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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Problem with limited with
Date: Fri, 26 Jul 2013 20:14:34 +0100
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Date: 2013-07-26T20:14:34+01:00
List-Id: <comp.lang.ada>

A poster on Stack Overflow[1] has a problem with mutually recursive
types. The essence to the problem is here:

   limited with Pak2;
   package Pak1 is
      type Pak2_T2_P is access all Pak2.T2;
      task type T1 is
         entry E1 (T2 : Pak2_T2_P);
      end T1;
   end Pak1;

   package Pak2 is
      task type T2 is
         entry E2;
      end T2;
   end Pak2;

   with Pak2;
   package body Pak1 is
      task body T1 is
         The_T2 : Pak2_T2_P;
      begin
         accept E1 (T2 : Pak2_T2_P) do
            The_T2 := T2;
         end E1;
         The_T2.E2;             --  compilation fails
         The_T2.all.E2;         --  compilation succeeds
      end T1;
   end Pak1;

The compiler's error message is
   invalid prefix in selected component "The_T2"
   dereference must not be of an incomplete type (RM 3.10.1)

I can see from 3.10.1(2.2) [2] that the dereference might be incomplete
- but (2.4) seems to say it should be OK.

I note that all the examples of using limited with use anonymous access
types, which can't be used as entry parameters; if I replace
   task body T1 is
      The_T2 : Pak2_T2_P;
   begin
by
   task body T1 is
      The_T2 : access Pak2.T2;
   begin
then the compilation succeeds.

Does this look like a GNAT bug?

[1]
http://stackoverflow.com/questions/17869802/using-limited-with-in-ada-to-avoid-circular-dependency
[2]
http://www.adaic.org/resources/add_content/standards/12rm/html/RM-3-10-1.html#p2.2