Evaluate an InputIterator sequence and return the last value in the sequence. The last_value class was the default Combiner template parameter type for signals in the original Signals library. Signals2 uses optional_last_value as the default, which does not throw. T result_type InputIterator InputIterator Attempts to dereference every iterator in the sequence [first, last). The result of the last successful iterator dereference. no_slots_error if no iterators were successfully dereferenced, unless the template type of last_value is void.