[Ada] Crash in interface derivation with null primitive

The frontend crashes processing the derivation of a tagged type whose
ultimate ancestor is an interface type I1 that has a null primitive,
implements another interface I2 derived from I2, and does not override
the null primitive.

2019-07-08  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* exp_disp.adb (Register_Primitive): When registering a
	primitive in the secondary dispatch table, handle primitive
	inherited through several levels of type derivation (required to
	properly handle inherited 'null' primitive).

gcc/testsuite/

	* gnat.dg/interface9.adb, gnat.dg/interface9_root-child.ads,
	gnat.dg/interface9_root.ads: New testcase.

From-SVN: r273204

gcc/ada/ChangeLog

+2019-07-08  Javier Miranda  <miranda@adacore.com>
+
+	* exp_disp.adb (Register_Primitive): When registering a
+	primitive in the secondary dispatch table, handle primitive
+	inherited through several levels of type derivation (required to
+	properly handle inherited 'null' primitive).
+
 2019-07-08  Bob Duff  <duff@adacore.com>
 
 	* doc/gnat_ugn/gnat_utility_programs.rst: Document handling of

gcc/ada/exp_disp.adb

@@ -7637,7 +7637,7 @@ package body Exp_Disp is
                      Unchecked_Convert_To (RTE (RE_Prim_Ptr),
                        Make_Attribute_Reference (Loc,
                          Prefix         =>
-                           New_Occurrence_Of (Alias (Prim), Loc),
+                           New_Occurrence_Of (Ultimate_Alias (Prim), Loc),
                          Attribute_Name => Name_Unrestricted_Access))));
 
             end if;

gcc/testsuite/ChangeLog

+2019-07-08  Javier Miranda  <miranda@adacore.com>
+
+	* gnat.dg/interface9.adb, gnat.dg/interface9_root-child.ads,
+	gnat.dg/interface9_root.ads: New testcase.
+
 2019-07-08  Ed Schonberg  <schonberg@adacore.com>
 
 	* gnat.dg/predicate9.adb: New testcase.

gcc/testsuite/gnat.dg/interface9.adb

+--  { dg-do compile }
+
+with Interface9_Root.Child;
+procedure Interface9 is
+   package R   is new Interface9_Root (Real => Float);
+   package RC  is new R.Child;
+
+begin
+   null;
+end Interface9;

gcc/testsuite/gnat.dg/interface9_root-child.ads

+generic
+package Interface9_Root.Child is
+    type Base_Type is abstract new Base_Interface with null record;
+
+    type Derived_Type is abstract new Base_Type and Derived_Interface
+      with null record; --  Test
+end Interface9_Root.Child;

gcc/testsuite/gnat.dg/interface9_root.ads

+generic
+   type Real is digits <>;
+package Interface9_Root is
+   type Base_Interface is limited interface;
+
+   procedure Primitive1 (B : in out Base_Interface) is abstract;
+   procedure Primitive2 (B : in out Base_Interface) is null;
+
+   type Derived_Interface is limited interface and Base_Interface;
+end Interface9_Root;