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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: bug or feature
References: <136dc796-8f60-4fc0-a2f8-70e0ca24c9f1@googlegroups.com>
Date: Mon, 08 Sep 2014 03:51:47 -0500
Message-ID: <854mwi1om4.fsf@stephe-leake.org>
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Date: 2014-09-08T03:51:47-05:00
List-Id: <comp.lang.ada>

Charly <carl.weierstrass@googlemail.com> writes:

>   1   with Ada.Text_IO;
>   2   with Datum_Pkg;
>   3  
>   4   procedure Main is
>   5 
>   6      subtype Datum is Datum_Pkg.Datum;
>   7      subtype Datum_Vector is Datum_Pkg.Datum_Vectors.Vector;
>   8     
>   9      D_Vec : Datum_Vector;
>  10     
>  11   begin
>  12   
>  13      for Dat : Datum_Pkg.Datum of D_Vec loop
>  14         Ada.Text_IO.Put_Line ("Datum=" & Dat.Image);
>  15      end loop;
>  16      
>  17      for Dat : Datum of D_Vec loop
>  18         Ada.Text_IO.Put_Line ("Datum=" & Dat.Image);
>  19      end loop;   
>  20      
>  21   end Main;
>
> Line 13 compiles without message but in line 17 I get an error message:
>    subtype indication does not match element type
> But this line should be equivalent to line 13 because of the subtype definition
> in line 6.

line 6 defines a _new_ subtype, _different_ from Datum_Pkg.Datum. They have
the same base type, but the error message says "subtype", not "base
type".

So you are out of luck.

In general, a subtype can add constraints, which is why this matters; if
the loop parameter has different constraints than the container object,
you'll get runtime errors.

It might be possible to allow a "no new constraints" subtype such as
yours to be considered to match, but I suspect there are problems with
that as well.

-- 
-- Stephe