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From: Simon Wright <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Generic instantiation with constrained subtype
Date: Sat, 23 Aug 2008 13:59:20 +0100
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Date: 2008-08-23T13:59:20+01:00
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I've been trying to write some test cases for the Booch Components and
came up with this.

The test case is written as a generic, to be instantiated with the
appropriate Container type. This is fine for the definite containers
(ie, ones like No_Problem below) but fails for the Bounded forms, which
use the Unconstrained/Constrained technique.

This is the example:

-----------------------------------------------------------------------
package Instantiation_Problem is

   type Base is tagged null record;

   type No_Problem is new Base with null record;

   type Unconstrained (Constraint : Positive)
   is new Base with null record;

   subtype Constrained is Unconstrained (Constraint => 10);

   generic
      type T is new Base with private;
   package G is end G;

   package NP is new G (No_Problem);

   package F is new G (Constrained);
   --                  ^--------------------18:24

   package U is new G (Unconstrained);
   --                  ^--------------------21:24

end Instantiation_Problem;
-----------------------------------------------------------------------

and GNAT (GCC 4.3.0) says

instantiation_problem.ads:18:24: constraint on actual is incompatible with formal
instantiation_problem.ads:18:24: instantiation abandoned
instantiation_problem.ads:21:24: actual subtype must be constrained
instantiation_problem.ads:21:24: instantiation abandoned

Am I going to have to rethink this? Have I used the wrong way of
specifying the generic parameter? Is GNAT confused? (note, at 21:24 it's
asking for a subtype, so why complain at 18:24?).

Ideally I'd like it if any solution could work with Ada 95 - these are
the Booch Ada 95 Components!

--S