You would'n come into a situation to divide by ex. 9 or 14, cause you would've already divided the number by their prime decomposers 2, 3 and 7. Thus no need to check for primality.

board is unsigned int. what happened when unsigned int 0 minus by one?

hint: x|=1 << (board[row][col] -1); is not shifting left by -1. it's something else

The for loop starts from the first prime number (2) and if n would be divisible by non-prime i, it also means that it is divisible by i prime factors. So n would be reduced (n /= i) in earlier loop steps, n % i == 0 can be true only for prime i because of it.

Yes, ease on the eyes

Can never overuse templates. 😉😂