爲什麼我不能在函數內聲明模板類型別名？

Question

爲什麼我不能在函數中聲明模板類型別名？

#include <vector> 

int main(){ 

    //type alias deceleration: 
    template <typename T> 
    using type = std::vector<T>; 

    //type instantiation: 
    type<int> t; 

} 

error: a template declaration cannot appear at block scope

爲什麼我們被迫把這些聲明塊範圍之外？

#include <vector> 

//type alias deceleration: 
template <typename T> 
using type = std::vector<T>; 

int main(){ 

    //type instantiation: 
    type<int> t; 
} 

Answer 1

該標準如此說。

從C++ 11標準（重點煤礦）：

14 Template

2 A template-declaration can appear only as a namespace scope or class scope declaration. In a function template declaration, the last component of the declarator-id shall not be a template-id. [ Note: That last component may be an identifier, an operator-function-id, a conversion-function-id, or a literal-operator-id. In a class template declaration, if the class name is a simple-template-id, the declaration declares a class template partial specialization (14.5.5). —end note ]