The coroutine caller type, used by the implementation.
typedef implementation_defined caller_type;
When using Boost.Coroutine v1, this type is:
typename coroutine<void()>::caller_type
When using Boost.Coroutine v2 (unidirectional coroutines), this type is:
pull_coroutine<void>
Header: asio/spawn.hpp
Convenience header: None