message: add support for enum classes
diff: @@ -122,6 +122,17 @@ using uncvref_t = remove_cv_t<remove_reference_t<T>>;
  template <bool If, typename Then, typename Else>
  using conditional_t = typename std::conditional<If, Then, Else>::type;
  +// Taken from http://stackoverflow.com/questions/26936640/how-to-implement-is-enum-class-type-trait
  +template <typename T>
  +using is_scoped_enum =
  + std::integral_constant<bool, not std::is_convertible<T, int>::value and
  + std::is_enum<T>::value>;
  +
  +template <typename T>
  +using is_unscoped_enum =
  + std::integral_constant<bool, std::is_convertible<T, int>::value and
  + std::is_enum<T>::value>;
  +
  // TODO update this doc
  /*!
  @brief unnamed namespace with internal helper functions
  @@ -130,6 +141,10 @@ using conditional_t = typename std::conditional<If, Then, Else>::type;
  namespace detail
  {
  +// Very useful construct against boilerplate (more boilerplate needed than in C++17: http://en.cppreference.com/w/cpp/types/void_t)
  +template <typename...> struct make_void { using type = void; };
  +template <typename... Ts> using void_t = typename make_void<Ts...>::type;
  +
  // Implementation of 3 C++17 constructs: conjunction, disjunction, negation.
  // This is needed to avoid evaluating all the traits in a condition
  //
  @@ -277,6 +292,7 @@ template <typename T, typename BasicJson>
  struct is_compatible_basic_json_type
  {
  static auto constexpr value =
  + is_unscoped_enum<T>::value or
  std::is_same<T, BasicJson>::value or
  std::is_constructible<typename BasicJson::string_t, T>::value or
  std::is_same<typename BasicJson::boolean_t, T>::value or
  @@ -1601,7 +1617,6 @@ class basic_json
  not detail::is_compatible_basic_json_type<
  uncvref_t<T>, basic_json_t>::value and
  not detail::is_basic_json_nested_class<uncvref_t<T>, basic_json_t, primitive_iterator_t>::value and
  - not std::is_enum<uncvref_t<T>>::value and
  not std::is_same<uncvref_t<T>, typename basic_json_t::array_t::iterator>::value and
  not std::is_same<uncvref_t<T>, typename basic_json_t::object_t::iterator>::value and
  detail::has_to_json<JSONSerializer, basic_json,
  @@ -1774,8 +1789,8 @@ class basic_json
  @since version 1.0.0
  */
  - // Quickfix, accept every enum type, without looking if a to_json method is provided...
  - template <typename T, enable_if_t<std::is_enum<T>::value, int> = 0>
  + // Constructor for unscoped enums (not enum classes)
  + template <typename T, enable_if_t<is_unscoped_enum<T>::value, int> = 0>
  basic_json(T val) noexcept
  : m_type(value_t::number_integer),
  m_value(static_cast<number_integer_t>(val))
  