public SynchronousCompletionAsyncResult(T result)
 {
     _result  = result;
     _source  = null;
     _isValid = true;
 }
 internal SynchronousCompletionAsyncResult(SynchronousCompletionAsyncResultSource <T> source)
 {
     _source  = source;
     _result  = default;
     _isValid = true;
 }