Is it possible to create a trait to answer if a type comes from std?

Question

After this question by utilizing ADL one can create a trait to answer if the passed type comes from our namespace:

#include 

namespace helper         


        

Answer 1

This seems to work:

#include 
#include 
#include 
#include 

namespace other { struct S{}; }

namespace my {
    template< class Type >
    void ref( Type&& ) {}

    template< class Type >
    auto ref_to( Type&& o )
        -> Type&
    { return o; }

    template< class Type >
    constexpr auto is_std_type()
        -> bool
    {
        using std::is_same;
        using std::declval;
        return not is_same< void, decltype( ref( ref_to( declval() ) ) )>::value;
    }

    struct Blah {};

    constexpr bool int_is_std       = is_std_type();
    constexpr bool blah_is_std      = is_std_type();
    constexpr bool other_is_std     = is_std_type();
    constexpr bool string_is_std    = is_std_type();
};

#include 
using namespace std;
auto main()
    -> int
{
    cout << boolalpha;
    cout << "int is std = " << my::int_is_std << "\n";
    cout << "blah is std = " << my::blah_is_std << "\n";
    cout << "other is std = " << my::other_is_std << "\n";
    cout << "string is std = " << my::string_is_std << "\n";
}