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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Interresting,
 possibly buggy behavior in GNAT generics w/ expression function.
Date: Fri, 29 Mar 2013 20:04:28 -0500
Organization: Jacob Sparre Andersen Research & Innovation
Message-ID: <kj5dn6$978$1@munin.nbi.dk>
References: <ac196de8-8d24-4302-8e3b-e0007d691095@googlegroups.com>
 <lysj3eir9b.fsf@pushface.org>
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Date: 2013-03-29T20:04:28-05:00
List-Id: <comp.lang.ada>

"Simon Wright" <simon@pushface.org> wrote in message 
news:lysj3eir9b.fsf@pushface.org...
> In Generic_Attribute_Set, you have two Image functions:
>
>    function Image(Value : Attribute_Values) return String
>
> (Attribute_Values is a generic formal scalar) and
>
>    function Image(Value : Positive) return String
>
> and you instantiate the package with Attribute_Values => Integer.
>
> What happens when you change the second Image's parameter to Integer is
> that GNAT chooses the wrong Image to call!

Something is wrong with this description, because as you have described it, 
any call on Image should be ambiguous and thus illegal. Specifically:

    generic
        type Attribute_Values is (<>);
    package Test_It is
        function Image(Value : Attribute_Values) return String;
        function Image(Value : Positive) return String;
   end Test_It;

   package A_Test is new Test_It (Integer);

This instantiation is legal. However, a call like
    A_Test.Image (10);
is ambiguous, because there is no possible way to know which routine should 
be called. It's certainly not possible to call the *wrong* one here, because 
they are exactly the same for resolution purposes.

Now, if they have different parameter names:
    function Image (Val : Positive) return String;
then
   A_Test.Image (Val => 10); -- Should work.
   A_Test.Image (10); -- Still ambiguous.

If GNAT is allowing these calls, *that's* the bug; what happens when you 
change subtypes is irrelevant.

Perhaps you guys would like to do as you are always griping to others and 
provide a complete enough example to see what you are really talking 
about???

                                      Randy.