使用Array类的索引方法时出错

Question

我创建了一个自定义类MyArray用于练习和学习。我在该类和一个内部数组中创建了一些方法。在类内部定义的内部数组上使用index(of: )方法时遇到错误。我班上的代码如下：

public class MyArray<Element> {
    private var factorialNumber: Double = 0
    private var arr: [Element] = []
    internal var instance: MyArray?

    private init() {}
    public func removeElement(_ item: Element) -> [Element] {
        var found = true
        while found {
            if arr.contains(item) { // ERROR: Missing argument label '"where:" in call
                let index = arr.index(of: item) // Error: cannot invoke index with an argument list of type (of: Element)
                arr.remove(at: index!)
            } else {
                found = false
            }
        }
        return arr
    }

    public func removeFirstOccurance(of value: Element) -> [Element] {
        let index : Int = arr.index(of: value)

        arr.remove(at: index!) // ERROR: Cannot force wrap value of non optional type Int
        return arr
    }
}