When a function is implemented as a coroutine (because it contains co_ statements in its body), there is an important type that determines things about how it behaves, called the "promise type". Now, this is an example of very unfortunate naming, since this type has nothing to do with the notion of promise objects (aka futures or deferreds), nor with the existing std::promise<> type. A "coroutine policy type" or just "coroutine type" would have been a much better name, but it is what it is.
alcinnz