Not quite what I was getting at: On an implementation with 32-bit ints, the code is valid – the values get promoted to 32 bit, added and then truncated to 16 bit. Yet on a platform with 16-bit ints (and microchips & unusual platforms is a frequently stated reason for using C), the addition overflows and result in UB.

Luckily most other languages haven't decided to copy C's implicit promotion rules & target-dependant integer sizes.