Proposed resolution:

This wording is relative to N4944.

1. Change [istreambuf.iterator.general] as indicated:

   
       constexpr istreambuf_iterator() noexcept;
       constexpr istreambuf_iterator(default_sentinel_t) noexcept;
       istreambuf_iterator(const istreambuf_iterator&) noexcept = default;
       ~istreambuf_iterator() = default;
       istreambuf_iterator(istream_type& s) noexcept;
       : istreambuf_iterator(s.rdbuf()) { }
       istreambuf_iterator(streambuf_type* s) noexcept;
       istreambuf_iterator(const proxy& p) noexcept;
       …
   
     private:
       streambuf_type* sbuf_;              // exposition only
       int_type c_{};                      // exposition only
     };
   }
   

2. Change [istreambuf.iterator.cons] as indicated:

   For each istreambuf_iterator constructor in this section, an end-of-stream iterator is constructed if and only if the exposition-only member sbuf_ is initialized with a null pointer value or if sbuf_->sgetc() returns traits_type::eof().

   constexpr istreambuf_iterator() noexcept;
   constexpr istreambuf_iterator(default_sentinel_t) noexcept;
   

   -1- Effects: Initializes sbuf_ with nullptr.

   istreambuf_iterator(istream_type& s) noexcept;

   -2- Effects: Initializes sbuf_ with s.rdbuf().

   istreambuf_iterator(streambuf_type* s) noexcept;
   

   [Drafting note: sgetc() can throw, but this function is noexcept. Should it swallow exceptions and create an end-of-stream iterator, to avoid weakening the exception spec of an existing function?]

   -3- Effects: Initializes sbuf_ with s. If s is not null, initializes c_ with s->sgetc(). Sets sbuf_ to null if sgetc exits via an exception, or if traits_type::eq_int_type(c_, traits_type::eof()) is true.

   istreambuf_iterator(const proxy& p) noexcept;
   

   -4- Effects: Initializes sbuf_ with p.sbuf_. If p.sbuf_ is not null, initializes c_ with p.keep_.

3. Change [istreambuf.iterator.ops] as indicated:

   charT operator*() const;
   

   -?- Preconditions: sbuf_ is not null.

   -1- Returns: The character obtained via the streambuf member sbuf_->sgetc(). traits_type::to_char_type(c_).

   -?- Throws: Nothing.

   istreambuf_iterator& operator++();
   

   -?- Preconditions: sbuf_ is not null.

   -2- Effects: As if by sbuf_->sbumpc(). Performs c_ = sbuf_->snextc(), then sets sbuf_ to null if traits_type::eq_int_type(c_, traits_type::eof()) is true.

   -3- Returns: *this.

   proxy operator++(int);
   

   -4- Returns: proxy(sbuf_->sbumpc(), sbuf_).
   Effects: Equivalent to:

   proxy p(**this, sbuf_);
   ++*this;
   return p;
   

   bool equal(const istreambuf_iterator& b) const;
   

   -5- Returns: bool(sbuf_) == bool(b.sbuf_).

   [Note: This is true if and only if both iterators are at end-of-stream, or neither is at end-of-stream, regardless of what streambuf object they use. — end note]

   template<class charT, class traits>
     bool operator==(const istreambuf_iterator<charT, traits>& a,
                     const istreambuf_iterator<charT, traits>& b);
   

   -6- Returns: a.equal(b).

   bool equal(const istreambuf_iterator& i, default_sentinel_t s) const;
   

   -7- Returns: i.equal(s) i.sbuf_ == nullptr.