Const in C++ arrays?

Question

C++ / C#

Hello, why doesn't CLion require the size of an array to be const?

#include <iostream>
#include <cstdlib>

using namespace std;

int main() {
    int n;

    cout << "Num: ";
    cin >> n;

    int arr[n + 1];

    for (int &i:arr) {
        i = 1;
    }

    for (int k = 2; k*k <= n; k++) {
        if (arr[k] == 1) {
            for (int i = k * k; i <= n; i += k) {
                arr[i] = 0;
            }
        }
    }

     if (arr[n] == 1) {
         cout << "Prime number";
     }
     else {
         cout << "Composite number";
     }

     return 0;

This code works correctly, why doesn't it throw an error?