As a enhancement to the -Wunused-value which may allready detected by the data-flow analysis.
int problem_funktion(int a)
{
   int b = 0; // This should cause a warning as the value 0 is never used
   if (__builtin_expect(((a > 0) && ((b = 5) != 0)), 1))
   {
      return(a*b);
   }
   return(a);
} 
Compiling with -Wunused-value imo it should give me a warning about assigning a unused value to b.