diff --git a/stl/inc/xutility b/stl/inc/xutility
index 880293a0d71..90654326b73 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -540,6 +540,8 @@ struct less_equal<void> {
 
 template <class _Fx>
 struct _Ref_fn { // pass function object by value as a reference
+    // _Ref_fn is an aggregate so it can be enregistered, unlike reference_wrapper
+
     template <class... _Args>
     constexpr decltype(auto) operator()(_Args&&... _Vals) { // forward function call operator
         if constexpr (is_member_pointer_v<_Fx>) {