使用带有部分类专门化的crtp?

我试着加入一个 operator [] 和一个班。我的问题是我已经对类进行了部分专门化,编译器不喜欢我不为派生类指定模板参数:

#include <iostream>
#include <type_traits>

using namespace std;

template <typename T>
struct mixin {
    template <typename U>
    void operator[](U u) {
        cout << u;
    }
};


template <typename T, typename = void>
struct derived : mixin<derived> {};


template <typename T>
struct derived<T, 
    typename enable_if<
        is_same<T, int>{}
    >::type> : mixin<derived> {};


int main() {
    derived<int> d;
    d[3.14];
}

随着叮当声这给:

test.cc:16:24: error: use of class template 'derived' requires template arguments
struct derived : mixin<derived> {};
                       ^~~~~~~
test.cc:16:8: note: template is declared here
struct derived : mixin<derived> {};
       ^
test.cc:23:22: error: use of class template 'derived' requires template arguments
    >::type> : mixin<derived> {};
                     ^~~~~~~
test.cc:16:8: note: template is declared here
struct derived : mixin<derived> {};
       ^

GCC的帮助更是微乎其微:

test.cc:16:31: error: type/value mismatch at argument 1 in template parameter list for ‘template<class T> struct mixin’
 struct derived : mixin<derived> {};
                               ^
test.cc:16:31: note:   expected a type, got ‘derived’
test.cc:23:29: error: type/value mismatch at argument 1 in template parameter list for ‘template<class T> struct mixin’
     >::type> : mixin<derived> {};
                             ^
test.cc:23:29: note:   expected a type, got ‘derived’
test.cc: In function ‘int main()’:

是在mixin子句中重新指定模板参数的唯一选项吗?