[Bug libstdc++/88151] New: Define iterator_traits::* for output iterators to actual values when possible (rather than to void)

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88151

            Bug ID: 88151
           Summary: Define iterator_traits::* for output iterators to
                    actual values when possible (rather than to void)
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

C++ Standard allows to have types other than `void` for output iterators:
http://eel.is/c++draft/iterator.traits#1

So std::iterator_traits could be specialized for std::*insert_iterator,
std::ostream_iterator, std::ostreambuf_iterator.

This could be useful for some users.