Index: tests/natools-string_slice_set_tests.adb
==================================================================
--- tests/natools-string_slice_set_tests.adb
+++ tests/natools-string_slice_set_tests.adb
@@ -828,10 +828,17 @@
          if Subset /= Set then
             Info_Fail (Report, Name, Reported, "Expected equal subset");
             Dump (Report, Set);
             Dump (Report, Subset);
          end if;
+
+         Subset.Add_Slice (First, First - 1);
+         if Subset /= Set then
+            Info_Fail (Report, Name, Reported, "Expected no-op Add_Slice");
+            Dump (Report, Set);
+            Dump (Report, Subset);
+         end if;
 
          Subset := Set.Subset (First + 1, Middle_First - 2);
          if Subset.To_String
            /= Parent_String (First + 1 .. Middle_First - 2)
          then