[DrJokepu]

So how about a hypothetical architecture that instead of using two's complement, stores negative numbers with a negative flag bit for some reason? Would MAX_INT+1 still be negative on such an architecture?

"We all know that in practice" type assumptions are exactly what make code difficult to port and/or maintain and developers should really stay away from them. Or at least, if you really want to make such assumptions (for example, for performance reasons), make sure that that (1) you know exactly what you're doing and (b) test for their validity at compile time or runtime and provide fallback code that always does the "right" thing.