diff --git a/libcxx/include/__algorithm/three_way_comp_ref_type.h b/libcxx/include/__algorithm/three_way_comp_ref_type.h
--- a/libcxx/include/__algorithm/three_way_comp_ref_type.h
+++ b/libcxx/include/__algorithm/three_way_comp_ref_type.h
@@ -27,6 +27,13 @@
   _Comp& __comp_;
   _LIBCPP_HIDE_FROM_ABI constexpr __debug_three_way_comp(_Comp& __c) : __comp_(__c) {}
 
+  template <class _Tp, class _Up>
+  _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(const _Tp& __x, const _Up& __y) {
+    auto __r = __comp_(__x, __y);
+    __do_compare_assert(0, __y, __x, __r);
+    return __r;
+  }
+
   template <class _Tp, class _Up>
   _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Tp& __x, _Up& __y) {
     auto __r = __comp_(__x, __y);
@@ -55,7 +62,7 @@
 
 // Pass the comparator by lvalue reference. Or in debug mode, using a
 // debugging wrapper that stores a reference.
-#  ifndef _LIBCPP_ENABLE_DEBUG_MODE
+#  ifdef _LIBCPP_ENABLE_DEBUG_MODE
 template <class _Comp>
 using __three_way_comp_ref_type = __debug_three_way_comp<_Comp>;
 #  else