STL thread detach, process hang

I am create another thread and expecting another thread terminates after main terminates. however the process hangs

std::mutex mtx;
std::condition_variable cv;
std::unique_ptr<std::thread> g_t;

void func()
{
    std::this_thread::sleep_for(std::chrono::seconds(3));
    std::cout << "func";
}

int main()
{
    std::thread t(func);
    t.detach();

    std::cout << "main";
    return 0;
}

I am confused, because the process shall terminates once all threads terminates.