函子"can't deduce template parameter"的模板方法

g++与std=c++14在函数类的模板方法（本身不是模板）上给我一个"无法推导模板参数'Key'"错误。我不明白为什么。代码看起来应该可以工作。

我正在实现一个23树，它有一个接受函子的级别顺序遍历方法。操作人员tree23代码基本上是这样的：

template<class Key, class Value> class tree23 {
public:
 class Node23 {
      friend class tree23<Key, Value>;             
    public:   
    // snip...
      friend std::ostream& operator<<(std::ostream& ostr, const Node23& node23)
      { 
        // snip... outputs the keys and values of node23 to ostr.
      }
   private:
        Node23 *parent;
        std::array<Key, 2> keys;
        std::array<Value, 2> values;
        std::array<std::unique_ptr<Node23>, 3> children;
        int totalItems; // either TwoNodeItems or ThreeNodeItems
        // snip...
 }; 
   template<typename Functor> void levelOrderTraverse(Functor f) const noexcept;
   // snip...
};

级别顺序遍历调用函子的函数调用运算符，并向其传递两个参数。

template<class Key, class Value> template<typename Functor> 
void tree23<Key, Value>::levelOrderTraverse(Functor f) const noexcept
{
   std::queue< std::pair<const Node23*, int> > queue; 
   Node23 *proot = root.get();
   if (proot == nullptr) return;
   auto initial_level = 1; // initial, top level is 1, the root.
   queue.push(std::make_pair(proot, initial_level));
   while (!queue.empty()) {
        std::pair<const Node23 *, int> pair_ = queue.front();
        const Node23 *current = pair_.first;
        int current_tree_level = pair_.second;
        // invokes functor's operator()(const Node23&, int)? 
        f(*current, current_tree_level);
        if (!current->isLeaf()) {
            for(auto i = 0; i < current->getChildCount(); ++i) {
               queue.push(std::make_pair(current->children[i].get(), current_tree_level + 1));  
            }
        }
        queue.pop(); 
   }
}

函子很简单：

class levelOrderPrinter {
  private:
    // snip...    
  public:
         levelOrderPrinter(std::ostream& ostr_lhs, int depth); 
         levelOrderPrinter(levelOrderPrinter&);
         levelOrderPrinter(const levelOrderPrinter&);
         template<class Key, class Value> 
         void operator()(const typename tree23<Key, Value>::Node23& node,
                         int current_level) noexcept;
};
template<class Key, class Value> 
void levelOrderPrinter::operator()(const typename tree23<Key, Value>::Node23& node,
                             int current_level) noexcept
{
   // Did level change?
   if (level != current_level) { 
       level = current_level;
       ostr << "nn" << "level = " <<  level; 
       // Provide some basic spacing to tree appearance.
       std::size_t num = tree_depth - level + 1;
       std::string str( num, ' ');
       ostr << str;
   }
   ostr <<  node;
}