> And one of those kinds of "blast radius" is traps,

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.

> N3863

Hm?

    Because implementation-defined behavior relies on the definition of unspecified behavior, it
    means that implementation-defined behavior requires two or more possibilities to be defined in
    the standard. However, a brief look at its uses in the standard clearly shows that there isn’t
    always a list of potential behaviors to pick from.

    N3783 5.2.1.2p1.1: Physical source file multibyte characters are mapped, in an implementation-
    defined manner, to the source character set (introducing new-line characters for end-of-line
    indicators) if necessary.

    N3783 5.2.1.3p1: A conforming implementation shall produce at least one diagnostic message
    (identified in an implementation-defined manner)

    N3783 5.2.2.2p1-2: In a freestanding environment (in which C program execution can take
    place without any benefit of an operating system), the name and type of the function called at
    program startup are implementation-defined.

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.