Signed integer overflow undefined behavior or not?

[Ola Fosheim Grøstad via Digitalmars-d]

On Friday, 13 November 2015 at 09:09:33 UTC, Don wrote:
> (Note that unlike the unsigned case, there is no simple 
> explanation of what happens).

Well, negative overflow for unsigned probably should be illegal 
too. Ada got this right by having:

32 bit signed integers monotonic
31 bit unsigned integers monotonic

That way you can transition between unsigned and signed without 
having negative values turned into positive ones and vice versa 
and have violations detected by verifier.

In addition Ada also provides explicit modular integers in user 
specified ranges.