cpp/experimental/flex barrier/arrive and drop

Arrive at the 's synchronization point and remove the current thread from the set of participating threads. It is unspecified whether this function blocks until the completion phase has ended. If the function blocks, the calling thread may be chosen to execute the completion phase.

The behavior is undefined if the calling thread is not in the set of participating threads of this.

Calls to synchronizes with the start of the completion phase of the. If the call blocks, then the completion of the completion phase synchronizes with the return from the call.

Calls to and  never introduce data races with themselves or each other.

Exceptions
Throws nothing.