Informs the caller whether or not the task group is currently in the midst of a cancellation. This does not necessarily indicate that the cancel method was called on the structured_task_group object (although such certainly qualifies this method to return true). It may be the case that the structured_task_group object is executing inline and a task group further up in the work tree was canceled. In cases such as these where the runtime can determine ahead of time that cancellation will flow through this structured_task_group object, true will be returned as well.
An indication of whether the structured_task_group object is in the midst of a cancellation (or is guaranteed to be shortly).