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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Type of subtraction operator
Date: Thu, 28 May 2009 14:23:52 -0700 (PDT)
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Date: 2009-05-28T14:23:52-07:00
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According to 4.5.3/2, each numeric type has a subtraction operator
with this specification:

function "-"(Left, Right : T) return T

This means that the type of difference is the same as the type of
operands.

Which means that if T is Natural, then I should expect
CONSTRAINT_ERROR when the result of subtraction is less than 0.
But it does not happen:

with Ada.Text_IO;
procedure Test is

   X : Natural := 5;
   Y : Natural := 7;
   Z : Integer := X - Y;   -- here the result is not in Natural'Range

begin
   Ada.Text_IO.Put_Line (Integer'Image (Z));
end Test;

The above happily prints -2.
I thought initially that it is a subtype that is responsible for this,
but defining My_Natural type as a completely new type (with
appropriate type cast when Z is initialized) gives the same result.

I conclude that the effective type of difference (the return type of
the "-" operator) is T'Base instead of T.
Is that right? Is this a bug in AARM or did I misunderstand something?

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Database Access Library for Ada: www.inspirel.com/soci-ada