Move-assign a basic_stream_file
from a file of another executor type.
template< typename Executor1> constraint< is_convertible< Executor1, Executor >::value, basic_stream_file & >::type operator=( basic_stream_file< Executor1 > && other);
This assignment operator moves a stream file from one object to another.
The other basic_stream_file
object from which the move will occur.
Following the move, the moved-from object is in the same state as if
constructed using the basic_stream_file(const executor_type&)
constructor.