Stackless coroutine is one of the big 4 feature of C++20. Stackless coroutine would allow programmers to write asynchronous program in synchronous style. However, the stackless coroutine in C++20 defines the syntax of the core language only. However, it didn't define coroutine library in C++20. So it would be hard for programmers to use C++20 coroutine directly. For user who want to use stackless coroutine part of async_simple diretly, he/she could visit Lazy. For user who want to know how coroutine works, C++ Coroutines.