Re: Unexpected discriminant check failure involving access types

[Mark Lorenzen <mark.lorenzen@gmail.com>]

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

Look at line 12. FOO_PTR is a constant object of an access type. The constant object is initialized to point to a discriminated record with discriminant 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 reproducer or is it a snippet from real code?

Regards,

Mark L