Defining types in private packages for use in parent spec

[Lutz Donnerhacke <lutz@iks-jena.de>]

How to solve the following problem? It's necessary to define the type Base
in the implementation defined private subpackage, because it varies on
various implementations.

------------------------------------------------------------------------
package Bigints is
   type Bigint is limited private;
private
   type Base_Array;
   type Base_Access is access Base_Array;
   type Bigint is limited record
      data : Base_Access;
   end record;
end Bigints;

with System;
private package Bigints.Implementation is
   type Base is mod System.Max_Binary_Modulus;
   procedure Demo(a : in out Bigint);
end Bigints.Implementation;

with Bigints.Implementation;
package body Bigints is
   subtype Base is Implementation.Base;
   type Base_Array is array(Positive range <>) of Base;
end Bigints;

package body Bigints.Implementation is
   procedure Demo(a : in out Bigint) is
   begin
      for i in a.data'Range loop
         null;
      end loop;
   end Demo;
end Bigints.Implementation;
------------------------------------------------------------------------

gcc -c bigints.adb
gcc -c bigints-implementation.adb
bigints-implementation.adb:5:17: prefix for "Range" attribute must be array
gnatmake: "bigints-implementation.adb" compilation error