c++ Difficulty initializing a struct using 'auto'

Question

Having difficulty initializing a struct consisting of a tree node and a bool using auto [aaa, aaa] syntax:

Identifier 'node' is undefined

I've tried using the struct type (NodeAndState) as well as decltype instead of auto, but nothing seems to work.

The code is exactly as in "Elements of Programming Interviews".

#include <memory>
#include <algorithm>
#include <iostream>
#include <stack>
#include <vector>

using namespace std;


template <typename T>
struct BinaryTreeNode
{
    T data;
    unique_ptr<BinaryTreeNode<T>> left, right;

    explicit BinaryTreeNode(const T& data) : data(data) {}
    
    BinaryTreeNode(T data, unique_ptr<BinaryTreeNode<T>>&& left,
        unique_ptr<BinaryTreeNode<T>>&& right) : data(data), left(move(left)),
        right(move(right)) {}
};


vector<int> InorderTraversal(const unique_ptr<BinaryTreeNode<int>>& tree) {
  vector<int> result;

  struct NodeAndState {
    const BinaryTreeNode<int>* node;
    bool left_subtree_traversed;
  };
  stack<NodeAndState> in_process(
      {{tree.get(), false}});
  while (!empty(in_process)) {
    // The problem is here
    auto [node, left_subtree_traversed] = in_process.top();
    in_process.pop();
    if (node) {
      if (left_subtree_traversed) {
        result.emplace_back(node->data);
      } else {
        in_process.push({node->right.get(), false});
        in_process.push({node, true});
        in_process.push({node->left.get(), false});
      }
    }
  }
  return result;
}