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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: I am leaving Ada :-( because of GNAT bugs
Date: Mon, 11 Dec 2017 16:47:26 -0600
Organization: JSA Research & Innovation
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"Simon Wright" <simon@pushface.org> wrote in message 
news:lyy3mctfad.fsf@pushface.org...
> bj.mooremr@gmail.com writes:
...
>> 9. In rdf-redland-query_results.ads, you declare a type derived from a
>> tagged type, but then try to apply a Pre'Class aspect. This apparently
>> is not legal, though the GPL 2016 compiler doesn't complain. I ran
>> into this, with my own code, that GPL 2017 told me with a compiler
>> error that the code was not legal.
>>
>> Pre'Class can only be applied to a root type apparently.
>
> See ARM 6.1.1(17.1/4), "Pre'Class shall not be specified for an
> overriding primitive subprogram of a tagged type T unless the Pre'Class
> aspect is specified for the corresponding primitive subprogram of some
> ancestor of T."

That was a rule change in the 2015 Corrigendum. It was made because the 
derived Pre'Class will be ored with the inherited one, and if you've 
inherited from a routine with no Pre'Class, you are oring with True -- which 
means your new Pre'Class will be completely ignored.

Pre'Class makes sense mainly for a root class; one generally will use 
dispatching calls within it so it can adjust as needed for child types. If 
you need a precondition that doesn't fit into that model, you should use Pre 
(and be aware that you are violating the basic LSP approach).

                            Randy.