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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Incomplete type generic formal interactions with
 Implicit_Dereference
Date: Mon, 22 Jan 2018 19:08:57 -0600
Organization: JSA Research & Innovation
Message-ID: <p461v9$3m6$1@franka.jacob-sparre.dk>
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Xref: reader02.eternal-september.org comp.lang.ada:50060
Date: 2018-01-22T19:08:57-06:00
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"Jere" <jhb.chat@gmail.com> wrote in message 
news:21b6b4fb-4648-419e-ae6c-c361d54eaa2f@googlegroups.com...
...
> Part of me thinks that since Some_Type is incomplete, that Ada wouldn't
> allow for an Implicit_Dereference declared on it.  I guess it depends on
> if it is meant to be evaluated in the compilation of the generic itself
> or at the point at which the function is called.
>
> Anyone have any thoughts on this?

I don't think we ever considered this case. But I think it is OK.

Specifically, 4.1.5(6/3) says that a generalized reference denotes a view 
equivalent to that of a dereference of the reference discriminant of the 
reference object. Thus, any Legality Rules that apply to the dereference of 
the discriminant apply to the generalized reference as well. For instance, 
this includes any accessibility rules (which was an important part of the 
design, as the accessibility rules prevent the access value from living 
longer than the object containing the access discriminant).

As such, any other Legality Rules, like the ones about incomplete types 
would also apply. Specifically, 3.10.1(10/3): "A prefix that denotes an 
object shall not be of an incomplete view."

So a use of an Implicit_Dereference wherever the type is incomplete would 
usually be illegal (there are a few ways that a dereference of an incomplete 
view can be used, mainly to be passed as a parameter to some routine with an 
access-to-incomplete parameter). But it would be OK where the type has been 
completed (in this case, outside of the generic instantiation, the way you 
intended to use this). This sounds like what you intended.

                                   Randy.