c++ - Recursion using template meta programming

Question

Can somebody explain to me, why the first call using the template function is falling into an infinite loop, while the second compile-time function runs correctly?

#include <iostream>
using namespace std;

template<int N, int M>
struct commondivs {                                              
  static const int val = (N<M) ? commondivs<N,(M-N)>::val : commondivs<(N-M),M>::val;
};

template<int N>
struct commondivs<N,N> {
  static const int val = N;
};


int commondiv(int N, int M){
    if(N==M){
        return N;
    }   
    return (N<M)?commondiv(N,(M-N)):commondiv((N-M),M);     
}

int main() {

    cout << commondivs<9,6>::val << endl;
    cout << commondiv(9,6) << endl;
    return 0;
}

question from:https://stackoverflow.com/questions/59034713/recursion-using-template-meta-programming

Answer 1

(N<M) ? commondivs<N,(M-N)>::val : commondivs<(N-M),M>::val

This line causes instantiation of both commondivs<N,(M-N)>::val and commondivs<(N-M),M>::val, even if the condition is known at compile time and one of the branches will never be taken.

Replace ? : with std::conditional_t, which doesn't have this limitation:

static const int val = std::conditional_t<N < M, commondivs<N,(M-N)>, commondivs<(N-M),M>>::val;