How to use std::pair as key in a std::map?

Question

How can I make the following code working? I get error 'no match for operator[]`. This is a situation in my application, appearantly there are ways to workaround but I would like to know if this is at all supported.

#include <unordered_map>
#include <utility>

std::pair<int, int> mypair(int a, int b)
{
    return std::pair<int, int>(a, b);
}

int main()
{
    std::unordered_map<std::pair<int, int>, int> mymap;
    mymap[mypair(2, 3)] = 5;
    return 0;
}