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            Move-assign a posix::basic_descriptor from a descriptor
            of another executor type.
          
template< typename Executor1> constraint_t< is_convertible< Executor1, Executor >::value, basic_descriptor & > operator=( basic_descriptor< Executor1 > && other);
This assignment operator moves a descriptor from one object to another.
                  The other posix::basic_descriptor 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_descriptor(const executor_type&)
            constructor.