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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.ada
Subject: Re: Problem with Position of the enumeration Type
Date: Wed, 23 Jan 2019 20:34:55 -0800
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Date: 2019-01-23T20:34:55-08:00
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Keith Thompson <kst-u@mib.org> writes:
> "Randy Brukardt" <randy@rrsoftware.com> writes:
>> "Simon Wright" <simon@pushface.org> wrote in message 
>> news:lyzhrruww2.fsf@pushface.org...
>> ...
>>> As Karl Müller <mtb23@gmx.de> hinted, you can use the non-standard GNAT
>>> implementation-defined attribute 'Enum_Rep[1]. The next page describes
>>> 'Enum_Val.
>>
>> It's Standard, just not until Ada 2020. See 
>> http://www.ada-auth.org/standards/2xrm/html/RM-13-4.html#p10.1.
>>
>> (Yes, borrowed from GNAT.)
>
> I'm glad to see this being standardized.

GNAT 7.30 has a bug in its implementation of the 'Enum_Val attribute.
In some cases, it appears to examine only the low-order bits of the
argument, causing it to yield a valid value when it should raise
Constraint_Error.

A demo:

    with Ada.Text_IO; use Ada.Text_IO;
    procedure Enum_Bug is
        type Enum is (Two, Four);
        for Enum use (2, 4);
    begin
        for I in 10 .. 11 loop
            begin
                Put(Integer'Image(I) & ": ");
                Put_Line(Enum'Image(Enum'Enum_Val(I)));
            exception
                when Constraint_Error =>
                    Put_Line("Constraint_Error");
            end;
        end loop;
    end Enum_Bug;

Expected output:

     10: Constraint_Error
     11: Constraint_Error

Actual output:

     10: TWO
     11: Constraint_Error

I'll also report this via https://www.adacore.com/community/contact

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */