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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Derivation + Access Discriminant = Headaches
Date: 1999/03/19
Message-ID: <m3r9qlvpuv.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 456415544
Sender: matt@mheaney.ni.net
NNTP-Posting-Date: Fri, 19 Mar 1999 00:31:57 PDT
Newsgroups: comp.lang.ada
Date: 1999-03-19T00:00:00+00:00
List-Id: <comp.lang.ada>

Below is a tagged type hierarchy.

The root is T.  Type T has no discriminant.

Type NT derives from T, and adds a discriminant that designates T'Class.

Type NNT derives from NT, and wants to replace the inherited
discriminant with a discriminant that designates NT'Class.

My expectation is that this should be legal, because NT is in T'Class,
and should therefore be allowed to replace the discriminant that
designates T'Class.

But when I try to compile NNT, my compiler is telling me the subtype of
the discriminant is wrong:

q.ads:5:14: subtype must be compatible with parent discriminant


Is this a compiler bug?  If not, what's the rationale for the
restriction?

Thanks,
Matt


--STX
package P is

   type T is abstract tagged limited private;

   type NT (O : access T'Class) is
     abstract new T with private;

private

   type T is abstract tagged limited null record;

   type NT (O : access T'Class) is new T with null record;

end P;


with P; use P;
package Q is

   type NNT (O : access NT'Class) is
     new NT with private;

private

   type NNT (O : access NT'Class) is
     new NT (O) with null record;

end Q;