The second part of the support is provided by header <boost/outcome/coroutine_support.hpp> (or <boost/outcome/experimental/coroutine_support.hpp> if you want Coroutine support for Experimental Outcome). This adds into namespace BOOST_OUTCOME_V2_NAMESPACE::awaitables (or BOOST_OUTCOME_V2_NAMESPACE::experimental::awaitables) these awaitable types suitable for returning from a Coroutinised function:

• eager<T>

  An eagerly evaluated Coroutine: invoking co_await upon a function returning one of these immediately begins the execution of the function now. If the function never suspends, the overhead is similar to calling an ordinary function.

• lazy<T>

  A lazily evaluated Coroutine (often named task<T> in most C++ Coroutine literature): invoking co_await upon a function returning one of these causes the function to be immediately suspended as soon as execution begins. Only resuming the execution of the coroutine proceeds execution.

• atomic_eager<T>

  eager<T> does not employ thread synchronisation during resumption of dependent coroutines which is fine if you do not traverse kernel threads during a suspend-resume cycle. If you do however potentially traverse kernel threads during suspend-resume, you ought to use atomic_eager<T> instead – this uses atomics to synchronise the setting and checking of state to ensure correctness.

• atomic_lazy<T>

  Same for lazy<T> as atomic_eager<T> is for eager<T>.