Proposed resolution:

This wording is relative to N4800.

1. Change class template istreambuf_iterator synopsis, [istreambuf.iterator], as indicated:

   template<class charT, class traits = char_traits<charT>>
   class istreambuf_iterator {
   public:
     using iterator_category = input_iterator_tag;
     using value_type        = charT;
     using difference_type   = typename traits::off_type;
     using pointer           = voidunspecified;
     using reference         = charT;
     […]
   };
   

[ 2019-10-30, Jonathan comments ]

Also, reverse_iterator::operator->() is now constrained and so not always defined, but reverse_iterator::pointer is defined unconditionally.

[ 2019-03-05, Daniel comments ]

With the acceptance of P1252R2 the committee decided to deprecate operator-> of move_iterator, interestingly without mentioning what should happen with its current pointer typedef (which is equal to the template parameter Iterator and thus never void). Effectively this is a very similar situation as for the here discussed istreambuf_iterator case and it seems attractive to me to solve both cases similarly.

[ 2019-03-05 Priority set to 3 after reflector discussion ]

[ 2019-03-03, Daniel provides concrete wording ]

The current working paper requires iterator_traits<Iter>::pointer to be void if Iter doesn't define operator-> ([iterator.traits]). We recently removed operator-> from istreambuf_iterator via LWG 2790, therefore either its pointer member should be void, or there should be a partial specialization of iterator_traits. Do we want to change unspecified to void in the class synopsis in [istreambuf.iterator]?