Discriminant ans tagged type ?!

I just do not understand why the following code compiles with the Aonix 
compiler and not with the Gnat GPL 2007?

     package A_Pkg is
         type A (L : Natural) is tagged null record;
     end A_Pkg;

     package A_Pkg.B_Pkg is
         type B is new A with record
             T : String(1..L);
         end record;
     end A_Pkg.B_Pkg;

     with A_Pkg.B_Pkg;
     procedure Strange is
     begin
         null;
     end Strange;

    => RM95 3.7 (18) ??