alt Hacker NewsNo, the behavior is undefined. That means, quoting the ISO C standard, "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirements".
A conforming implementation could reject it at compile time, or generate code that traps, or generate code that set a to 137, or, in principle, generate code that reformats your hard drive. Some of these behaviors are unlikely, but none are forbidden by the language standard.
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.