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#include <iostream>
 
namespace ex
{
    struct Test
    {
    };
 
    std::istream &operator>>(std::istream &is, Test &t)
    {
        std::cout << "Example namespace" << std::endl;
        return is;
    }
}
 
void f(ex::Test &t);
 
int main()
{
    ex::Test t;
    std::cin >> t;
    f(t);
}
 
std::istream &operator>>(std::istream &is, ex::Test &t)
{
    std::cout << "Global namespace" << std::endl;
    return is;
}
 
void f(ex::Test &t)
{
    std::cin >> t; //error: ambiguous
}
 

prog.cpp: In function ‘void f(ex::Test&)’:
prog.cpp:33:17: error: ambiguous overload for ‘operator>>’ in ‘std::cin >> t’
prog.cpp:33:17: note: candidates are:
prog.cpp:25:15: note: std::istream& operator>>(std::istream&, ex::Test&)
In file included from /usr/include/c++/4.7/iostream:41:0,
                 from prog.cpp:1:
/usr/include/c++/4.7/istream:866:5: note: std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&&, _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = ex::Test] <near match>
/usr/include/c++/4.7/istream:866:5: note:   no known conversion for argument 1 from ‘std::istream {aka std::basic_istream<char>}’ to ‘std::basic_istream<char>&&’
prog.cpp:9:19: note: std::istream& ex::operator>>(std::istream&, ex::Test&)

Standard output is empty