From ce0db9b968fb1488f8d2d2f9c7c56b178f70dbfb Mon Sep 17 00:00:00 2001
From: Michele Caini <michele.caini@gmail.com>
Date: Mon, 12 Oct 2020 10:28:10 +0200
Subject: [PATCH] type_list: removed type_list_size[_v] in favor of
 type_list::size (breaking changes) and added operator+ for type lists

---
 src/entt/core/type_traits.hpp  | 62 ++++++----------------------------
 src/entt/entity/organizer.hpp  | 12 +++----
 test/entt/core/type_traits.cpp |  5 +--
 3 files changed, 19 insertions(+), 60 deletions(-)

diff --git a/src/entt/core/type_traits.hpp b/src/entt/core/type_traits.hpp
index 4f370aa40..d71376175 100644
--- a/src/entt/core/type_traits.hpp
+++ b/src/entt/core/type_traits.hpp
@@ -96,44 +96,16 @@ template<std::size_t N>
 inline constexpr choice_t<N> choice{};
 
 
-/*! @brief A class to use to push around lists of types, nothing more. */
-template<typename...>
-struct type_list {};
-
-
-/*! @brief Primary template isn't defined on purpose. */
-template<typename>
-struct type_list_size;
-
-
 /**
- * @brief Compile-time number of elements in a type list.
+ * @brief A class to use to push around lists of types, nothing more.
  * @tparam Type Types provided by the type list.
  */
 template<typename... Type>
-struct type_list_size<type_list<Type...>>
-        : std::integral_constant<std::size_t, sizeof...(Type)>
-{};
-
-
-/**
- * @brief Helper variable template.
- * @tparam List Type list.
- */
-template<class List>
-inline constexpr auto type_list_size_v = type_list_size<List>::value;
-
-
-/*! @brief Primary template isn't defined on purpose. */
-template<typename...>
-struct type_list_cat;
-
-
-/*! @brief Concatenates multiple type lists. */
-template<>
-struct type_list_cat<> {
-    /*! @brief A type list composed by the types of all the type lists. */
-    using type = type_list<>;
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
 };
 
 
@@ -141,24 +113,10 @@ struct type_list_cat<> {
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.
  * @tparam Other Types provided by the second type list.
- * @tparam List Other type lists, if any.
+ * @return A type list composed by the types of both the type lists.
  */
-template<typename... Type, typename... Other, typename... List>
-struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
-    /*! @brief A type list composed by the types of all the type lists. */
-    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
-};
-
-
-/**
- * @brief Concatenates multiple type lists.
- * @tparam Type Types provided by the type list.
- */
-template<typename... Type>
-struct type_list_cat<type_list<Type...>> {
-    /*! @brief A type list composed by the types of all the type lists. */
-    using type = type_list<Type...>;
-};
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) { return {}; }
 
 
 /**
@@ -166,7 +124,7 @@ struct type_list_cat<type_list<Type...>> {
  * @tparam List Type lists to concatenate.
  */
 template<typename... List>
-using type_list_cat_t = typename type_list_cat<List...>::type;
+using type_list_cat_t = decltype((type_list<>{} + ... + List{}));
 
 
 /*! @brief Primary template isn't defined on purpose. */
diff --git a/src/entt/entity/organizer.hpp b/src/entt/entity/organizer.hpp
index 4abc834cb..8f7cababb 100644
--- a/src/entt/entity/organizer.hpp
+++ b/src/entt/entity/organizer.hpp
@@ -381,8 +381,8 @@ public:
         track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
 
         vertices.push_back({
-            type_list_size_v<typename resource_type::ro>,
-            type_list_size_v<typename resource_type::rw>,
+            resource_type::ro::size,
+            resource_type::rw::size,
             name,
             nullptr,
             callback,
@@ -414,8 +414,8 @@ public:
         track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
 
         vertices.push_back({
-            type_list_size_v<typename resource_type::ro>,
-            type_list_size_v<typename resource_type::rw>,
+            resource_type::ro::size,
+            resource_type::rw::size,
             name,
             &value_or_instance,
             callback,
@@ -443,8 +443,8 @@ public:
         track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
 
         vertices.push_back({
-            type_list_size_v<typename resource_type::ro>,
-            type_list_size_v<typename resource_type::rw>,
+            resource_type::ro::size,
+            resource_type::rw::size,
             name,
             payload,
             func,
diff --git a/test/entt/core/type_traits.cpp b/test/entt/core/type_traits.cpp
index 2b76445e7..8bfe1f231 100644
--- a/test/entt/core/type_traits.cpp
+++ b/test/entt/core/type_traits.cpp
@@ -46,9 +46,10 @@ TEST(TypeTraits, TypeList) {
     using type = entt::type_list<int, char>;
     using other = entt::type_list<double>;
 
-    static_assert(entt::type_list_size_v<type> == 2u);
-    static_assert(entt::type_list_size_v<other> == 1u);
+    static_assert(type::size == 2u);
+    static_assert(other::size == 1u);
 
+    static_assert(std::is_same_v<decltype(type{} + other{}), entt::type_list<int, char, double>>);
     static_assert(std::is_same_v<entt::type_list_cat_t<type, other, type, other>, entt::type_list<int, char, double, int, char, double>>);
     static_assert(std::is_same_v<entt::type_list_cat_t<type, other>, entt::type_list<int, char, double>>);
     static_assert(std::is_same_v<entt::type_list_cat_t<type, type>, entt::type_list<int, char, int, char>>);