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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Ada design bug or GNAT bug?
Date: Sat, 20 Jun 2015 20:55:49 +0200
Organization: cbb software GmbH
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Xref: news.eternal-september.org comp.lang.ada:26390
Date: 2015-06-20T20:55:49+02:00
List-Id: <comp.lang.ada>

Is it intentional that there is no way to derive from a more specific type
privately? A key design pattern is no more possible in GNAT GPL 2015:

package P1 is
   type T1 is tagged null record;
   type T2 is new T1 with null record;
   function Foo return not null access T2;
end P1;

with P1; use P1;
package P2 is
   type T3 is new T1 with private;
private
   type T3 is new T2 with null record;
   overriding function Foo return not null access T3; -- Illegal!
end P2;

p2.ads:5:09: type must be declared abstract or "Foo" overridden
p2.ads:5:09: "Foo" has been inherited from subprogram at p1.ads:4
p2.ads:6:24: private function with tagged result must override visible-part
function
p2.ads:6:24: move subprogram to the visible part (RM 3.9.3(10))

[ Since when access types became tagged? ]

There seem no way to implement T3 by deriving from T2. *Any* declaration of
Foo would be illegal.

1. It cannot be inherited
2. A public declaration is not an overriding
3. A private declaration must be public

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de