Q: Access subprogram with class type parameters

[Rush Kester <Rush.Kester@jhuapl.edu>]

This is my first attempt at using subprogram access types and class
programming.

Why doesn't Default_Get_Procedure match Get_Procedure in the following?

with text_io;
procedure test is
   type Values is
      tagged record
         null;
      end record;
   type Bool_Values          is new Values
      with record
         Bool_Value : Boolean;
      end record;
   package bool_io is new text_io.enumeration_io(boolean);

   type Get_Procedure is access procedure (
     File : in text_io.file_type;
     Value : out values'class);

   procedure Default_Get_Procedure(
     File : in text_io.file_type;
     Value : out Bool_Values) is

   begin
     bool_io.get(File, Value.Bool_Value);
   end Default_Get_Procedure;

 Get_Message : Get_Procedure := Default_Get_Procedure'Access; -- what's
wrong?
-- ...
 File : text_io.file_type;
begin
  null;
end test;