new_type: implement multiply-assignment

4 files changed, 118 insertions, 5 deletions

diff --git a/doc/src/index.rst b/doc/src/index.rst
index 591a168..4c13412 100644
--- a/doc/src/index.rst
+++ b/doc/src/index.rst
@@ -320,6 +320,14 @@ Arithmetic Operators
 
    **enablement:** This operator shall be available iff. a) :cpp:type:`new_type<BaseType, TagType, DerivationClause>::base_type` is multipliable using the operator :literal:`*` and b) :cpp:type:`DerivationClause` includes :cpp:var:`Arithmetic`.
 
+.. cpp:function:: template<typename BaseType, \
+                  typename TagType, \
+                  auto DerivationClause> \
+                  constexpr new_type<BaseType, TagType, DerivationClause> & operator*=(new_type<BaseType, TagType, DerivationClause> const & lhs, new_type<BaseType, TagType, DerivationClause> const & rhs)
+
+   **noexcept specification:** This operator shall be noexcept iff. :cpp:type:`new_type<BaseType, TagType, DerivationClause>::base_type` is nothrow multiply-assignable.
+
+   **enablement:** This operator shall be available iff. a) :cpp:type:`new_type<BaseType, TagType, DerivationClause>::base_type` is multiply-assignable using the operator :literal:`-=` and b) :cpp:type:`DerivationClause` includes :cpp:var:`Arithmetic`.
 
 .. cpp:function:: template<typename BaseType, \
                   typename TagType, \
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:
@@ -436,6 +443,26 @@ namespace nt
   }
 
   /**
+   * @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
    *
    * @note This operator is only available if the derivation clause of the passed in nt::new_type objects contains nt::Arithmetic and the base
diff --git a/test/src/arithmetic_suite.cpp b/test/src/arithmetic_suite.cpp
index 75a7f46..c7051e2 100644
--- a/test/src/arithmetic_suite.cpp
+++ b/test/src/arithmetic_suite.cpp
@@ -202,7 +202,7 @@ inline namespace division_tests
 
 }  // namespace division_tests
 
-inline namespace compound_addition
+inline namespace compound_arithmetic
 {
 
   auto addition_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_an_instance_of_the_same_new__type() -> void
@@ -239,7 +239,24 @@ inline namespace compound_addition
     ASSERT_EQUAL(elhs -= erhs, (lhs -= rhs).decay());
   }
 
-}  // namespace compound_addition
+  auto multiplication_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_an_instance_of_the_same_new__type() -> void
+  {
+    using type_alias = nt::new_type<int, struct tag, deriving(nt::Arithmetic)>;
+    ASSERT((std::is_same_v<type_alias &, decltype(std::declval<type_alias &>() *= std::declval<type_alias const &>())>));
+  }
+
+  auto multiplication_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_the_correct_value_with_respect_to_the_base_type()
+      -> void
+  {
+    using type_alias = nt::new_type<int, struct tag, deriving(nt::Arithmetic)>;
+    auto elhs = 42;
+    auto erhs = 18;
+    auto lhs = type_alias{elhs};
+    auto rhs = type_alias{erhs};
+    ASSERT_EQUAL(elhs *= erhs, (lhs *= rhs).decay());
+  }
+
+}  // namespace compound_arithmetic
 
 auto arithmetic_suite() -> std::pair<cute::suite, std::string>
 {
@@ -279,13 +296,13 @@ auto arithmetic_suite() -> std::pair<cute::suite, std::string>
           KAWAII(division_of_two_instances_of_a_new__type_deriving_arithmetic_produces_an_instance_of_the_same_new__type),
           KAWAII(division_of_two_instances_of_a_new__type_deriving_arithmetic_produces_the_correct_value_with_respect_to_the_base_type),
 
-          /// Compound Addition Tests
+          /// Compound Arithmetic Tests
           KAWAII(addition_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_an_instance_of_the_same_new__type),
           KAWAII(addition_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_the_correct_value_with_respect_to_the_base_type),
-
-          /// Compound Addition Tests
           KAWAII(subtraction_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_an_instance_of_the_same_new__type),
           KAWAII(subtraction_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_the_correct_value_with_respect_to_the_base_type),
+          KAWAII(multiplication_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_an_instance_of_the_same_new__type),
+          KAWAII(multiplication_assignment_of_two_instances_of_a_new__type_deriving_arithmetic_produces_the_correct_value_with_respect_to_the_base_type),
 
           // clang-format on
       },