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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Generic oddness
Date: Sun, 12 Apr 2020 18:38:07 +0200
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Date: 2020-04-12T18:38:07+02:00
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Per Jakobsen <pdj@knaldgas.dk> writes:
> Declaring the package List_Type from Doubly_Linked_Lists gives me
> "abstract subprogram not allowed as generic actual" unless I declare
> the "A_Intf" type in a package separate from the List_Type
> declaration, or if I instantiate a workaround (dummy) generic before
> (as in the below MWE):
>
> --- gnatchop ---------------------
> generic
>    type A is private;
> package Gen is
>    pragma Pure;
> end Gen;
> package Intf is
>    type Intf_Type is interface;
>    procedure Do_Stuff (I : in out Intf_Type) is abstract;
> end Intf;
> with Ada.Containers.Doubly_Linked_Lists;
> with Intf;
> with Gen;
>
> package Test is
>    use Intf;
>
>    type A_Intf is new Intf_Type with
>       record
>          A : Natural;
>       end record;
>
>    overriding
>    procedure Do_Stuff (I : in out A_Intf);
>
>    package Workaround is new Gen (A => A_Intf);
>    --  Commenting the above line will cause the
>    --  next line to fail !?!?
>    package List_Type is
>       new Ada.Containers.Doubly_Linked_Lists (Element_Type => A_Intf,
>                                               "="          => "=");
> end Test;
> package body Test is
>
>    overriding
>    procedure Do_Stuff (I : in out A_Intf) is
>    begin
>       null;
>    end Do_Stuff;
>
> end Test;
> --- gnatchop end ---------------------
>
> Unpack with gnatchop, then compile with gnatmake test.adb. Then
> comment the Workaround package instantiation and recompile.
>
> Using GNATMAKE Community 2019 (20190517-83).
>
> Is there a sane explanation for this behavior?

I think I have a partial explanation.  Instantiating the package Gen
freezes the type Test.A_Intf.  Similarly, if you declare A_Intf in a
package and then instantiate Doubly_Linked_Lists in another package, the
instantiation of Doubly_Linked_Lists sees a frozen type as the actual.

The part that I don't understand is why you can instantiate Gen but not
Doubly_Linked_Lists with the same actual type as parameter; I think the
instantiation of Doubly_Linked_Lists should freeze the type A_Intf just
like package Workaround does.

-- 
Ludovic Brenta.
Multi-channel structures transfer the thinkers/planners throughout the
organization. In the same time, the enabler will be well equipped to
harness an alternative.