标签发送过程中发生错误

#include <iterator>
#include <forward_list>
#include <string>
#include <vector>

using namespace std;

template<typename C>
using Iterator_type = typename C::iterator;

template<typename Iter>
using Iterator_category = typename iterator_traits<Iter>::iterator_category;

template<typename Ran>
void sort_helper(Ran beg, Ran end, random_access_iterator_tag){
    sort(beg,end);
}

template<typename For>
void sort_helper(For beg, For end, forward_iterator_tag){
    vector<decltype(*beg)> vc{beg, end};
    sort(vc.begin(),vc.end());
    copy(vc.begin(),vc.end(),beg);
}

template<typename C>
void sort(C& c){
    using Iter = Iterator_type<C>;
    sort_helper(c.begin(),c.end(),Iterator_category<Iter>{});
}

void test(vector<string>& vs, forward_list<double>& lls){
    sort(vs);
    sort(lls);
}

我得到一个编译错误——形成指向引用类型double&

来自Bjarne Stroustrup的代码——C++编程语言 5.4.2.1迭代器_特征