Downcasting facility refactored so the user does not have to write the boilerplate code anymore

src/include/units/bits/downcasting.h

@@ -30,6 +30,7 @@ namespace units {
   template<typename BaseType>
   struct downcast_base {
     using base_type = BaseType;
+    friend auto downcast_guide(downcast_base);
   };
 
   template<typename T>
@@ -39,13 +40,33 @@ namespace units {
       } &&
       std::derived_from<T, downcast_base<typename T::base_type>>;
 
+  template<typename Target, Downcastable T>
+  struct downcast_helper : T {
+    friend auto downcast_guide(typename downcast_helper::downcast_base) { return Target(); }
+  };
+
+  namespace detail {
+
+    template<typename T>
+    concept bool has_downcast = requires {
+        downcast_guide(std::declval<downcast_base<T>>());
+    };
+
+    template<typename T>
+    constexpr auto downcast_target_impl()
+    {
+      if constexpr(has_downcast<T>)
+        return decltype(downcast_guide(std::declval<downcast_base<T>>()))();
+      else
+        return T();
+    }
+
+  }
+
+  template<Downcastable T>
+  using downcast_target = decltype(detail::downcast_target_impl<T>());
+
   template<Downcastable T>
   using downcast_base_t = T::base_type;
 
-  template<Downcastable T>
-  struct downcast_traits : std::type_identity<T> {};
-
-  template<Downcastable T>
-  using downcast_traits_t = downcast_traits<T>::type;
-
 }  // namespace units