why i use of deleted function 'void std::ref(const _Tp&&) [with _Tp = int]'

#include<iostream>
#include<utility>
#include<tuple>
#include<functional>
using namespace std;
int main()
{
    int i = 0;
    auto p = make_pair(ref(i), ref(i++));
    p.first++;
    p.second++;
    cout << "i = " << i << endl;
}

for example if i use ref() like this, the compiler will say use of deleted function 'void std::ref(const _Tp&&) [with _Tp = int]' however if my code is following #include<iostream> #include<utility> #include<tuple> #include<functional> using namespace std; int main() { int i = 0; auto p = make_pair(ref(i), ref(++i)); p.first++; p.second++; cout << "i = " << i << endl; } I will successfully get the output "i = 3" so I can't understand why I get so different answers