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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 19:56:05 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2013-03-29T19:56:05-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1ex3dm2hk2j54$.1uiyljwziv0hi$.dlg@40tude.net...
...
>> But, do you think it should have failed at instantiation?
>
> It should fail to compile, instantiation time is too late. Generic
> contracts are much too weak.

This doesn't make much sense, as you then go on to suggest adding a formal 
contract which would have the effect of rejecting an instantiation if the 
type actually had Foo. That's exactly what Ada does, so I don't see any 
advantage of a complication.

The problem you are having is that you are viewing the contract of a generic 
in too narrow of a fashion. The contract includes not only the things in the 
generic part, but also a number of properties expressed in the rest of the 
specification. After all, almost all Ada legality rules are rechecked in 
instance specifications, and these too have to be considered part of the 
contract.

                                      Randy.