为什么在使用模板别名时，嵌套struct的静态指针的定义报告错误

Question

以下代码是一个类模板。在课堂之外，我定义了静态成员。但是，我使用两种方法来定义PrimeBlock *静态成员，方法 - 1使用模板别名，并报告重新声明错误;方法-2可以正常工作。我想知道为什么方法 - 1不起作用。

template <typename tree_type>
class Tree {
    struct PrimeBlock {
        vector<tree_type*> vec;
    };

    static tree_type* ROOT;
    static PrimeBlock* primeBlock;
};

template <typename tree_type>
tree_type* ROOT = nullptr;

// method - 1 template alias for definition - error
template <typename tree_type>
using PrimeBlock = typename Tree<tree_type>::PrimeBlock;
template <typename tree_type>
PrimeBlock<tree_type>* Tree<tree_type>::primeBlock = nullptr; // this cannot work due to redeclaration error

// method - 2 - it works
//template <typename tree_type>
//typename Tree<tree_type>::PrimeBlock* Tree<tree_type>::primeBlock = nullptr; // however, this could work, why?

方法-1产生的错误消息

error: conflicting declaration ‘PrimeBlock<RtreeVariant>* Tree<RtreeVariant>::primeBlock’
 PrimeBlock<RtreeVariant>* Tree<RtreeVariant>::primeBlock = nullptr;

note: previous declaration as ‘Tree<RtreeVariant>::PrimeBlock* Tree<RtreeVariant>::primeBlock’
 static PrimeBlock* primeBlock;

error: declaration of ‘Tree<RtreeVariant>::PrimeBlock* Tree<RtreeVariant>::primeBlock’ outside of class is not definition [-fpermissive]
 PrimeBlock<RtreeVariant>* Tree<RtreeVariant>::primeBlock = nullptr;