Visibility of Indexing aspects

[Jere <jhb.chat@gmail.com>]

I was going through an Ada library and noticed that they
were able to add an indexing aspect to a type in the private
section of a package but have the indexing work for types
declared using the public view of the type.  It's a neat feature,
but I wasn't sure if this was intentional or a GNAT bug?
I searched through sections 13.1 and 4.1.6 of the RM, but no
mention that I could find of the private/public requirements
for the Variable_Indexing and Constant_Indexing aspects.

Is there something I missed in the RM, is this a GNAT bug, or
something else?

Reference example compiled in GCC 8.1:

with Ada.Text_IO; use Ada.Text_IO;

procedure jdoodle is
    package Containers is

      type Container is private;
      
    private

      type Container_Array is array(1..20) of aliased Integer;
      type Container is tagged record
         Data : Container_Array := (others => 0);
      end record
        with Variable_Indexing => Reference;
      
      type Reference_Holder
         (Element : not null access Integer)
      is limited null record
         with Implicit_Dereference => Element;
      
      function Reference
         (Self  : aliased in out Container;
          Index : Positive)
          return Reference_Holder
      is (Element => Self.Data(Index)'Access);
      
    end Containers;
   
    C : aliased Containers.Container;
begin
    for Index in 1 .. 20 loop
        C(Index) := Index;
        Put_Line(Integer'Image(C(Index)));
    end loop;
end jdoodle;