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+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ Problem with ``reference`` and old/new iterator category correspondance
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+.. _N1550: http://www.boost-consulting.com/writing/n1550.html
+.. _N1530: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1530.html
+
+:Author: David Abrahams and Jeremy Siek
+:Contact: dave@boost-consulting.com, jsiek@osl.iu.edu
+:Organization: `Boost Consulting`_, Indiana University Bloomington
+:date: $Date$
+:Copyright: Copyright David Abrahams, Jeremy Siek 2003. Use, modification and
+      distribution is subject to the Boost Software License,
+      Version 1.0. (See accompanying file LICENSE_1_0.txt or copy
+      at http://www.boost.org/LICENSE_1_0.txt)
+
+.. _`Boost Consulting`: http://www.boost-consulting.com
+
+==============
+ Introduction
+==============
+
+The new iterator categories are intended to correspond to the old
+iterator categories, as specified in a diagram in N1550_. For example,
+an iterator categorized as a mutable Forward Iterator under the old
+scheme is now a Writable, Lvalue, and Foward Traversal iterator.
+However, there is a problem with this correspondance, the new iterator
+categories place requirements on the ``iterator_traits<X>::reference``
+type whereas the standard iterator requirements say nothing about the
+``reference`` type . In particular, the new Readable Iterator
+requirements say that the return type of ``*a`` must be
+``iterator_traits<X>::reference`` and the Lvalue Iterator requirements
+says that ``iterator_traits<X>::reference`` must be ``T&`` or ``const
+T&``.
+
+
+====================
+ Proposed Resolution
+====================
+
+Change the standard requirements to match the requirements of the new
+iterators. (more details to come)
+
+
+==========
+ Rationale
+==========
+
+The lack of specification in the standard of the ``reference`` type is
+certainly a defect. Without specification, it is entirely useless in a
+generic function. The current practice in the community is generally
+to assume there are requirements on the ``reference`` type, such as
+those proposed in the new iterator categories.
+
+There is some danger in *adding* requirements to existing concepts.
+This will mean that some existing iterator types will no longer meet
+the iterator requirements. However, we feel that the impact of this is
+small enough to warrant going ahead with this change.
+
+An alternative solution would be to leave the standard requirements as
+is, and to remove the requirements for the ``reference`` type in the
+new iterator concepts. We are not in favor of this approach because it
+extends what we see as a defect further into the future.