I try to modify vector value use pass by reference in following code.

#include <iostream>
#include <vector>

void func(int a, int b, int& c) {
    c = a + b;
}

int main() {
    std::vector<int> v = {1, 2, 3, 4};
    func(3, 4, v[0]);  // is this valid???
    
    for(int i : v) {
        std::cout << i << " ";
    }
    // Output be: 7 2 3 4

    return 0;
}

When I ask some LLM models like gpt3.5, claude, it says this code is incorrect,but I can get the right answer when I actually run the code. By the way, GPT4o says this is correct. Can someone tells me what’s going on here?