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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Indefinite Containers of Indefinite Private Types
Date: Tue, 04 Aug 2015 08:40:36 +0300
Organization: Tidorum Ltd
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Xref: news.eternal-september.org comp.lang.ada:27350
Date: 2015-08-04T08:40:36+03:00
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On 15-08-04 05:23 , Jeffrey R. Carter wrote:
> This is legal:
>
> package Indefinite1 is
>     type T (<>) is private;
> private -- Indefinite1
>     type T is new String;
> end Indefinite1;
>
> with Indefinite1;
> with Ada.Containers.Indefinite_Vectors;
>
> package Indefinite2 is
>     use type Indefinite1.T;
>
>     package Lists is new Ada.Containers.Indefinite_Vectors
>        (Index_Type => Positive, Element_Type => Indefinite1.T);
> end Indefinite2;
>
> But this isn't:
>
> with Ada.Containers.Indefinite_Vectors;
>
> package Indefinite1 is
>     type T (<>) is private;
> private -- Indefinite1
>     package Lists is new Ada.Containers.Indefinite_Vectors
>        (Index_Type => Positive, Element_Type => T);
>
>     type T is new String;
> end Indefinite1;
>
> It seems to me that both instantiations have all the information they need.
> Probably I'm missing something, but why is the latter illegal?

I'm not sure that I have understood your question correctly -- are you 
asking for the RM rule that makes this illegal, or for the reason behind 
that rule?

At the point of instantiation in the second case, "T" is a partial view 
of the type. RM 7.3(5) says "... Similarly, before the full declaration, 
the name of the partial view cannot be used in a generic_instantiation 
or in a representation item."

As to why this rule exists, I suspect it has to do with the "rechecks" 
discussed in the annotated RM 12.3(11) where the legality of an 
instantiation is checked by comparing the actual types with the _full_ 
declaration of the generic (not only with the declarations of the 
generic formals). It might be difficult to apply these rechecks if the 
actual type is only a partial view.

The second case becomes legal if the full declaration of type T is moved 
to occur before the instantiation -- as happens in the first case -- but 
you probably knew that.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .