One of the tricky things is that undefined behavior is commonly understood as having an unlimited "blast radius" and implementation-defined behavior is commonly understood as not permitting that kind of "blast radius." So any extension point that needed that unlimited "blast radius" had to be undefined behavior. (This has changed somewhat in modern times, but it does hold a little bit today--I'd point you to the discussion of N3863 at the most recent meeting, but I don't have any minutes that I feel really captured that discussion well enough.)
And one of those kinds of "blast radius" is traps, and I suspect the original motivation for making signed integer overflow undefined behavior was precisely to preserve the ability to trap on signed integer overflow, which couldn't be implementation-defined. But the records of the discussions leading up to C89 and C99 are just too fragmentary for me to be certain.
And one of those kinds of "blast radius" is traps, and I suspect the original motivation for making signed integer overflow undefined behavior was precisely to preserve the ability to trap on signed integer overflow, which couldn't be implementation-defined. But the records of the discussions leading up to C89 and C99 are just too fragmentary for me to be certain.