Re: type casting tagged types in the wrong

Re: type casting tagged types in the wrong

[James A. Squire]

Scott Moody <scott@PLATO.DS.BOEING.COM> wrote:

> I run this same program with GNAT 3.03 on solaris
> and it reports the error correctly:
>
> tmp.adb:24:14: downward conversion of tagged objects not allowed
> gnatmake: *** compilation failed.
>
> ----source:
[source snipped]

I just ran it myself against the same compiler on the same platform and
to my chagrin I got the same error message.

I had figured that the bug would be consistant.  The place where I found
it
was in the Alert system that is partially coded in the Rationale.  I
added code for the other subunits besides Handle, and then I switched
the overloads of Handle so that the one for Medium Alerts called the one
for High Alerts instead of the one for Alerts.  I made sure the subunit
for Set_Alarm actually tried to read the extra field.  In this case
there was no error on the line:

Handle (HA => High_Alert(MA));

I will send this off to report@gnat.com.
--
James Squire
MDA Avionics Tools & Processes
ja_squire@csehp3.mdc.com
"one of these days I'm going to better myself by going to Knight school"

Re: type casting tagged types in the wrong direction

[Tucker Taft]

James A. Squire (m193884@CSEHP3.MDC.COM) wrote:
: I have a question:

: Say I have something like:

: type A is tagged
:   record
:     X : ABC;
:     Y : DEF;
:   end record;

: type B is new A with
:   record
:     W : GHI;
:   end record;

: type C is new B with
:   record
:     Z : JKL;
:   end record;


: OK.  Now, let's say that I forgot which type extends off of which, and I
: write a statement like this:

:   Jim  : C;
:   Jane : B;
:   Save : JKL;

: begin
:   ...
:   Jim := C (Jane);

This is illegal, by RM95 4.6(21-23).

:   Save := Jim.Z;
:   ...
: end;

: GNAT 3.03 compiles this statement.  Is that really right?  

No.  GNAT should have complained at compile time about the conversion
from B to C.  You may convert from B'Class to C (and then a run-time
check is performed), or you may convert from C to B, but a direct
conversion from B to C is illegal.

: ... Is Jim.Z defined?

Jim.Z is "defined," but the conversion on the previous line
is illegal, so who knows what code GNAT is generating for the
assignment to Jim.

: According to the LRM 4.6, the above is called a "view conversion".  This
: raises the question, how do you take a type C view of a type B entity
: when type C looks for additional information?

You can't.

: Should a validated Ada95 compiler flag this usage?  I realize that GNAT
: is not validated.

GNAT is validated, but validation doesn't guarantee bug-free
(nor does anything else I know of ;-).

This looks like a bug in GNAT, pure and simple.  (It also *suggests*
that another ACVC test might be in order ;-).

: I would appreciate any expert or otherwise opinions that any of you have
: to offer.
: Thank you.
: --
: James Squire
: MDA Avionics Tools & Processes
: ja_squire@csehp3.mdc.com
: "one of these days I'm going to better myself by going to Knight school"

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA

Re: type casting tagged types in the wrong

[Scott Moody]

I run this same program with GNAT 3.03 on solaris
and it reports the error correctly:

tmp.adb:24:14: downward conversion of tagged objects not allowed
gnatmake: *** compilation failed.


----source:
procedure tmp is

 type A is tagged
   record
     X : integer;
     Y : integer;
   end record;

 type B is new A with
   record
     W : integer;
   end record;

 type C is new B with
   record
     Z : integer;
   end record;

   Jim  : C;
   Jane : B;
   Save : integer;

 begin
   Jim := C (Jane);
   Save := Jim.Z;
 end;