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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: tagged primitive operation and freezing
Date: Sat, 13 Nov 2010 11:24:30 +0100
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Date: 2010-11-13T11:24:30+01:00
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troll writes on comp.lang.ada:
> generic
>   type Object_T is private;
> procedure blah ( Param : Object_T );
>
> -------
>
> with blah;
> package oops is
>   type a is tagged null record;  -- any tagged type
>
>  -- must be a type extension to induce failure:
>   type Flight_T is new a with null record;  -- fail
>
>   function b (Flight : in Flight_T) return String;
>
>   procedure c is new blah (Object_T => Flight_T);
>
> end;

I think the reason why this is illegal is because c is a primitive
operation of Flight_T; at the same time, using Flight_T as an actual
generic parameter freezes Flight_T just before the declaration of this
new primitive operation.  Therefore the declaration of c is illegal.

You can work around this by wrapping Flight_T in a package:

generic
  type Object_T is private;
procedure blah ( Param : Object_T );

with blah;
package oops is
  type a is tagged null record;  -- any tagged type

  package Flight is
  -- must be a type extension to induce failure:
     type T is new a with null record;
     function b (Flight : in T) return String;
  end Flight; -- this freezes Flight_T

  procedure c is new blah (Object_T => Flight.T); -- non-primitive

end oops;

-- 
Ludovic Brenta.