/in/foo.cc: In function 'int main()':
/in/foo.cc:9:44: warning: iteration 999 invokes undefined behavior [-Waggressive-loop-optimizations]
  for(int i = 1; i <= 1000; i++)a[i+1] = a[i] + 2;
                                         ~~~^
/in/foo.cc:9:19: note: within this loop
  for(int i = 1; i <= 1000; i++)a[i+1] = a[i] + 2;
                 ~~^~~~~~~