Index: checks.adb
===================================================================
--- checks.adb	(revision 220439)
+++ checks.adb	(working copy)
@@ -6,7 +6,7 @@
 --                                                                          --
 --                                 B o d y                                  --
 --                                                                          --
---          Copyright (C) 1992-2014, Free Software Foundation, Inc.         --
+--          Copyright (C) 1992-2015, Free Software Foundation, Inc.         --
 --                                                                          --
 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
 -- terms of the  GNU General Public License as published  by the Free Soft- --
@@ -5521,10 +5521,14 @@
                   return;
                end if;
 
-            --  Ditto if the prefix is an explicit dereference whose designated
-            --  type is unconstrained.
+            --  Ditto if prefix is simply an unconstrained array. We used
+            --  to think this case was OK, if the prefix was not an explicit
+            --  dereference, but we have now seen a case where this is not
+            --  true, so it is safer to just suppress the optimization in this
+            --  case. The back end is getting better at eliminating redundant
+            --  checks in any case, so the loss won't be important.
 
-            elsif Nkind (Prefix (P)) = N_Explicit_Dereference
+            elsif Is_Array_Type (Atyp)
               and then not Is_Constrained (Atyp)
             then
                Activate_Range_Check (N);