From 05abaa3d00393165e7794a150ad6c4b5ac5dd37e Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Thu, 2 Jan 2020 15:42:12 +0100
Subject: new_type: implement multiply-assignment

---
 include/newtype/impl/type_traits_extensions.hpp | 61 +++++++++++++++++++++++++
 include/newtype/new_type.hpp                    | 27 +++++++++++
 2 files changed, 88 insertions(+)

(limited to 'include')

diff --git a/include/newtype/impl/type_traits_extensions.hpp b/include/newtype/impl/type_traits_extensions.hpp
index 5104306..6224d29 100644
--- a/include/newtype/impl/type_traits_extensions.hpp
+++ b/include/newtype/impl/type_traits_extensions.hpp
@@ -887,6 +887,67 @@ namespace nt::impl
     template<typename T>
     auto constexpr is_nothrow_subtract_assignable_v = is_nothrow_subtract_assignable<T>::value;
 
+    /**
+     * @brief A trait to test if a given type is multiply-assignable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-multiply-assignable T
+     */
+    template<typename T, typename = void>
+    struct is_multiply_assignable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is multiply-assignable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for multiply-assignable T
+     */
+    template<typename T>
+    struct is_multiply_assignable<T, std::void_t<decltype(std::declval<T &>() *= std::declval<T const &>())>> : std::true_type
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is multiply-assignable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_multiply_assignable_v = is_multiply_assignable<T>::value;
+
+    /**
+     * @brief A trait to test if a given type is noexcept multiply-assignable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the base case for non-noexcept multiply-assignable or non-multiply-assignable T
+     */
+    template<typename T, typename = void>
+    struct is_nothrow_multiply_assignable : std::false_type
+    {
+    };
+
+    /**
+     * @brief A trait to test if a given type is noexcept multiply-assignable
+     *
+     * @tparam T The type to test
+     * @note This specialization forms the case for multiply-assignable T detemining if T is noexcept multiply-assignable
+     */
+    template<typename T>
+    struct is_nothrow_multiply_assignable<T, std::void_t<decltype(std::declval<T &>() *= std::declval<T const &>())>>
+        : std::bool_constant<noexcept(std::declval<T &>() *= std::declval<T const &>())>
+    {
+    };
+
+    /**
+     * @brief A variable template to test if a given type is noexcept multiply-assignable
+     *
+     * @tparam T The type to test
+     */
+    template<typename T>
+    auto constexpr is_nothrow_multiply_assignable_v = is_nothrow_multiply_assignable<T>::value;
+
   }  // namespace compound_arithmetic
 
 }  // namespace nt::impl
diff --git a/include/newtype/new_type.hpp b/include/newtype/new_type.hpp
index 8d55f61..a7ab4af 100644
--- a/include/newtype/new_type.hpp
+++ b/include/newtype/new_type.hpp
@@ -50,6 +50,13 @@ namespace nt
         -> std::enable_if_t<DerivationClauseV(nt::Arithmetic) && impl::is_subtract_assignable_v<BaseTypeT>,
                             new_type<BaseTypeT, TagTypeT, DerivationClauseV> &>;
 
+    template<typename BaseTypeT, typename TagTypeT, auto DerivationClauseV>
+    auto constexpr friend
+    operator*=(new_type<BaseTypeT, TagTypeT, DerivationClauseV> & lhs,
+               new_type<BaseTypeT, TagTypeT, DerivationClauseV> const & rhs) noexcept(impl::is_nothrow_multiply_assignable_v<BaseTypeT>)
+        -> std::enable_if_t<DerivationClauseV(nt::Arithmetic) && impl::is_multiply_assignable_v<BaseTypeT>,
+                            new_type<BaseTypeT, TagTypeT, DerivationClauseV> &>;
+
     using super = impl::new_type_move_assignment<BaseType, TagType>;
 
   public:
@@ -435,6 +442,26 @@ namespace nt
     return {lhs.decay() * rhs.decay()};
   }
 
+  /**
+   * @brief Multiply two instances of the same nt::new_type, modifying the left-hand side
+   *
+   * @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 multiplyable.
+   * @param lhs The left-hand side of the multiplyition
+   * @param rhs The right-hand side of the multiplyition
+   * @return a reference to the the modified value
+   */
+  template<typename BaseType, typename TagType, auto DerivationClause>
+  auto constexpr
+  operator*=(new_type<BaseType, TagType, DerivationClause> & lhs,
+             new_type<BaseType, TagType, DerivationClause> const & rhs) noexcept(impl::is_nothrow_multiply_assignable_v<BaseType>)
+      -> std::enable_if_t<DerivationClause(nt::Arithmetic) && impl::is_multiply_assignable_v<BaseType>,
+                          new_type<BaseType, TagType, DerivationClause> &>
+  {
+    lhs.m_value *= rhs.m_value;
+    return lhs;
+  }
+
   /**
    * @brief Divide two instances of the same nt::new_type
    *
-- 
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