Compile Error: Use constexpr to declare std::array size

I am learning constexpr and, for my understanding, constexpr tells the compiler to calculate functions during compile time instead of running time. I used the following code for testing but bumped into an error I really don't understand. Can you please explain why?

#include <iostream>
#include <array>
using namespace std;

constexpr int foo(int i)
{
    return i + 5;
}

int main()
{
    int i = 10;
    std::array<int, foo(5)> arr; // OK
    // But...
    std::array<int, foo(i)> arr1; // Error
}

The error is: the value of 'i' is not usable in a constant expression. Why? i is declared beforehand why does it have to be a const?