[Bug libstdc++/81857] New: istreambuf_iterator not work as input iterator

[abominable-snowman at yandex dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81857

            Bug ID: 81857
           Summary: istreambuf_iterator not work as input iterator
           Product: gcc
           Version: 6.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: abominable-snowman at yandex dot ru
  Target Milestone: ---

istreambuf_iterator not increment g-pointer of istream, so character will be
extracted twice (or more, under some occasion), so it not work as single-pass
input iterator:

  std::stringstream s;
  char b[] = "c2ee3d09-43b3-466d-b490-db35999a22cf";
  char r[] = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
  //          012345678901234567890123456789012345

  s << b;
  EXAM_CHECK( !s.fail() ); // pass
  std::copy_n( std::istreambuf_iterator<char>(s), 36, r );
  EXAM_CHECK( !s.fail() ); // pass
  EXAM_CHECK( memcmp(b, r, 36) == 0 ); // pass
  EXAM_CHECK( s.tellg() == 35 ); // pass ?!
  char c = 'q';
  std::copy_n( std::istreambuf_iterator<char>(s), 1, &c );
  EXAM_CHECK( !s.fail() ); // pass ?!
  EXAM_CHECK( c == 'f' ); // pass ?!
  EXAM_CHECK( s.tellg() == 35 ); // pass ?!