From 9950b2ea348ea6ce8da5abaed4c03ced6cc85a47 Mon Sep 17 00:00:00 2001
From: Michele Caini <michele.caini@gmail.com>
Date: Tue, 15 Dec 2020 12:20:20 +0100
Subject: [PATCH] type_traits: added type_list_element[_t] and
 value_list_element[_v]

---
 src/entt/core/type_traits.hpp  | 76 ++++++++++++++++++++++++++++++++++
 test/entt/core/type_traits.cpp |  8 ++++
 2 files changed, 84 insertions(+)

diff --git a/src/entt/core/type_traits.hpp b/src/entt/core/type_traits.hpp
index 5caa6994c..635a93354 100644
--- a/src/entt/core/type_traits.hpp
+++ b/src/entt/core/type_traits.hpp
@@ -132,6 +132,44 @@ struct type_list {
 };
 
 
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename Type, typename... Other>
+struct type_list_element<Index, type_list<Type, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Type First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_element<0u, type_list<Type, Other...>> {
+    /*! @brief Searched type. */
+    using type = Type;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+
 /**
  * @brief Concatenates multiple type lists.
  * @tparam Type Types provided by the first type list.
@@ -265,6 +303,44 @@ struct value_list {
 };
 
 
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>>
+{};
+
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+
 /**
  * @brief Concatenates multiple value lists.
  * @tparam Value Values provided by the first value list.
diff --git a/test/entt/core/type_traits.cpp b/test/entt/core/type_traits.cpp
index c3c514e1a..fdf69adeb 100644
--- a/test/entt/core/type_traits.cpp
+++ b/test/entt/core/type_traits.cpp
@@ -54,6 +54,10 @@ TEST(TypeTraits, TypeList) {
     static_assert(entt::type_list_contains_v<type, int>);
     static_assert(entt::type_list_contains_v<type, char>);
     static_assert(!entt::type_list_contains_v<type, double>);
+
+    static_assert(std::is_same_v<entt::type_list_element_t<0u, type>, int>);
+    static_assert(std::is_same_v<entt::type_list_element_t<1u, type>, char>);
+    static_assert(std::is_same_v<entt::type_list_element_t<0u, other>, double>);
 }
 
 TEST(TypeTraits, ValueList) {
@@ -67,6 +71,10 @@ TEST(TypeTraits, ValueList) {
     static_assert(std::is_same_v<entt::value_list_cat_t<value, other, value, other>, entt::value_list<0, 2, 1, 0, 2, 1>>);
     static_assert(std::is_same_v<entt::value_list_cat_t<value, other>, entt::value_list<0, 2, 1>>);
     static_assert(std::is_same_v<entt::value_list_cat_t<value, value>, entt::value_list<0, 2, 0, 2>>);
+
+    static_assert(entt::value_list_element_v<0u, value> == 0);
+    static_assert(entt::value_list_element_v<1u, value> == 2);
+    static_assert(entt::value_list_element_v<0u, other> == 1);
 }
 
 TEST(TypeTraits, IsEqualityComparable) {