_order

I'm not sure where precisely this sequencing exception to the default "eval order undefined" rule is given, but after the 24(!) sequencing rules they do give this "++i + i++" as an explicit example of undefined behavior.

Interestingly that page says that since C++17 f(++i, ++i) is "unspecified" rather than "undefined", whatever that means, and presumably plus(++i, i++) would be too, which seems a bit inconsistent.