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From: ncohen@watson.ibm.com (Norman H. Cohen)
Subject: Re: Record aggregate question (language lawyer needed!)
Date: 1996/06/21
Message-ID: <4qf5el$1633@watnews1.watson.ibm.com>#1/1
X-Deja-AN: 161452941
distribution: world
references: <mcriley.835361851@BIX.com>
organization: IBM T.J. Watson Research Center
keywords: record aggregate bug
reply-to: ncohen@watson.ibm.com
newsgroups: comp.lang.ada
Date: 1996-06-21T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <mcriley.835361851@BIX.com>, mcriley@BIX.com (mcriley on BIX) writes: 
|>
|> The following test program is accepted without question by one Ada
|> compiler and executes without any qualms, while another warns of
|> a constraint_error at runtime.  Which is correct, or, is the exact
|> implementation left up to the compiler implementor?
|>
|> procedure Rec_Test is
|>
|>    type Task_Record_Type is
|>      record
|>          Originator : String (1 .. 20);
|>          Header_Dtg : String (1 .. 12);
|>      end record;
|>
|>     Task_Record : Task_Record_Type := (others => (others => ' '));
|>
|> begin
|>   null;
|> end rec_test;
|>
|> The relevant part of the LRM is 4.3.1[1]: "A component association
|> with the choice others [...] is only allowed if the represented
|> components are all of the same type."  Well the components are
|> both of the same _type_, but obviously the subtype constraints vary.

Since you quote the Ada-83 RM, Ada-83 AI-00244 applies.  The summary of
that AI is: 

   In a record aggregate, a component association having multiple choices
   denoting components of the same type is considered equivalent to a
   sequence of single choice component associations representing the same
   components.

Thus, what you have written is equivalent to

   Task_Record:  Task_Record_Type :=
                    (Originator => (others => ' '),
                     Header_Dtg => (others => ' ') );

It should work without problem.  Perhaps the compiler that raises
Constraint_Error (or threatens to) predates AI-00244, which received
final approval in November 1986.

The URL for the full text of AI83-00244 is: 

ftp://sw-eng.falls-church.va.us/public/AdaIC/standards/83com/ai-00244-bi.wj

(By the way, the Ada-95 equivalent of the rule in RM83-4.3.1(1) is in the
last sentence of RM95-4.3.1(16).  The effect of AI83-00244 is captured in
RM95-4.3.1(20).)

--
Norman H. Cohen    ncohen@watson.ibm.com