GNAT Error message

[Thierry&Annick <tajz@libertysurf.fr>]

Consider the following (incorrect) code "test.ada" :

generic
  type Index_Type is (<>);
package Generic_Array is
  type Instance_Type is array (Index_Type) of Integer;
  procedure Fill (This : out Instance_Type);
end Generic_Array;

package body Generic_Array is
  procedure Fill (This : out Instance_Type) is
  begin
    for Index in Index_Type loop
      This (Index) := Integer (Index - This'First);   -- Line 13
    end loop;
  end Fill;
end Generic_Array;

with Generic_Array;
procedure Check is
  type Int_Type is range 0 .. 10;
  package Int_Array is new Generic_Array (Int_Type);
begin
  null;
end Check;


The GNAT error message after gnatchop'ed (gnatchop -q -r -w) is :
test.ada:13:38: invalid operand types for operator "-"
test.ada:13:38: left operand has type "Index_Type" defined at test.ada:2

test.ada:13:38: right operand has type "Index_Type" defined at
test.ada:2
gnatmake: "check.adb" compilation error

Ok, my fault : Index_Type should be "range <>" to have a "-" operator,
but the error message is not clear.
Anybody knows what will this message be with others compilers ?