if ((int) ((unsigned) a + (unsigned) b) < 0 )
        complain();

if (a^b < 0) overflow=0; /* opposite signs can't overflow */
else if (a>0) overflow=(b>INT_MAX-a);
else overflow=(b<INT_MIN-a);

overflow = (a+b<a);

-4,294,967,295 < sum < 4,294,967,295

4,294,967,295 + 1 = -4,294,967,295

Do the addition normally, then check the result (e.g. if (a+23<23) overflow).