why "a.at(-2)" not judge Array overbounds

The book says that std::array is safer and simpler than other methods of the assignment.

Here's my code:

#include<iostream>
#include<array>
using namespace std;
int main(){
    array<int,5> a = {1,2,3,4,5};
    a.at(-2)=100;
    cout<<a[-2]<<endl;
    return 0;
}

Why is there no warning or error in this code?