deafult型变量模板

我正在尝试创建一个默认指针类型的参数包:

template<int*... T>
void f() {}

我发现这个代码有错误:

int main(){
    int* a = new int(5);
    f<a>();
}

int main(){
    f<nullptr>();
}

为什么?

错误:

./example.cpp: In function 'int main()': ./example.cpp:6:7: error: the
value of 'a' is not usable in a constant expression
     f<a>();
       ^ ./example.cpp:5:10: note: 'a' was not declared 'constexpr'
     int* a = new int;
          ^ ./example.cpp:6:10: error: no matching function for call to 'f<a>()'
     f<a>();
          ^ ./example.cpp:2:6: note: candidate: 'template<int* ...T> void f()'  void f() {}
      ^ ./example.cpp:2:6: note:   template argument deduction/substitution failed: ./example.cpp:6:10: error: 'a' is not a
valid template argument because 'a' is a variable, not the address of
a variable
     f<a>();
          ^ Compiler returned: 1

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