$ cat track_number.ads
package Track_Number is
   type Number_Type is abstract tagged private;
   function "=" (Left, Right : Number_Type) return Boolean is abstract;
private
   type Number_Type is abstract tagged null record;
end Track_Number;
$ gnatmake -gnatwh track_number.ads
gcc -c -gnatwh track_number.ads
track_number.ads:3:13: warning: declaration of "=" hides one in package Standard
$