[COMMITTED] ada: Raise Tag_Error when Ada.Tags operations are called with No_Tag

["Marc Poulhiès" <poulhies@adacore.com>]

From: Piotr Trojanek <trojanek@adacore.com>

Implement missing behavior of RM 13.9 (25.1/3): Tag_Error is raised by a
call of Interface_Ancestor_Tags and Is_Descendant_At_Same_Level, if any
tag passed is No_Tag. This change also fixes Descendant_Tag, which
relies on Is_Descendant_At_Same_Level. The remaining operations already
worked properly.

gcc/ada/

	* libgnat/a-tags.adb
	(Interface_Ancestor_Tags): Raise Tag_Error on No_Tag.
	(Is_Descendant_At_Same_Level): Likewise.

Tested on x86_64-pc-linux-gnu, committed on master.

---
 gcc/ada/libgnat/a-tags.adb | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/gcc/ada/libgnat/a-tags.adb b/gcc/ada/libgnat/a-tags.adb
index d663a41f5a8..a9af942fc64 100644
--- a/gcc/ada/libgnat/a-tags.adb
+++ b/gcc/ada/libgnat/a-tags.adb
@@ -554,13 +554,18 @@ package body Ada.Tags is
    -----------------------------
 
    function Interface_Ancestor_Tags (T : Tag) return Tag_Array is
-      TSD_Ptr     : constant Addr_Ptr :=
-                      To_Addr_Ptr (To_Address (T) - DT_Typeinfo_Ptr_Size);
-      TSD         : constant Type_Specific_Data_Ptr :=
-                      To_Type_Specific_Data_Ptr (TSD_Ptr.all);
-      Iface_Table : constant Interface_Data_Ptr := TSD.Interfaces_Table;
-
+      TSD_Ptr     : Addr_Ptr;
+      TSD         : Type_Specific_Data_Ptr;
+      Iface_Table : Interface_Data_Ptr;
    begin
+      if T = No_Tag then
+         raise Tag_Error;
+      end if;
+
+      TSD_Ptr     := To_Addr_Ptr (To_Address (T) - DT_Typeinfo_Ptr_Size);
+      TSD         := To_Type_Specific_Data_Ptr (TSD_Ptr.all);
+      Iface_Table := TSD.Interfaces_Table;
+
       if Iface_Table = null then
          declare
             Table : Tag_Array (1 .. 0);
@@ -731,7 +736,10 @@ package body Ada.Tags is
       Ancestor   : Tag) return Boolean
    is
    begin
-      if Descendant = Ancestor then
+      if Descendant = No_Tag or else Ancestor = No_Tag then
+         raise Tag_Error;
+
+      elsif Descendant = Ancestor then
          return True;
 
       else
-- 
2.34.1