Follow-up for muc++ meetup 2016-03-31

#include <type_traits>
#include <utility>

template<typename T>
struct wrong
{
  static constexpr auto value = false;
};

struct not_integral
{
  template <typename T = void>
  static void _()
  {
    static_assert(wrong<T>::value, "parameter for test has to be an integral value");
  }
};

struct ok
{
};

template<typename Check>
struct bad_statement
{
  bad_statement()
  {
    Check::_();
  }
};

template<typename Check>
using is_ok = std::enable_if_t<std::is_same<Check, ok>::value>;

template<typename Check>
using is_not_ok = std::enable_if_t<not std::is_same<Check, ok>::value>;


template <typename T>
using check = std::conditional_t<std::is_integral<T>::value,
      ok,
      bad_statement<not_integral>>;

/*
 * TEST FUNCTION AND ITS IMPLEMENTATION
 * ------------------------------------
 */
template<typename Check, typename T, typename = is_ok<Check>>
auto test_impl(const Check&, T t) -> void
{
  // Do something
  return;
};

template<typename Check, typename T, typename = is_not_ok<Check>>
auto test_impl(const Check&, T t) -> Check;

template <typename T>
auto test(T t) -> decltype(test_impl(check<T>{}, t))
{
  return test_impl(check<T>{}, t);
};


/*
 * MAIN
 * ------------------------------------
 */
int main()
{
  static_assert(std::is_same<void, decltype(test(7))>::value, "");
  static_assert(std::is_same<bad_statement<not_integral>, decltype(test(""))>::value, "");
  test("");
}