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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: derived formal types and known discriminants
Date: Fri, 23 Jun 2017 12:26:11 -0500
Organization: JSA Research & Innovation
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Date: 2017-06-23T12:26:11-05:00
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<sbelmont700@gmail.com> wrote in message 
news:a9316065-923c-439e-9176-8f3eb469c601@googlegroups.com...
>Can any language lawyer explain the reasoning behind precluding known 
>discriminants
>for general formal derived types?

I don't know, I can only guess. And my guess would be that discriminants 
that are added/removed would be a problem. Within the generic, you could 
declare objects giving those discriminant values, but they might not be 
right. For a type with unknown discriminants, you can't declare an object at 
all, so that's easier.

But why using the same matching rules used for formal private types wouldn't 
work, I don't know. Or maybe it just didn't seem worth worrying about, given 
that formal derived types are mainly useful to give Steve Baird a rare 
mechanism for the amazing semantic issues that he comes up with. (Yes, 
formal tagged derived types have some uses in deep type hierarchies, and 
that's about it, because no one declares other kinds of derived types in the 
first place.)

                                      Randy.