无法将类型'[（Measurement，[MeasurementValue]）？]'的值转换为闭包结果类型'（Measurement，[MeasurementValue]）？'

Question

class func measurementsFromMeasurementsDB(_ measurementsDB: [CIMeasurement]) -> [(Measurement, [MeasurementValue])] {

    var measurements: [(Measurement, [MeasurementValue])?] = measurementsDB.map {return measurementsFromMeasurementsDB([($0)])}

    measurements = measurements.filter {$0 != nil}
    return measurements.map {return $0!}
}