Weird Collection behaviour

weird-collection-behaviour.swift

import UIKit

public struct NonEmptyArray<Element> {
    fileprivate var elements: [Element]
    
    public init(elements: Element...) {
        self.elements = elements
    }
    
    public var count: Int {
        return elements.count
    }
    
    public var first: Element {
        return elements.first!
    }
}

extension NonEmptyArray: Collection {
    public typealias Index = Int
    
    public var startIndex: Int {
        return 0
    }
    
    public var endIndex: Int {
        return count
    }
    
    public func index(after i: Int) -> Int {
        return i + 1
    }
}

// MARK: MutableCollection
extension NonEmptyArray: MutableCollection {
    public subscript(_ index: Int) -> Element {
        get {
            return elements[index]
        }
        set {
            elements[index] = newValue
        }
    }
}

let array = NonEmptyArray(elements: 1)
let first: Int! = array.first // Calls Collection.first
let otherFirst = array.first // Calls NonEmptyArray.first