Accessibility and possible Gnat bug

Accessibility and possible Gnat bug

[Michael B.]

In the previous discussion about accessibility I was pointed to the 
paper "Accessibility rules OK!" of John Barnes (ACM SIGAda Ada Letters 
Volume XV Issue 1, Jan./Feb. 1995) which contains the following example:

procedure Example6  is
    type T is new Integer;

    procedure P1(XP1 : access T) is
       type A is access all T;
       Ptr: A := null;

       procedure P2(XP2: access T) is
       begin
          Ptr := A(XP2);
       end P2;

    begin
       P2(XP1);
    end P1;

    X : aliased T;

begin
    declare
       Y : aliased T;
     begin
       declare
          Z : aliased T;
        begin
          P1(X'Access);
          P1(Y'Access);
          P1(Z'Access);   -- Raises Program_Error.
        end;
     end;
end Example6;

In the paper the program is expected to run without errors.
I compiled it with Gnat 4.9 on Debian 8.0, ran it and got an 
Program_Error on the marked line.

Is this correct in Ada 2012 or not? I would like to hear your opinion 
about that before I write a possibly unnecessary bug report to AdaCore.

Regards,

Michael

Re: Accessibility and possible Gnat bug

[Tero Koskinen]

19.12.2014, 17:35, Michael B. kirjoitti:
> In the previous discussion about accessibility I was pointed to the
> paper "Accessibility rules OK!" of John Barnes (ACM SIGAda Ada Letters
> Volume XV Issue 1, Jan./Feb. 1995) which contains the following example:
>
> procedure Example6  is
...
>           P1(Z'Access);   -- Raises Program_Error.
...
> end Example6;
>
> In the paper the program is expected to run without errors.
> I compiled it with Gnat 4.9 on Debian 8.0, ran it and got an
> Program_Error on the marked line.
>
> Is this correct in Ada 2012 or not? I would like to hear your opinion
> about that before I write a possibly unnecessary bug report to AdaCore.

I don't know what the standard actually says, but Irvine ICCAda
generates executable, which does not raise exception when
the code is compiled in Ada 2005 mode.

Also, Janus/Ada generates executable, which does not raise exception,
when Janus/Ada uses Ada 95 mode.

GNAT GPL 2013 always generates executable, which raises exception.
(Tested with -gnat95 and -gnat2012 flags).

Yours,
  Tero

Re: Accessibility and possible Gnat bug

[Michael B.]

That's an empirical argument for the bug thesis.
Thank you for testing.

Regards,

Michael


On 12/19/14 18:34, Tero Koskinen wrote:
> I don't know what the standard actually says, but Irvine ICCAda
> generates executable, which does not raise exception when
> the code is compiled in Ada 2005 mode.
>
> Also, Janus/Ada generates executable, which does not raise exception,
> when Janus/Ada uses Ada 95 mode.
>
> GNAT GPL 2013 always generates executable, which raises exception.
> (Tested with -gnat95 and -gnat2012 flags).