segfault when `handler` called before `result.get()` about asio

[breese]

Consider using io_service::post() to call the handler. That also ensures that the handler is always called from a correct thread.

[chriskohlhoff]

Technically what you're doing does not meet the requirements for asynchronous operations (http://think-async.com/Asio/asio-1.10.2/doc/asio/reference/asynchronous_operations.html) which state that the result will be delivered using post().

However, this does seem like a useful thing to support specifically for coroutines, so I have committed a change in 6394ab6. But please be warned that your async_foobar() will violate people's expectations about how an asynchronous operation will behave.