ostream_iterator with map pairs

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I must be doing something wrong, but I expected this to compile and work:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <map>

std::ostream& operator<<(
    std::ostream& out,
    const std::pair<size_t, size_t>& rhs)
{
    return out << rhs.first << ", " << rhs.second;
}

int main()
{
    std::map<size_t, size_t> m;

    for (size_t i = 0; i < 10; ++i)
    {
        m[i] = 2U*i;
    }

    std::copy(m.begin(), m.end(),
        std::ostream_iterator<const std::pair<size_t, size_t>>(std::cout,
"\n"));

    return 0;
}

....but I get a compile error where ostream_iterator is instantiated
both on g++ 4.6.3 and VS 2013.

What am I missing?
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