The following is a simplified version of a pattern I sometimes see in my students' code:
bool foobar(int a, int b)
if (a < b) return true;
This should do the trick:
#pragma warning (error: 4715).
/we4715 command line option (see /w, /W0, /W1, /W2, /W3, /W4, /w1, /w2, /w3, /w4, /Wall, /wd, /we, /wo, /Wv, /WX (Warning Level) (courtesy of Tom Sigerdas)).