dependent_typename.cpp

#include <iostream>


template<int N>
struct A {
    struct B {
        B(int x) : x(x) {}
        int x;
    };
};


template <int N>
std::ostream& operator<<(std::ostream& os, const typename A<N>::B& b) {
    return os << b.x;
}


int main(int argc, char const *argv[])
{
    A<0>::B b(42);
    // std::cout << b;            // does not work
    operator<<<0>(std::cout, b);  // works
    return 0;
}