Change in GCC 5.1.0

[Simon Wright <simon@pushface.org>]

GCC 4.9.1 (and presumably GNAT GPL 2014) allowed variable indexing on a
Set, even though there was no Variable_Indexing aspect in the spec.

GCC 5.1.0 doesn't.

So this code (from Gprbuild GPL 2014) compiled and, presumably, worked
when it shouldn't have:

   with Ada.Containers.Ordered_Sets;
   procedure Iteration is
      type Slave is new Integer;
      package Slave_S is new Ada.Containers.Ordered_Sets
        (Element_Type => Slave);
      Pool : Slave_S.Set;
      procedure Iterate (Proc : access procedure (S : in out Slave)) is
      begin
         for C in Pool.Iterate loop
            declare
               S : Slave := Slave_S.Element (C);
            begin
               Proc (S);
               Pool (C) := S;     -- <<<<<<<<<<<<<<< wrong
            end;
         end loop;
      end Iterate;
   begin
      null;
   end Iteration;

The thing about an (ordered) set is that replacing an element must
involve re-ordering the set, in case the element's position has
changed. So the compilable code in this case is

               Pool.Replace_Element (C, S);

That said, that looks awfully like tampering to me (if the order
changes, anyway). I await Gprbuild GPL 2015 with interest.