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.

Because enforcing undefined behavior on this test-compiler means you can build your software on any standards-compliant compiler and have it work the same. In theory.