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From: "Alex R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: Bug with variant derived from interface?
Followup-To: comp.lang.ada
Date: Thu, 04 Jun 2009 10:14:21 +0200
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Date: 2009-06-04T10:14:21+02:00
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Adam Beneschan wrote:

> On Jun 3, 10:09 am, "Alex R. Mosteo" <alejan...@mosteo.com> wrote:
>> Before reporting a bug to Adacore I'd like to have your opinion. With the
>> following test program I get the error below in both GPL2008/2009.
>>
>> procedure Bugcase is
>>
>> --  type Base is abstract tagged null record;
>> --  This works
>>
>> type Base is interface;
>> --  This does not work.
>>
>> type Derived_Ok (X : Boolean) is new Base with null record;
>>
>> type Derived_Also_Ok (X : Boolean) is new Base with record
>> Y : Integer;
>> end record;
>>
>> type Derived_Bad (X : Boolean) is new Base with record
>> --  Above line is the one failing.
>> case X is
>> when others => null;
>> end case;
>> end record;
>>
>> begin
>> null;
>> end Bugcase;
>>
>> $ gnatmake bugcase.adb
>> gcc -c bugcase.adb
>> bugcase.adb:14:09: call to abstract function must be dispatching
>> gnatmake: "bugcase.adb" compilation error
> 
> I don't see any reason this program should be illegal.  The message
> involving a "call to abstract function" is certainly wrong, because
> there aren't any abstract function calls involved.  (Just as a wild
> guess, I'm guessing that the abstract function GNAT is complaining
> about is "=", and there's some special subroutine that tries to
> compose the equality function for variant records that isn't handling
> this case right...  did I come close???)

No idea... but certainly the error, which I've only seen before in 
statements and not in declarations, hints at the compiler being astray. I'll 
report it and see.