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#include <iostream>
 
namespace implementation
{
	namespace win
	{
		class A{};
	}
	namespace posix
	{
		class A{};
	}
}
 
namespace ns
{
	#ifdef _WIN32
		using A = implementation::win::A;
	#else
		using A = implementation::posix::A;
	#endif
 
	// dump ns::A to any stream
	template<class OStream>
	OStream& operator << (OStream& os, const A& a)
	{
		os << "A";	// fake
		return os;
	}
}
 
int main() 
{
	ns::A a;
	std::cout << a;
	return 0;
}

prog.cpp: In function 'int main()':
prog.cpp:35:15: error: cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'
  std::cout << a;
               ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from prog.cpp:1:
/usr/include/c++/4.9/ostream:602:5: note: initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = implementation::posix::A]'
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^

Standard output is empty