Confusion about undefined behaviour in C++

I read this answer about undefined behaviour, where I saw following statement:

++++++i;     // UB, parsed as (++(++(++i)))

I don't think it is undefined behaviour. I have a doubt, Is it really UB in C++? If yes, then How?

Also, I made program and compiled using g++ prog.cpp -Wall -Wextra -std=gnu++1z -pedantic command, it's working fine without any warning. It's give an expected output.

#include <iostream>
using namespace std;

int main()
{
    int i = 0;
    cout<<++++++i<<endl;
}