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The return type if a function.
typedef see_below result_type;
          The type of result_type
          is based on the type T
          of the wrapper's target object:
        
T is a pointer to
              function type, result_type
              is a synonym for the return type of T;
            T is a class type
              with a member type result_type,
              then result_type is
              a synonym for T::result_type;
            result_type
              is not defined.
            
          Header: asio/bind_cancellation_slot.hpp
        
          Convenience header: asio.hpp