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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Profile mismatch?
Date: Thu, 13 Mar 2014 16:35:40 +0000
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Date: 2014-03-13T16:35:40+00:00
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adambeneschan@gmail.com writes:

> On Thursday, March 13, 2014 5:06:40 AM UTC-7, Simon Wright wrote:
>> Given the code below
>> 
>>    package Profile is
>>       type A1 is abstract tagged limited null record;
>>       procedure P (This : not null access A1);
>>       type A2 is abstract new A1 with null record;
>>       procedure P (This : access A2);              -- mismatch
>>       type T1 is new A1 with null record;
>>       procedure P (This : access T1);              -- mismatch
>>       type T2 is new A2 with null record;
>>       procedure P (This : not null access T2);     -- mismatch
>>       type T3 is new T2 with null record;
>>       procedure P (This : access T3);              -- mismatch
>>    end Profile;
>> 
>> should a compiler recognise the mismatches of the null exclusions where
>> indicated?
>> 
>> GNAT (4.8, 4.9, GPL 2013) accepts this code without comment. I've
>> seen very similar cases (structurally, but with many more
>> subprograms) which GNAT didn't accept, saying
>> 
>>    not subtype conformant with operation inherited at line 42
>>    type of "This" does not match
>> 
>> I've not yet tried to track this down.
>
> All of the access parameters are automatically "not null", because
> they are controlling parameters; a parameter of type "access T" is a
> controlling parameter if it's for a primitive subprogram of type T
> where T is tagged.
>
> In Ada 95, which didn't have "not null", the rules said that
> parameters in that position were required to be non-null.  When "not
> null" was added to Ada 2005, the decision was made to allow, but not
> require, the "not null" on controlling parameters, for backward
> compatibility.  That's not the case for parameters in other positions,
> which is why you may see "not subtype conformant" errors.

This is what GNAT is doing (I take it you don't actually mean
"position"; in the first case "access T" had to be non-null in Ada 95,
and in the second case it's non-controlling parameters).

> "Full conformance" requires that the actual presence or absence of
> "not null" be the same, whether or not the null exclusion is implied.
> However, this is needed only in some contexts, such as when a
> subprogram is declared without a body in one place, and the same
> subprogram's body appears later.  Overriding doesn't require full
> conformance, I think; I think it only requires subtype conformance,
> although I'd have to look it up and I can't do so right now.

My code has subtype conformance, I think.

Thanks!