Question about subtype of loop index variable

[Paul Graham <pgraham@cadence.com>]

I have a question about the type and subtype of a loop index variable. 
I'll just
give an example:

with Text_Io; use Text_Io;
procedure Test_Range is
   package Int_Io is new Integer_Io(Integer); use Int_Io;
   type T1 is range 0 .. 7;
   type T2 is array(T1 range <>) of Integer;
   X : T2(0 .. 3);
   Y : Integer;
begin
   Y := 0;
   for I in X'First - 1 .. X'Last + 1 loop
      Y := Y + 1;
   end loop;
   Put("Y = ");
   Put(Y);
   New_Line;
end;

My question is, what is the type (and subtype) of loop index I?  Since
the
expressions X'First - 1 and X'Last + 1 are of type T1, then
I is also of type T1.  But the values X'First - 1 and X'Last + 1 lie
outside
the bounds of T1, so I would think that a range constraint error would
occur
as I is assigned these values.  Yet gnat 3.13 compiles and executes this
code 
without error.  Am I misunderstanding something?

Paul