Index: sem_util.adb
===================================================================
--- sem_util.adb	(revision 150111)
+++ sem_util.adb	(working copy)
@@ -7142,19 +7142,33 @@ package body Sem_Util is
       Last_Assignment_Only : Boolean := False)
    is
    begin
+      --  ??? do we have to worry about clearing cached checks?
+
       if Is_Assignable (Ent) then
          Set_Last_Assignment (Ent, Empty);
       end if;
 
-      if not Last_Assignment_Only and then Is_Object (Ent) then
-         Kill_Checks (Ent);
-         Set_Current_Value (Ent, Empty);
+      if Is_Object (Ent) then
+         if not Last_Assignment_Only then
+            Kill_Checks (Ent);
+            Set_Current_Value (Ent, Empty);
 
-         if not Can_Never_Be_Null (Ent) then
-            Set_Is_Known_Non_Null (Ent, False);
-         end if;
+            if not Can_Never_Be_Null (Ent) then
+               Set_Is_Known_Non_Null (Ent, False);
+            end if;
+
+            Set_Is_Known_Null (Ent, False);
 
-         Set_Is_Known_Null (Ent, False);
+            --  Reset Is_Known_Valid unless type is always valid, or if we have
+            --  a loop parameter (loop parameters are always valid, since their
+            --  bounds are defined by the bounds given in the loop header).
+
+            if not Is_Known_Valid (Etype (Ent))
+              and then Ekind (Ent) /= E_Loop_Parameter
+            then
+               Set_Is_Known_Valid (Ent, False);
+            end if;
+         end if;
       end if;
    end Kill_Current_Values;
 
Index: sem_util.ads
===================================================================
--- sem_util.ads	(revision 150113)
+++ sem_util.ads	(working copy)
@@ -820,9 +820,9 @@ package Sem_Util is
    --  clear the Is_True_Constant flag, since that only gets reset if there
    --  really is an assignment somewhere in the entity scope). This procedure
    --  also calls Kill_All_Checks, since this is a special case of needing to
-   --  forget saved values. This procedure also clears Is_Known_Non_Null flags
-   --  in variables, constants or parameters since these are also not known to
-   --  be valid.
+   --  forget saved values. This procedure also clears the Is_Known_Null and
+   --  Is_Known_Non_Null and Is_Known_Valid flags in variables, constants or
+   --  parameters since these are also not known to be trustable any more.
    --
    --  The Last_Assignment_Only flag is set True to clear only Last_Assignment
    --  fields and leave other fields unchanged. This is used when we encounter
@@ -839,8 +839,8 @@ package Sem_Util is
       Last_Assignment_Only : Boolean := False);
    --  This performs the same processing as described above for the form with
    --  no argument, but for the specific entity given. The call has no effect
-   --  if the entity Ent is not for an object. Again, Last_Assignment_Only is
-   --  set if you want to clear only the Last_Assignment field (see above).
+   --  if the entity Ent is not for an object. Last_Assignment_Only has the
+   --  same meaning as for the call with no Ent.
 
    procedure Kill_Size_Check_Code (E : Entity_Id);
    --  Called when an address clause or pragma Import is applied to an entity.