C++: What is std::coroutine (C++20)?

1#include <coroutine>
2#include <iostream>
3
4// Generator coroutine
5struct Generator {
6    struct promise_type {
7        int current_value;
8        Generator get_return_object() {
9            return Generator{std::coroutine_handle<promise_type>::from_promise(*this)};
10        }
11        std::suspend_always initial_suspend() { return {}; }
12        std::suspend_always final_suspend() noexcept { return {}; }
13        std::suspend_always yield_value(int value) {
14            current_value = value;
15            return {};
16        }
17        void return_void() {}
18        void unhandled_exception() { std::terminate(); }
19    };
20
21    std::coroutine_handle<promise_type> handle;
22    bool next() { handle.resume(); return !handle.done(); }
23    int value() { return handle.promise().current_value; }
24};
25
26Generator count(int n) {
27    for (int i = 0; i < n; i++) {
28        co_yield i;
29    }
30}