Relaxing Requirement on Iterator++ result
  -----------------------------------------              26-Jan-95
  by Nathan Myers  <myersn@roguewave.com>               X3J16/95-0021
     Rogue Wave Software                                 WG21/0621
 
  The standard iterator istreambuf_iterator does not conform to the
  requirements in table 16 of Clause 17 describing Input Iterators.
  In particular, its operator++(int) returns not the iterator types
  themselves, but an object which is convertible to that type and
  defines some operations also performed by that type.
 
  This is necessary for performance, and does not affect the correctness
  of the algorithms which use the iterators.  This proposal is to relax
  the requirements mentioned, and also similar requirements in tables
  17, 18, 19, and 20 on operators ++ and --.  Specifically, I would
  change the notation of the result type of expressions like "r++" from
  "X&" to "convertible to const X&".  Furthermore, I would add to the
  requirements, for each expression r++, r--, a corresponding expression
  like *r++, *r--, specified to yield the same result as *a in the same
  table, and with semantics like the composition of the two operators.
 
  The reason this makes a difference is that the result of r++ can
  implement operator* directly, without first converting to the iterator
  type; and allowing it permits a much more efficient iterator.
 
  If this proposal is not accepted, we will need to change the
  specification of the istreambuf_iterator, at a cost in performance.
  Further, user iterators would be overconstrained at a similar cost in
  performance.