The Ada front-end expects the back-end to handle bitfields of
arbitrary type.  In the following example, a bitfield of struct type:

package Bit_Struct is
   type R is record
      A : Integer;
      B : String (1 .. 3);
   end record;

   type S is record
      An_R : R;
   end record;
   for S use record
      An_R at 0 range 4 .. 60;
   end record;

   An_S : constant S := (An_R => (A => 0, B => "Yo!"));
end;