PVS-Studio warning V521 for GDCM

bool Sorter::StableSort(
  std::vector<std::string> const & filenames)
{
  ....
  std::vector< SmartPointer<FileWithName> >::iterator
    it2 = filelist.begin();

  for( Directory::FilenamesType::const_iterator it =
         filenames.begin();
       it != filenames.end(), it2 != filelist.end();
       ++it, ++it2)
  {
  ....
}

This suspicious code was found in GDCM project by PVS-Studio static code analyzer.
Warning message is:
V521 Such expressions using the ',' operator are dangerous. Make sure the expression is correct. gdcmsorter.cxx 82

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/