gnat_rm.texi: Add note on Old giving warning when applied to constant

2008-05-28  Robert Dewar  <dewar@adacore.com>

	* gnat_rm.texi: Add note on Old giving warning when applied to constant

	* sem_attr.adb (Analyze_Attribute, case Old): Give warning if prefix is
	a constant

From-SVN: r136078

gcc/ada/gnat_rm.texi

@@ -5803,6 +5803,10 @@ package body Old_Pkg is
 end Old_Pkg;
 @end smallexample
 
+@noindent
+Note that it is allowed to apply 'Old to a constant entity, but this will
+result in a warning, since the old and new values will always be the same.
+
 @node Passed_By_Reference
 @unnumberedsec Passed_By_Reference
 @cindex Parameters, when passed by reference

gcc/ada/sem_attr.adb

@@ -3501,6 +3501,13 @@ package body Sem_Attr is
             Error_Attr ("attribute % cannot apply to limited objects", P);
          end if;
 
+         if Is_Entity_Name (P)
+           and then Is_Constant_Object (Entity (P))
+         then
+            Error_Msg_N
+              ("?attribute Old applied to constant has no effect", P);
+         end if;
+
          --  Check that the expression does not refer to local entities
 
          Check_Local : declare