Looking under the hood of awaitables and coroutines

Aside from cppcoro, the standard library offers two more trivial awaitables: suspend_never and suspend_always. By looking at them, we can see how to implement our own awaitables when needed:

struct suspend_never {
    constexpr bool await_ready() const noexcept { return true; }
    constexpr void await_suspend(coroutine_handle<>) const noexcept {}
    constexpr void await_resume() const noexcept {}
};

struct suspend_always {
    constexpr bool await_ready() const noexcept { return false; }
    constexpr void await_suspend(coroutine_handle<>) const noexcept {}
    constexpr void await_resume() const noexcept {}
};

When typing co_await, you tell the compiler to first call the awaiter's await_ready(). If it says the awaiter is ready by returning true, await_resume() will get called. The return type of await_resume() should be the type the awaiter is actually producing. If the awaiter was not ready, the program will instead...