add restrict for variant

iguana/json_reader.hpp

@@ -512,13 +512,11 @@ IGUANA_INLINE bool from_json_variant_impl(U &value, It it, It end, It &temp_it,
   }
 }
 
-template <size_t Idx, typename T>
-using variant_element_t = std::remove_reference_t<decltype(std::get<Idx>(
-    std::declval<std::remove_reference_t<T>>()))>;
-
 template <typename U, typename It, size_t... Idx>
 IGUANA_INLINE void from_json_variant(U &value, It &it, It &end,
                                      std::index_sequence<Idx...>) {
+  static_assert(!has_same_variant_type_v<std::remove_reference_t<U>>,
+                "don't allow same type in std::variant");
   bool r = false;
   It temp_it = it;
   It temp_end = end;

iguana/json_writer.hpp

@@ -245,6 +245,8 @@ IGUANA_INLINE void to_json_impl(Stream &s, T &&t) {
 template <bool Is_writing_escape, typename Stream, typename T,
           std::enable_if_t<variant_v<T>, int>>
 IGUANA_INLINE void to_json_impl(Stream &s, T &&t) {
+  static_assert(!has_same_variant_type_v<std::remove_reference_t<T>>,
+                "don't allow same type in std::variant");
   std::visit(
       [&s](auto value) {
         to_json_impl<Is_writing_escape>(s, value);

iguana/util.hpp

@@ -139,6 +139,20 @@ struct is_variant<std::variant<T...>> : std::true_type {};
 template <typename T>
 constexpr inline bool variant_v = is_variant<std::remove_cvref_t<T>>::value;
 
+template <typename T>
+struct has_same_variant_type : std::false_type {};
+
+template <typename T, typename... Us>
+struct has_same_variant_type<std::variant<T, Us...>>
+    : std::disjunction<std::is_same<T, Us>...> {};
+
+template <typename T>
+constexpr bool has_same_variant_type_v = has_same_variant_type<T>::value;
+
+template <size_t Idx, typename T>
+using variant_element_t = std::remove_reference_t<decltype(std::get<Idx>(
+    std::declval<std::remove_reference_t<T>>()))>;
+
 template <typename T>
 constexpr inline bool refletable_v = is_reflection_v<std::remove_cvref_t<T>>;