Surprising but useful.

C++20 Coroutines back to basics — Restrictions and rules for legal coroutines

by Gajendra Gulgulia

From the article:

In my previous articles on coroutines, I attempted to explain the mechanics of coroutine suspension, resuming it, promise_type, awaiters and awaitables. In this article, I would like to answer two very basic questions:

• What are the restrictions on coroutines ?
• When can a coroutine usage be legal in terms of member, non-member functions of class/structs, lambda expressions and so on ?