Comment by wizzwizz4

If the buggy implementation relies indirectly on the assumption that 2^n - 1 is composite, by performing a calculation that's only valid for composite values on a prime value, there won't be a separate path for the failing case. If the Mersenne numbers don't affect flow control in a special way in either implementation, there's no reason for the path coverage heuristic to produce a case that distinguishes the implementations.