Warning for different pointer signedness

[Joe Buck]

On Mon, Dec 20, 2004 at 10:21:38PM +0100, Gabriel Dos Reis wrote:
> Pointers-to-object comparaison make sense (from the relevant standards
> point of view) only if they
> 
>   (i)   point into the same object; or
>   (ii)  are (or one of them is) one-past-the-end of the same object; or
>   (iii) are (or one of them is) null.
> 
> You then realize that if the pointers are not of the same type or cannot be
> implicitly converted to each other, then the comparison become
> invalid.

But there is the interesting fact that pointers that differ only in
the signed-ness of the pointed-to object can alias each other.  That
seems to imply that testing for equality makes sense, even without casting.