warning: operation on 'zero' may be undefined

[Robert Dewar]

It is confusing to describe this problem in terms of ambiguity, or
from the point of view of where the increment can occur. The point
is that the semantics is undefined. Period. You can then ask why
was this made undefined, and the answer is that it is a simple
general rule that captures all the real nasty cases, and so what
if it captures some non-nasty cases.