Re: Bus error

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Georg Bauhaus <bauhaus.rm.tsoh@maps.futureapps.de> writes:

> Dmitry A. Kazakov wrote:
>> On Fri, 29 Jun 2007 17:44:22 +0200, Georg Bauhaus wrote:
>>
>>> Dmitry A. Kazakov wrote:
>>>>    function Very_Positive return Integer is
>>>>    begin
>>>>        return -1;
>>>>    end Very_Positive;
>>>>
>>>>    Oops : Positive renames Very_Positive; -- This is OK!

> Oops will at some point raise Constraint error;...

Dmitry's complaint is the above will NOT raise C_E.
The subtype of Oops is Integer, not Positive,
and its value is -1.  Oops is a constant (even though
it doesn't say so).

The fact that "Positive" is completely ignored by the compiler is
clearly a language design flaw.

From AARM-8.5.1:

6 An object_renaming_declaration declares a new view [of the renamed
object] whose properties are identical to those of the renamed
view. [Thus, the properties of the renamed object are not affected by
the renaming_declaration. In particular, its value and whether or not it
is a constant are unaffected; similarly, the constraints that apply to
an object are not affected by renaming (any constraint implied by the
subtype_mark of the object_renaming_declaration is ignored).]

    6.a   Discussion: Because the constraints are ignored, it is a good
      idea
          to use the nominal subtype of the renamed object when writing
      an
          object_renaming_declaration.

>...how can a compiler
> be supposed to known that some Integer function will *always*
> return non-Positives?

Indeed!  Likewise, how is the _programmer_ supposed to know?

>...What makes you think that the programmer
> who has written the Oops doesn't know what he is
> doing using a subtype, not a new type?
>
> I think it is a program design fault, if a fault at all (or a flaw as
> Bub Duff notes).

Yes, I agree with you and with my evil twin Bub ;-) that it is a flaw in
the program.  The programmer should have written:

    Oops : Integer renames Very_Positive;

But it's exactly the sort of flaw that the compiler ought to be required
to detect, and give an error at compile time.  (Note that the compiler
doesn't need to look inside the body of Very_Positive to detect the
"flaw".)

Actually, the programmer really should have written:

    Oops : constant Integer := Very_Positive;

but that's a different story.

- Bob