decltype delay type

#include <iostream>

using namespace std;

template<typename T1,typename T2>
auto f(T1 v1[], T2 v2[], const size_t count)->decltype(v1[0]*v2[0])
{
    decltype(v1[0] * v2[0]) sum{};
    for (size_t i{};i < count;i++)
    {
        sum += v1[i] * v2[i];
    }
    return sum;
}

void decltypeDemo()
{
    double x[]{ 100.5,99.5,88.7,77.8 };
    short y[]{ 3,4,5,6 };
    long z[]{ 11L,12L,13L,14L };
    size_t n{ _countof(x) };

    cout << "Result type is " << typeid(f(x, y, n)).name() << endl;
    cout << "Result is " << f(x, y, n) << endl;
    auto result = f(z, y, n);
    cout << "Result type is " << typeid(result).name() << endl;
    cout << "Result is " << result << endl;
}

int main()
{
    decltypeDemo();
}