I'm trying to make my code run fine through MemorySanitizer. I'm running into a case where the look_left/right routines might call set_min() with an uninitialized i_left/i_right index. This is fine because set_min() is called with sqr_d_candidate == sqr_d, so the uninitialized index is never assigned. But changing the arg to a const & avoids reading the value from memory and thus makes MemorySanitizer happy.