message: Take `const T& node` argument in `is_leaf()`
diff: @@ -191,7 +191,7 @@ struct TreeNodeTraits : trees_internal::FeatureTraits {
  // Returns true when `node` is a leaf, false otherwise.
  template <class T, //
  std::enable_if_t<TreeNodeTraits<T>::available>* = nullptr>
  -bool is_leaf(T& node) {
  +bool is_leaf(const T& node) {
  return node.Children().empty();
  }
  