> let implementations define the behavior themselves That's literally the definition of implementation-defined behavior. Undefined behavior really means undef

> let implementations define the behavior themselves That's literally the definition of implementation-defined behavior. Undefined behavior really means undefined; in terms of the C language, there are no constraints on behavior. Sure, you might get a result one way on one implementation, but if you rely on that you're technically writing a dialect of C, and need to let the compiler know using flags.