问Swift 3:错误:对Swift‘>成员的模糊引用

EN

error: ambiguous reference to member '>'
        let moveDirection = dx > 0 ? .right : .left

enum MoveDirection {
    case none
    case left
    case right
}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    guard let touch = touches.first else {
        return;
    }

    let location = touch.location(in: humanPlayerScreen)
    let previousLocation = touch.previousLocation(in: humanPlayerScreen)

    let dx = location.x - previousLocation.x
    let dy = location.y - previousLocation.y
    let moveDirection = dx > 0 ? .right : .left // error

    ...
}

Swift.>:5:13: note: found this candidate
public func >(lhs: UInt8, rhs: UInt8) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: Int8, rhs: Int8) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: UInt16, rhs: UInt16) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: Int16, rhs: Int16) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: UInt32, rhs: UInt32) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: Int32, rhs: Int32) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: UInt64, rhs: UInt64) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: Int64, rhs: Int64) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: UInt, rhs: UInt) -> Bool
            ^
Swift.>:5:13: note: found this candidate
public func >(lhs: Int, rhs: Int) -> Bool
            ^
Foundation.Date:95:24: note: found this candidate
    public static func >(lhs: Date, rhs: Date) -> Bool
                       ^
Foundation.IndexPath:51:24: note: found this candidate
    public static func >(lhs: IndexPath, rhs: IndexPath) -> Bool
                       ^
Foundation.IndexSet.Index:5:24: note: found this candidate
    public static func >(lhs: IndexSet.Index, rhs: IndexSet.Index) -> Bool
                       ^
CoreMedia.>:1:13: note: found this candidate
public func >(time1: CMTime, 

time2: CMTime) -> Bool
            ^
Swift.>:10:13: note: found this candidate
public func ><T : Comparable>(lhs: T, rhs: T) -> Bool
            ^
Swift.>:1:13: note: found this candidate
public func ><T : FloatingPoint>(lhs: T, rhs: T) -> Bool
            ^
Swift.>:1:13: note: found this candidate
public func ><T : _SwiftNewtypeWrapper where T.RawValue : Comparable>(lhs: T, rhs: T) -> Bool
            ^
Swift.>:12:13: note: found this candidate
public func ><A : Comparable, B : Comparable>(lhs: (A, B), rhs: (A, B)) -> Bool
        ^
Swift.>:12:13: note: found this candidate
public func ><A : Comparable, B : Comparable, C : Comparable>(lhs: (A, B, C), rhs: (A, B, C)) -> Bool
            ^
Swift.>:12:13: note: found this candidate
public func ><A : Comparable, B : Comparable, C : Comparable, D : Comparable>(lhs: (A, B, C, D), rhs: (A, B, C, D)) -> Bool
            ^
Swift.>:12:13: note: found this candidate
public func ><A : Comparable, B : Comparable, C : Comparable, D : Comparable, E : Comparable>(lhs: (A, B, C, D, E), rhs: (A, B, C, D, E)) -> Bool
            ^
Swift.>:12:13: note: found this candidate
public func ><A : Comparable, B : Comparable, C : Comparable, D : Comparable, E : Comparable, F : Comparable>(lhs: (A, B, C, D, E, F), rhs: (A, B, C, D, E, F)) -> Bool
            ^
Swift.Comparable:158:24: note: found this candidate
    public static func >(lhs: Self, rhs: Self) -> Bool
                       ^
Swift.LazyFilterIndex<Base>:7:24: note: found this candidate
    public static func >(lhs: LazyFilterIndex<Base>, rhs: LazyFilterIndex<Base>) -> Bool
                       ^