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Date: Mon, 10 Aug 2015 06:14:40 -0700 (PDT)
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Subject: Re: Unexpected discriminant check failure involving access types
From: Mark Lorenzen <mark.lorenzen@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:27403
Date: 2015-08-10T06:14:40-07:00
List-Id: <comp.lang.ada>

On Monday, August 10, 2015 at 2:38:23 PM UTC+2, Markus Sch=F6pflin wrote:
> Given the following piece of code:
>=20
> ---%<---
>       1  procedure TEST
>       2  is
>       3     type T is (T1, T2);
>       4
>       5     type RECORD_T (X : T :=3D T1) is record
>       6        null;
>       7     end record;
>       8
>       9     type PTR_T is access RECORD_T;
>      10
>      11     FOO : RECORD_T;
>      12     FOO_PTR : constant PTR_T :=3D new RECORD_T;
>      13
>      14     FOO1 : constant RECORD_T :=3D (X =3D> T1);
>      15     FOO2 : constant RECORD_T :=3D (X =3D> T2);
>      16  begin
>      17     FOO :=3D FOO1;
>      18     FOO :=3D FOO2;
>      19
>      20     FOO_PTR.all :=3D FOO1;
>      21     FOO_PTR.all :=3D FOO2;
>      22  end;
> --->%---
>=20
> When compiled and executed, I get:
>=20
>  > ./test
>=20
> raised CONSTRAINT_ERROR : test.adb:21 discriminant check failed
>=20
> Can anyone please explain me why I get a discriminant check error when us=
ing=20
> access types? I would have expected it to work the same as for the non-ac=
cess=20
> case.
>=20
> TIA, Markus

Look at line 12. FOO_PTR is a constant object of an access type. The consta=
nt object is initialized to point to a discriminated record with discrimina=
nt T1, since this is the default discriminant of objects of type RECORD_T. =
In line 21 it goes horribly wrong and you try to change the object pointed =
to, to a record with discriminant T2.

What problem are you trying to solve? Is the above code just a simple repro=
ducer or is it a snippet from real code?

Regards,

Mark L