C++: What is std::coroutine?

1#include <coroutine>
2#include <generator>
3
4std::generator<int> fibonacci() {
5    int a = 0, b = 1;
6    while (true) {
7        co_yield a; // suspend and yield value
8        auto temp = a;
9        a = b;
10        b = temp + b;
11    }
12}
13
14for (int n : fibonacci()) {
15    if (n > 100) break;
16    std::cout << n << " ";
17}