bugfix: use explicit cast to underlying_type

include/opaque_typedef/opaque_typedef.hpp

@@ -115,6 +115,8 @@ namespace types {
      */
     constexpr value_type const & get() const { return value_; }
     constexpr explicit operator value_type const &() const { return value_; }
-    constexpr underlying_type operator*() const { return value_; }
+    constexpr underlying_type operator*() const {
+      return static_cast<underlying_type>(value_);
+    }
   };
 }