Unchecked_Union record inside an other record - trouble

[Victor Porton <porton@narod.ru>]

I am trying to interface my program with a C library.

The below program does not compile:

main.adb:19:14: unconstrained subtype in component declaration

If this does not work, what should I do to interface a union inside a struct 
in a C library?

-- main.adb
with Interfaces.C; use Interfaces.C;

procedure Main is

   type Kind is (First, Second);
   
   type T(K: Kind) is
      record
         case K is
            when First  => I: int;
            when Second => C: char;
         end case;
      end record
   with Unchecked_Union, Convention=>C;

   type T2 is
      record
         Z: int;
         Ob: T;
      end record
   with Convention=>C;
   
begin
   null;
end;

-- 
Victor Porton - http://portonvictor.org