Ios 如何正确扩展RxCocoa.Driver？

我想像这样扩展驱动程序：

extension Driver where E == Array<String>? {

    var dashedLabel: UILabel {
        let label = UILabel()
        map({ $0?.joined(separator: "-") }).drive(label.rx.text)
        // yes, the above is never disposed
        // what I am interested about is this error:
        // Referencing instance method 'drive' on
        // 'SharedSequenceConvertibleType'
        // requires the types
        // 'SharedSequence<S, Optional<Array<String>>>.SharingStrategy'
        // (aka 'S') and 'DriverSharingStrategy' be equivalent
        return label
    }

}

公共结构SharedSequence:SharedSequenceConvertibleType

public typealias Driver<E> = SharedSequence<DriverSharingStrategy, E>

public struct SharedSequence<S: SharingStrategyProtocol, Element> : SharedSequenceConvertibleType