模板和自变量相关的查找

#include <iostream>

class Foo
{};

namespace NS
{
    class Stream
    {};

    template <typename T>
    Stream& operator << ( Stream& s, T t)
    {
        std::cerr << \"Namespace call!\\n\";
        return s;
    }
}

template<typename STREAM>
STREAM& operator << ( STREAM& s, Foo f )
{
    std::cerr << \"Global NS call\";
    return s;
}

/**
* This function (as opposed to the one above) is not ambiguous.  Why?

NS::Stream& operator << ( NS::Stream& s, Foo f )
{
    std::cerr << \"Global NS call\";
    return s;
}

*/

int main()
{
    Foo f;
    NS::Stream s;

    s << f;
    return 0;
}

test11.cpp: In function ‘int main()’:
test11.cpp:28: error: ambiguous overload for ‘operator<<’ in ‘s << f’
test11.cpp:18: note: candidates are: STREAM& operator<<(STREAM&, Foo) [with STREAM = NS::Stream]
test11.cpp:13: note:                 NS::Stream& NS::operator<<(NS::Stream&, T) [with T = Foo]