message: fix C2678 error on VS2017 15.5
diff: @@ -51,7 +51,7 @@ class IndicesRowIterator
  return tmp;
  }
  - reference operator*() { return iter_->ix()(row_idx_, 0); }
  + reference operator*() const { return iter_->ix()(row_idx_, 0); }
  pointer operator->() { return &iter_->ix()(row_idx_, 0); }
  