IMO both approaches would be useful. One nice thing about outlawing undefined behaviour rather than defining it, though, it that you'll be writing portable code

IMO both approaches would be useful. One nice thing about outlawing undefined behaviour rather than defining it, though, it that you'll be writing portable code - if you have a code base that's filled with all all sorts of once-was-undefined-but-now-is-defined behaviour, you can never build that code on another compiler.