is_convertible, C++ (clang)

is_convertible

//clang 3.8.0

#include <iostream>
#include <type_traits>
#include <utility>

template<typename...>
struct or_;

template<>
struct or_<>
    : public std::false_type
{ };

template<typename _B1>
struct or_<_B1>
    : public _B1
{ };

template<typename _B1, typename _B2>
struct or_<_B1, _B2>
    : public std::conditional<_B1::value, _B1, _B2>::type
{ };

template<typename _B1, typename _B2, typename _B3>
struct or_<_B1, _B2, _B3>
    : public std::conditional<_B1::value, _B1, or_<_B2, _B3>>::type
{ };

typedef char (& yes_type)[2];
typedef char (& no_type )[1];

template <typename U>
U& declval();

namespace detail
{
    template <typename Tp1, typename Tp2>
    struct is_convertible_helper
    {
    private:
        template <typename U>
        static yes_type is_convertible_check(U const &);
        template <typename U>
        static no_type  is_convertible_check(...);

public:
        enum
        {
            value = sizeof( is_convertible_check<Tp2>(declval<Tp1>()) ) - 1
        };
    };
    template <typename Tp>
    struct is_convertible_helper<Tp, Tp>
    {
   public:
        enum
        {
            value = 1
        };
    };
} // detail
                                          
template <typename Tp1, typename Tp2>
struct is_convertible
    : std::integral_constant<bool, detail::is_convertible_helper<Tp1, Tp2>::value>
{ };

class C
{
    C() = default;
};

int main()
{
    std::cout << is_convertible<C, C>::value;
}


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