why for-loop isn't a compile time expression and extended constexpr allows for-loop in a constexpr function

I wrote code like this

#include <iostream>
using namespace std;
constexpr int getsum(int to){
    int s = 0;
    for(int i = 0; i < to; i++){
        s += i;
    }
    return s;
}
int main() {
    constexpr int s = getsum(10);
    cout << s << endl;
    return 0;
}

I understand that it works because of extended constexpr. However in this question why-isnt-a-for-loop-a-compile-time-expression, the author gave his code as follow:

#include <iostream>
#include <tuple>
#include <utility>

constexpr auto multiple_return_values()
{
    return std::make_tuple(3, 3.14, "pi");
}

template <typename T>
constexpr void foo(T t)
{
    for (auto i = 0u; i < std::tuple_size<T>::value; ++i)
    {
        std::get<i>(t);
    }    
}

int main()
{
    constexpr auto ret = multiple_return_values();
    foo(ret);
}

It could not compile by GCC5.1, however, after replacing std::get<i>(t); with something without a specified template parameter his code did work. After reading answers under this question I found their main point is constexpr for creates trouble for the compiler so for-loop is used at run-time. So it makes me confused, on one hand, for-loop is at run-time, on the other, the loop is in a constexpr function, so it must be calculated at compile time, so there seems to be a contradiction. I just wonder where did I make a mistake.