Re: GNAT or Language Problems?

["Randy Brukardt" <randy@rrsoftware.com>]

"Jeffrey R.Carter" <spam.jrcarter.not@spam.acm.org.not> wrote in message 
news:u76n8o$3873$1@dont-email.me...
> On 2023-06-23 11:55, Randy Brukardt wrote:
>>
>> Sigh. :Let's start over. My theory was that the problem came from an 
>> attempt
>> by AdaCore to implement AI22-0041-1, a post Ada-2022 binding 
>> interpretation.
>> That AI adds a static compatibility requirement to 7.4(6/3) (making it
>> 7.4(6/6)). (It of course could just be a bug, too, but AI22-0041-1 seems
>> like the only sane reason to be changing the matching code in a working
>> compiler that passes the ACATS.)
>>
>> There is *no* version of the RM currently that includes the wording from
>> post-Ada 2022 AIs (because of we had to wait for ISO to publish the thing
>> first). You can only find that in the >50 approved AIs.
>
> Thanks for this clarification. I didn't realize I would have to look at 
> the AI itself. I'm using -gnat12, in hopes that the Ada-12 rules would be 
> applied, but that doesn't seem to work.
>
> [New version of 7.4(6):]
>
>>    a.. If the deferred constant declaration includes a subtype_indication
>> that defines a subtype S1, then the subtype_indication in the full
>> declaration shall define a subtype S2 that is statically compatible with 
>> S1
>> (see 4.9.1). If S1 is a constrained subtype, the constraint defined by S2
>> shall statically match the constraint defined by S1. [Redundant: If the
>> subtype S1 of the deferred constant is unconstrained, then the full
>> declaration is still allowed to impose a constraint.]
>
> As my case doesn't involve the concepts involved in the other PPs, aliased 
> and null exclusion, I presume that this PP applies. The essential parts of 
> the code are
>
>    type B_String (Max_Length : Positive := 1_000) is tagged limited 
> private;
>    Null_B_String : constant B_String;
>    ...
>    Null_B_String : constant B_String := (Max_Length => 1, others => <>);
>
> I would think that the subtype indications in the two, being identical, 
> would be statically compatible. But my understanding of this may be wrong.
>
> I'm not sure whether the deferred declaration has a constrained subtype, 
> which would result in the constraint from the initialization expression of 
> the full declaration not matching. But that rule has existed since Ada 95.
>
> I think it would be a mistake for the language to require explicit 
> discriminant associations for this case.
>
> So I'm still not able to tell if this is a compiler error or intended by 
> the language.

A subtype is always supposed to statically match itself (even though that 
isn't clear from them RM wording).

B_String is an unconstrained type, which technically has a "null constraint" 
(see 3.2(7/2), rather unfortuate terminology, IMHO(. These always statically 
match, so they're always statically compatible as well.

So this appears to be a compiler bug. It might be related to the relatively 
new rule that Object_Size must match for static matching to work -- but that 
only applies to non-default Object_Size values (and there are none of those 
given here).

                     Randy.