The compiler could do those things too (-Wsign-compare is a thing; nothing for signed index though), but that won't change the existing code that uses "for (int ..." (incl. 637K C source files on github), and those warnings could be false positives too (e.g. signed indexes into lookup tables, or keeping using an int32_t after having asserted somewhere before (possibly out of the compiler's vision) that it's non-negative).