cpp/coroutine/noop coroutine promise

is the promise type of no-op coroutines.

A no-op coroutine behaves as if it
 * does nothing other than the control flow of a coroutine, and
 * is suspended immediately after beginning and resumption, and
 * has a coroutine state such that destroying the state is no-op, and
 * never reaches its final suspended point if there is any std referring to it.

No-op coroutines can be started by std, and controlled by the coroutine handle it returns. The returned coroutine handle is of type std, which is a synonym for.

Some operations of a no-op coroutines are determined no-op at compile time through the type.