From 9c95106af2fbaa9f5f8f605ee7df54cc90356df6 Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Tue, 31 Dec 2019 16:54:49 +0100
Subject: new_type: implement addition via nt::Arithmetic

---
 include/newtype/impl/type_traits_extensions.hpp | 66 +++++++++++++++++++++++++
 include/newtype/new_type.hpp                    | 21 ++++++++
 2 files changed, 87 insertions(+)

(limited to 'include')

diff --git a/include/newtype/impl/type_traits_extensions.hpp b/include/newtype/impl/type_traits_extensions.hpp
index ae0c7d5..d6dbfd6 100644
--- a/include/newtype/impl/type_traits_extensions.hpp
+++ b/include/newtype/impl/type_traits_extensions.hpp
@@ -513,6 +513,72 @@ namespace nt::impl
 
   }  // namespace iostreamable
 
+  inline namespace arithmetic
+  {
+
+    /**
+     * @brief A trait to test if a given type is addable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-addable T
+     */
+    template<typename T, typename = void>
+    struct is_addable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is input streamable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for addable T
+     */
+    template<typename T>
+    struct is_addable<T, std::void_t<decltype(std::declval<T const &>() + std::declval<T const &>())>> : std::true_type
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is addable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_addable_v = is_addable<T>::value;
+
+    /**
+     * @brief A trait to test if a given type is noexcept addable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-noexcept addable or non-addable T
+     */
+    template<typename T, typename = void>
+    struct is_nothrow_addable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is noexcept addable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for addable T detemining if T is noexcept addable
+     */
+    template<typename T>
+    struct is_nothrow_addable<T, std::void_t<decltype(std::declval<T const &>() + std::declval<T const &>())>>
+        : std::bool_constant<noexcept(std::declval<T const &>() + std::declval<T const &>())>
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is noexcept addable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_nothrow_addable_v = is_nothrow_addable<T>::value;
+
+  }  // namespace arithmetic
+
 }  // namespace nt::impl
 
 #endif
\ No newline at end of file
diff --git a/include/newtype/new_type.hpp b/include/newtype/new_type.hpp
index 77b531e..8228a94 100644
--- a/include/newtype/new_type.hpp
+++ b/include/newtype/new_type.hpp
@@ -325,6 +325,27 @@ namespace nt
   {
     return input >> target.m_value;
   }
+
+  /// @section Arithmetic operators
+
+  /**
+   * @brief Add two instances of the same nt::new_type
+   *
+   * @note This operator is only available if the derivation clause of the passed in nt::new_type objects contains nt::Arithmetic and the base
+   * type is addable.
+   * @param lhs The left-hand side of the addition
+   * @param rhs The right-hand side of the addition
+   * @return a new instance of the same nt::new_type
+   */
+  template<typename BaseType, typename TagType, auto DerivationClause>
+  auto constexpr
+  operator+(new_type<BaseType, TagType, DerivationClause> const & lhs, new_type<BaseType, TagType, DerivationClause> const & rhs) noexcept(
+      impl::is_nothrow_addable_v<BaseType> && std::is_nothrow_copy_constructible_v<BaseType>)
+      -> std::enable_if_t<DerivationClause(nt::Arithmetic) && impl::is_addable_v<BaseType>, new_type<BaseType, TagType, DerivationClause>>
+  {
+    return {lhs.decay() + rhs.decay()};
+  }
+
 }  // namespace nt
 
 #endif
-- 
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