Dispatching problem.

[petter.fryklund.konsult@dynamics.saab.se (Petter Fryklund)]

We have the following:

package A is
  type Msg;
  type Msg_Ptr is access all Msg'Class;
  type Msg is abstract tagged null record;
  ... other declarations

  type M1 is new Msg with ....
  type M1_Ptr is access all M1;
  type M2 is new Msg with ....
  type M2_Ptr is access all M2;
  ... other declarations

  function X (Param : Integer) return Msg_Ptr;
end A;

with A;
package B is 
  type Msg is abstract new A.Msg with null record;
  type Msg_Ptr is access all Msg;
  type B1 is new A.M1 with null record;
  type B1_Ptr is access all B1;
  type B2 is new A.M1 with null record;
  type B2_Ptr is access all B2;

  procedure Y (MP : access B1);
  procedure Y (MP : access B2); 
end B;

with A;
with B;
procedure Main is

   X : A.Msg_Ptr;
   Y : A.Msg_Ptr;
begin
   X := A.X (1);        -- Building a A.M1
   Y := A.X (2);        -- Building a A.M2
   B.Y (B.Msg_Ptr (X)); -- causes Constraint_Error Tag Check Failed to be raised. 
end Main;

How can we dispatch in package B?