Yields "foo.adb:12:04: warning: types for unchecked conversion have different sizes"

foo.adb

with Ada.Unchecked_Conversion;
with Interfaces.C;
with Ada.Text_IO;

procedure Foo is
   type Bar is (Kol, Ra, Bi);
   for Bar use (Kol => 1, Ra => 2, Bi => 3);
   for Bar'Size use Interfaces.C.int'Size;
   
   subtype Cocktail_Bar is Bar range Kol .. Ra;
   
   function Convert is new Ada.Unchecked_Conversion (Source => Interfaces.C.int,
                                                     Target => Cocktail_Bar);
   Value : Interfaces.C.int := 2;
begin
   Ada.Text_IO.put (Convert (Value)'Img);
end Foo;