Suggested warning: "negating an expression of unsigned type does not yield a negative value"

[Jamie Lokier]

Falk Hueffner wrote:
> > The C and C++ standards require that unsigned values obey modulo 2**N
> > arithmetic, so the value of -x is rigorously defined.
> 
> Sure it is. But it is not what is intended. Example: x = 5, then
> -x=4294967291, i.e., p will be advanced by 4294967291 bytes, which is
> way beyond the legal range of p, but happens to work anyway on 32 bit
> architectures (but not on 64 bit architectures).

Good point.  That's elusive enough that I saw ptr[-x] and thought it
was was fine; I am now humbly corrected.

-- Jamie