N3863 is one of those papers where what the paper proposed and what was actually discussed in the meeting ended up being rather different things, and it's the meeting discussion that I'm pointing to, not the paper itself.

> Wait. An implementation-defined behaviour isn't allowed to be trap/termination of execution? Okay, make it unspecified then, with the options of trapping or calculating some unspecified (not indeterminate) integer value.

Unspecified behavior also isn't allowed to be a trap. The difference between unspecified and implementation-defined is whether or not the compiler has to document its choice; the difference between unspecified and undefined is whether or not the choice is limited.