code.c:
#define n 5

#define add(x, y) ((x) + (y) >= n ? ((x) + (y) - n) : ((x) + (y)))
#define sum(x, y) add(add(u[x], v[x]), add(u[y], v[y]))

int main(void){
 int x, y, z;
 unsigned short u[n], v[n];

 // With
 // unsigned int u[n], v[n];
 // there is no problem.

 return sum(x, sum(y, z)) != sum(sum(x, y), sum(x, z)) ||
         sum(sum(x, y), z) != sum(sum(x, z), sum(y, z)) ||
         sum(x, y) != sum(y, x);
}

dmc code.c -o
Internal error: gflow 386
--- errorlevel 1

Without -o option it compiles fine.
Tested with DMC v8.49.

Thanks,
Brian