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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Compiler bug
Date: Tue, 1 Apr 2008 04:52:50 -0700 (PDT)
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Date: 2008-04-01T04:52:50-07:00
List-Id: <comp.lang.ada>

Consider a trivial example with composition of interfaces:

with Ada.Text_IO;

procedure A is

   package Stuff is

      type Base_1 is interface;
      procedure P_1 (X : in Base_1) is abstract;

      type Base_2 is interface;
      procedure P_2 (X : in Base_2) is abstract;

      type Middle is interface and Base_1 and Base_2;

      type Concrete is new Middle with null record;
      procedure P_1 (X : in Concrete);
      procedure P_2 (X : in Concrete);

      function Make_Concrete return Concrete;

   end Stuff;

   package body Stuff is

      procedure P_1 (X : in Concrete) is
      begin
         Ada.Text_IO.Put_Line ("Concrete.P_1");
      end P_1;

      procedure P_2 (X : in Concrete) is
      begin
         Ada.Text_IO.Put_Line ("Concrete.P_2");
      end P_2;

      function Make_Concrete return Concrete is
         C : Concrete;
      begin
         return C;
      end Make_Concrete;

   end Stuff;

   use Stuff;

   B_1 : Base_1'Class := Make_Concrete;
   B_2 : Base_2'Class := Make_Concrete;

begin
   B_1.P_1;
   B_2.P_2;
end;

$ gnatmake a
$ ./a
Concrete.P_1
Concrete.P_1
$

In other words, both calls dispatched to the same procedure.

Some experiments led me to the interesting observation:
Changing this:

      type Middle is interface and Base_1 and Base_2;

to this:

      type Middle is interface and Base_2 and Base_1;

results in:

$ ./a
Concrete.P_2
Concrete.P_2

Again, both calls dispatched to the same procedure, but now I know
that in both cases the dispatch goes to the *first* progenitor of the
Middle interface. Obviously against 3.9.4-1/b.

Things get particularly funny when the signatures of P_1 and P_2 are
different (say, they have different sets of parameters).

It all looks like the v-table got messed up.

$ gnatmake --version
GNATMAKE 4.4.0 20080314 (experimental) [trunk revision 133226]

It is the newest version I could find for my system.

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com