There is a way for this not to terminate:

  while (new_capacity < required) {
    new_capacity *= 2;
  }

1. All variables are unsigned (due to being size_t). So we don't worry about overflow UB.

2. new_capacity * 2 always produces an even number, whether truncating or not.

3. Supppose required is SIZE_MAX, the highest value of size_t; note that this is an odd number.

4. Therefore new_capacity * 2 is always < required; loop does not terminate.