message: Fix LayoutFunctionIterator's increment and decrement operators
diff: @@ -369,9 +369,9 @@ class LayoutFunctionIterator {
  return *this;
  }
  - iterator operator++() { return *this += 1; }
  + iterator& operator++() { return *this += 1; }
  - iterator operator--() { return *this -= 1; }
  + iterator& operator--() { return *this -= 1; }
  iterator operator++(int) {
  auto tmp = *this;
  