这是Swift 4编译器错误吗?

我写了一个小游戏来演示如何对数组坐标进行排序,以找到最接近的5个。我使用高阶函数,将坐标映射到也包含距离的结构,对其进行排序,然后选择前5项。然而,我无法将前5名作为同一复合语句的一部分进行最后的挑选。

代码如下:

    import Foundation
    import CoreLocation

    let currentLatitide = 19.1553902
    let currentLongitude = 72.8528602
    struct CoordStruct: CustomStringConvertible {
        let coord: CLLocationCoordinate2D
        let distance: Double

        var description: String {
            return "lat: " + String(format: "%.4f",coord.latitude) +
                ", long: " + String(format: "%.4f",coord.longitude) + ", distance: " + distance.description
        }
    }

    let location = CLLocation(latitude: currentLatitide, longitude: currentLongitude)

     let points =   [[19.5,71.0],[18.5,72.0],[19.15,72.85],[19.1,75.0],[19.2,70.0],[19.3,70.0],[19.4,70.0],[19.6,70.0],[19.7,70.2],[19.9,70.3],[25,62.0],[24.5,73.4],[23.5,65.0],[21.5,68.0],[20.5,69.0]]

    let structs: [CoordStruct] = points.map //This is where the error shows up.
        {
        let thisCoord = CLLocationCoordinate2D(latitude: $0[0], longitude: $0[1])
        let thisLocation = CLLocation(latitude:$0[0], longitude: $0[1])
        let distance = location.distance(from: thisLocation)
        return CoordStruct(coord: thisCoord, distance: distance)
        }
        .sorted { $0.distance < $1.distance }
        //----------------------------------
        .prefix(5)  //This won't compile
        //----------------------------------
    let first5Structs = structs.prefix(5)


    first5Structs.forEach { print($0) }

let test = points.map { $0[1] }
    .sorted { $0 < $1 }
    .prefix(5)

print(test)

请参见标记的行 //This won't compile 。在该行未注释的情况下,编译器在引用map语句时会说“对成员‘map’的引用不明确”。如果我注释掉有问题的一行,并在另一行上添加前缀,编译器会很高兴。错误消息毫无意义,这似乎是Swift编译器的典型特征,但代码似乎应该编译。我错过了什么?