> A lot of C optimization could work that way - relying on cooperation between the compiler and programmers.

That is precisely how it works already. The reason your code has no bounds checks is exactly because the compiler can assume that you have done your part and ensured that all indices are in bounds. This is what "the compiler can ignore UB" is all about.

The signed integer kerfuffle is just the same: The compiler assumes your cooperation in ensuring, beforehand, that your signed arithmetic never overflows. Its part of the bargain is generating the best possible code it can. Another part of its bargain is offering you the -fwrapv flag to communicate more about your expectations. A third part of the bargain is offering you sanitizers that can inform you that you have done something you probably didn't want.