[msebor]

Some instances of undefined behavior at translation time can effectively be avoided in practice by tightening up requirements on implementations to diagnose them. But strictly speaking, because the standard allows compilers to continue to chug along even after an error and emit object code with arbitrary semantics, turning even such straightforward instances into constraint violations (i.e., diagnosable errors) doesn't prevent UB.

It might seem like defining the semantics for signed overflow would be helpful but it turns out it's not, either from a security view or for efficiency. In general, defining the behavior in cases that commonly harbor bugs is not necessarily a good way to fix them.