How is an abs function that can return a negative value not a flat-out bug‽ Seriously? It may still not be cool to end up with true_abs(MIN_INT) - 1 rather than the "real" value, but at least it won't be negative. That's really bad.
Either way, you're going to have the incorrect number. When debugging, I'd rather have something that is very incorrect rather than having a mysterious off-by-one error.
Edit: The C99 standard acknowledges that abs(INT_MIN) isn't representable, so using abs(INT_MIN) is undefined.
