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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Community Input for the Maintenance and Revision of the Ada
 Programming Language
Date: Wed, 13 Sep 2017 21:41:18 +0200
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Xref: news.eternal-september.org comp.lang.ada:48109
Date: 2017-09-13T21:41:18+02:00
List-Id: <comp.lang.ada>

On 09/13/2017 08:34 PM, Egil H H wrote:
> On Wednesday, September 13, 2017 at 6:34:43 PM UTC+2, Jeffrey R. Carter wrote:
>>
>> No. If you supply a discriminant, the object is constrained to that subtype.
>> Only is you do not supply a discriminant is the object unconstrained.
>>
> 
> That's wrong.
> 
> "If a discriminated type has default_expressions for its discriminants, then unconstrained variables of the type are permitted, and the values of the discriminants can be changed by an assignment to such a variable."
> See RM 3.7 (28)
> http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-3-7.html

Let's try it:

procedure Egilhh is
    type R (D : Integer := 0) is null record;

    C : R (D => 3);
begin -- Egilhh
    C := (D => 4);
end Egilhh;

$ gnatmake egilhh.adb
gcc-6 -c egilhh.adb
egilhh.adb:6:06: warning: incorrect value for discriminant "d"
egilhh.adb:6:06: warning: "Constraint_Error" will be raised at run time
gnatbind-6 -x egilhh.ali
gnatlink-6 egilhh.ali
$ ./egilhh

raised CONSTRAINT_ERROR : egilhh.adb:6 discriminant check failed

Unconstrained variables of the type are permitted, but so are constrained 
variables of the type. You declare an unconstrained variable by not providing a 
discriminant. You declare a constrained variable by providing the discriminant. 
This has been true since Ada 83. See the examples at the end of ARM 3.7.1:

15
Large   : Buffer(200);  --  constrained, always 200 characters
                         --   (explicit discriminant value)
Message : Buffer;       --  unconstrained, initially 100 characters
                         --   (default discriminant value)

-- 
Jeff Carter
"If a sperm is wasted, God gets quite irate."
Monty Python's the Meaning of Life
56