PVS-Studio warning V567 for tesseract-ocr

void TabVector::Evaluate(....) {
  ....
  int num_deleted_boxes = 0;
  ....
  ++num_deleted_boxes = true;
  ....
}

This suspicious code was found in tesseract-ocr project by PVS-Studio static code analyzer.
Warning message is:
V567 Undefined behavior. The 'num_deleted_boxes' variable is modified while being used twice between sequence points. libtesseract303 tabvector.cpp 735

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/