Neither GCC 4.6.0 (release) and 4.7 svn (r181390) warn about the impossible comparison in

bool f(bool b)
   {
   return (b == 'a');
   }

The optimizer figures out it is always false and will optimize it to

        xorl    %eax, %eax
        ret

From the description in the manual I would have expected -Wtype-limits to detect this case.