I always believed the difference between unspecified and undefined behavior was specifically tuned so that unspecified behavior was fine to use un-portably, whe

I always believed the difference between unspecified and undefined behavior was specifically tuned so that unspecified behavior was fine to use un-portably, whereas undefined behavior was never meant to be used. Hence the wording of the standard beinh carefully chosen. Of course, this makes it surprising that relying on 2s complement is undefined rather than unspecified. As well as other examples like left shifting 2^31. From this story, it sounds like the distinction between undefined and unspecified behavior is newer than I thought, and perhaps more about optimization from the beginning?