I’m not sure I agree. If signed overflow is UB, loops like this can be optimized the hell out of. The most obvious way would be to unroll it and eliminate the l

I’m not sure I agree. If signed overflow is UB, loops like this can be optimized the hell out of. The most obvious way would be to unroll it and eliminate the loop (and loop variable) entirely, but you can also do things like vectorize it, maybe turn it in to just a small number of SIMD instructions. The performance gains are potentially enormous if this is in a hot loop. With your magic int that traps on overflow, you couldn’t do that if the compiler was forced to rely on that behaviour. This is exactly why signed overflow is UB in C, and I don’t think that’s an unreasonable case for a language like C. To be clear, my point is that this program is incorrect if overflow happens regardless of whether overflow is UB or not. So you might as well make it UB and optimize the hell out of it.