no member named 'sum' in 'std::slice_array

Here's a code that can't be compiled with any at least c+++11-conform compiler (tried at godbolt):

#include <valarray>
#include <vector>
#include <iostream>

class Whatever {
public:
    int sliceSum(const std::vector<int>& nums, int k) const {
        std::valarray<int> data(nums.size());
        for (int i = 0; i < nums.size(); ++i) {
            data[i] = nums[i];
        }
        std::cout << typeid(decltype(data[std::slice(0, k, 1)])).name() << std::endl;
        return data[std::slice(0, k, 1)].sum();
    }
};

class Matrix {
    std::valarray<int> data;
    int dim;
 public:
    Matrix(int r, int c) : data(r*c), dim(c) {}
    int& operator()(int r, int c) {return data[r*dim + c];}
    int trace() const {
        std::cout << typeid(decltype(data[std::slice(0, dim, 1)])).name() << std::endl;
        return data[std::slice(0, dim, 1)].sum();
    }
};
int main()
{
    Matrix m(3,3);
    int n = 0;
    for(int r=0; r<3; ++r)
       for(int c=0; c<3; ++c)
           m(r, c) = ++n;
    Whatever s;
    s.sliceSum({1,2,3}, 3);
    m.trace();
}

Class Matrix is taken from https://en.cppreference.com/w/cpp/numeric/valarray/slice and class Whatever is mine.

So I don't understand what the problem is and why types are different in two identic (?) cases.