Trying to use Ada 2012 features

test.adb

with Ada.Text_IO;

procedure Test is
   
   type Int_Array is array (1 .. 64) of aliased Integer;
   
   subtype Index is Integer range 1 .. 4;
   
   type Ref_Element (Data : not null access Integer) is limited null record
     with Implicit_Dereference => Data;
   
   package Foo is
      type My_Type is tagged private
        with Variable_Indexing => Element;
      
      function Element (Object : aliased in out My_Type; X, Y, Z : Index) return Ref_Element;
   
   private
      type My_Type is tagged record
         Elements : Int_Array := (others => 0);
      end record;
   end Foo;
      
   package body Foo is
      function Element (Object : aliased in out My_Type; X, Y, Z : Index) return Ref_Element is
      begin
         return Ref_Element'(Data => Object.Elements (X + (Y - 1) * 4 + (Z - 1) * 16)'Access);
      end Element;
   end Foo;

   Object : aliased Foo.My_Type;
   Cur    : Integer;
begin
   Object (1, 1, 1) := 4;
   Object (3, 1, 2) := 16;
   
   for I in Index'Range loop
      Cur := Object (I, I, I);
      Ada.Text_IO.Put_Line (Cur'Img);
   end loop;
end Test;

Compiler output with GNATMAKE GPL 2012 (20120509):
gcc -c -gnat12 test.adb
test.adb:25:04: container cannot be indexed with "Universal_Integer"
test.adb:26:04: container cannot be indexed with "Universal_Integer"
test.adb:29:17: container cannot be indexed with "Index"
a-textio.ads:205:14: aspect Indexing requires a function that applies to type "My_Type"
a-textio.ads:206:14: aspect Indexing requires a function that applies to type "My_Type"
a-textio.ads:239:14: aspect Indexing requires a function that applies to type "My_Type"
a-textio.ads:240:14: aspect Indexing requires a function that applies to type "My_Type"
gnatmake: "test.adb" compilation error

New output:
test.adb:14:32: "Element" is undefined

New output:

test.adb:34:04: missing argument for parameter "Y" in call to "Element" declared at line 16
test.adb:35:04: missing argument for parameter "Y" in call to "Element" declared at line 16
test.adb:38:14: missing argument for parameter "Y" in call to "Element" declared at line 16