Index: sem_warn.adb
===================================================================
--- sem_warn.adb	(revision 251773)
+++ sem_warn.adb	(working copy)
@@ -1382,16 +1382,22 @@
                   --  deal with case where original unset reference has been
                   --  rewritten during expansion.
 
-                  --  In some cases, the original node may be a type conversion
-                  --  or qualification, and in this case we want the object
-                  --  entity inside.
+                  --  In some cases, the original node may be a type
+                  --  conversion, a qualification or an attribute reference and
+                  --  in this case we want the object entity inside. Same for
+                  --  an expression with actions.
 
                   UR := Original_Node (UR);
                   while Nkind (UR) = N_Type_Conversion
                     or else Nkind (UR) = N_Qualified_Expression
                     or else Nkind (UR) = N_Expression_With_Actions
+                    or else Nkind (UR) = N_Attribute_Reference
                   loop
-                     UR := Expression (UR);
+                     if Nkind (UR) = N_Attribute_Reference then
+                        UR := Prefix (UR);
+                     else
+                        UR := Expression (UR);
+                     end if;
                   end loop;
 
                   --  Don't issue warning if appearing inside Initial_Condition