Fixed returned nullptr typo and removed lambda return type declaration

Author: pawel.wojciechowski

include/graphqlservice/GraphQLService.h

@@ -506,6 +506,16 @@ struct is_union<Union<Types...>> : std::true_type
 {
 };
 
+// helper type for the visitor #4
+template <class... Ts>
+struct overloaded : Ts...
+{
+	using Ts::operator()...;
+};
+// explicit deduction guide (not needed as of C++20)
+template <class... Ts>
+overloaded(Ts...) -> overloaded<Ts...>;
+
 template <typename T>
 struct GraphQLBuilder
 {
@@ -552,14 +562,19 @@ struct GraphQLBuilder
 			if (u)
 				return std::visit(
 					[]<typename V>(V&& arg) {
-						if constexpr(std::is_same_v<typename std::remove_reference_t<U>::element_type,std::monostate>){
+						using model_t =
+							typename GraphQLUnion<typename std::remove_reference_t<U>::element_type,
+								typename T::element_type>::model_map::
+								find<typename std::remove_reference_t<V>::element_type>;
+						if constexpr (std::is_same_v<model_t, std::monostate>)
+						{
 							throw std::logic_error("Unsupported variant type");
-						}else{
+							return std::shared_ptr<typename T::element_type>();
+						}
+						else
+						{
 							return GraphQLBuilder<T>::build(
-								GraphQLBuilder<std::shared_ptr<typename GraphQLUnion<
-									typename std::remove_reference_t<U>::element_type,
-									typename T::element_type>::model_map::find<typename std::
-										remove_reference_t<V>::element_type>>>::build(std::move(arg)));
+								GraphQLBuilder<std::shared_ptr<model_t>>::build(std::move(arg)));
 						}
 					},
 					std::move(u->value));