Strange function look up

When trying to get to grips with std::ostream_iterator I have come up with the following code which doesn't compile (under gcc 5.3 or clang 3.6).

#include <iostream>
#include <iterator>

namespace temp {
struct Point {
    int x;
};
}

//namespace temp {
//namespace std {
std::ostream& operator<<(std::ostream& s, temp::Point p) {
    return s << p.x;
}
//}

int main(int argc, char** argv) {
    temp::Point p{1};
    std::ostream_iterator{std::cout} = p;
    //std::cout << p;
    std::cout << std::endl;

    return 0;
}

When operator<< is in global scope, compilation throws up a host of template instantiation errors.

However, std::cout << p works fine. And, if operator<< is declare in namespace temp or namespace std, the code compiles and runs as one would expect.

My question is why does a global operator<< not work?