extension Sequence where Element: Numeric {
    public func sum() -> Element {
        return reduce(0, +)
    }
}

extension Sequence {
    public func sum<T>() -> T where T: Numeric, Element == T? {
        var sum: T = 0

        for value in self {
            if let value = value {
                sum += value
            }
        }

        return sum
    }
}

extension DataSeries: BidirectionalCollection {
    public typealias Value = Any?
    public typealias Element = Value
    ...
}

let series: DataSeries

let s = series.sum()

*****:70:24: error: ambiguous reference to member 'sum()'
        XCTAssertEqual(series.sum(), 948.872)
                       ^~~~~~
Statistics.Sequence:2:17: note: found this candidate
    public func sum() -> Self.Element
                ^
Statistics.Sequence:2:17: note: found this candidate
    public func sum<T>() -> T where T : Numeric, Self.Element == T?