Input archive: Add implicit serialization operator

include/cereal/cereal.hpp

@@ -700,6 +700,16 @@ namespace cereal
         return *self;
       }
 
+      //! Implicit conversion for serialization constant types
+      // Such as "const int value = ar;"
+      template <typename T, typename = std::enable_if_t<std::is_default_constructible_v<T> && std::is_copy_constructible_v<T>>>
+      operator T()
+      {
+        T value{};
+        process( value );
+        return value;
+      }
+
       //! Serializes any data marked for deferment using defer
       /*! This will cause any data wrapped in DeferredData to be immediately serialized */
       void serializeDeferments()