expression function bug or think?

[georg bauhaus <rmhost.bauhaus@maps.arcor.de>]

When I compile the following program with GNAT GPL 2011 on Mac,
I get confusingly different behavior depending on whether
or not I suppress checks. *Suppressing* the checks results in the
desired
program behavior. (The effect  reminds me of potential for Ariane 5
gossip—or simply of me being dense.)  When checks are *disabled*
(-gnatp), the program runs as expected and prints fib(10) = 55.
When checks are enabled (no -gnatp) I get a constraint error
on L.6.  Optimization does not affect the result, only -gnatp.

Is the program correct, at least in theory?

raised CONSTRAINT_ERROR : fib.ada:6 range check failed

package Functions is
function Fib (N : Natural) return Natural is
    (case N is
     when 0 => 0,
     when 1 => 1,
     when others => Fib(N-1) + Fib(N-2));  -- L.6
end Functions;

with Functions;
with Ada.Text_IO;

procedure Run_Fib is
    N : Natural;
begin
    if Functions.Fib(0) /= 0 then
         raise Program_Error;
    end if;
    N := Functions.Fib(10);
    Ada.Text_IO.Put_Line ("fib(10) =" & Natural'Image(N));
end Run_Fib;

The debugger says that N=0 at L.6. Therefore, my guess is that
translating in normal Ada mode (implying range checks) breaks
the instruction sequence for case somewhere.  (I haven't understood
the asm yet, takes time and effort since I don't
read that every day.)

Maybe there is a hint in 
http://gcc.gnu.org/ml/gcc-patches/2010-10/txt00134.txt
of
http://gcc.gnu.org/ml/gcc-patches/2010-10/msg00579.html

Is this a bug?