Subtype of limited type non-limited?

Subtype of limited type non-limited?

[Florian Weimer]

The following code results in a GNAT error message:

access_discriminant.ads:21:36: access discriminants allowed 
                               only for limited types

GNAT assumes that the subtype of a limited type is non-limited (at
least under these special circumstances).  Is this really correct?

with Ada.Streams;

package Access_Discriminant is

   subtype Root_Stream_Type is Ada.Streams.Root_Stream_Type;

   type Read_Write is new Root_Stream_Type with null record;
   --  The following works:
   --  type Read_Write is new Ada.Streams.Root_Stream_Type with null record;

   procedure Read
     (Stream : in out Read_Write;
      Item   : out    Ada.Streams.Stream_Element_Array;
      Last   : out    Ada.Streams.Stream_Element_Offset);
   procedure Write
     (Stream : in out Read_Write;
      Item   : in     Ada.Streams.Stream_Element_Array);

   type Message_Sender is null record;

   type Mail_Body_Stream (Sender : access Message_Sender)
   is new Read_Write with null record;

end Access_Discriminant;

Re: Subtype of limited type non-limited?

[Tucker Taft]

Florian Weimer wrote:
> 
> The following code results in a GNAT error message:
> 
> access_discriminant.ads:21:36: access discriminants allowed
>                                only for limited types
> 
> GNAT assumes that the subtype of a limited type is non-limited (at
> least under these special circumstances).  Is this really correct?

Nope, this looks like a bug.  All subtypes and derivatives of a limited
type are limited, as are all types with a limited subcomponent.
This is sometimes summarized as "limited types are poison." ;-)

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA