Conditional conformance in swift 4.1 for optional for some common protocols

optional-conditional-collection.swift

extension Optional: Collection where Wrapped: Collection {
    public typealias Element = Wrapped.Element
    public typealias Index = Wrapped.Index

    public func index(after i: Wrapped.Index) -> Wrapped.Index {
        switch self {
        case let value?:
            return value.index(after: i)
        case nil:
            fatalError() // TODO: ???
        }
    }

    public subscript(position: Wrapped.Index) -> Wrapped.Element {
        switch self {
        case let value?:
            return value[position]
        case nil:
            preconditionFailure("Fatal error: Index out of range")
        }
    }

    public var startIndex: Wrapped.Index {
        switch self {
        case let value?:
            return value.startIndex
        case nil:
            fatalError() // TODO: ???
        }
    }

    public var endIndex: Wrapped.Index {
        switch self {
        case let value?:
            return value.endIndex
        case nil:
            fatalError() // TODO: ???
        }
    }
}

optional-conditional-sequence.swift

extension Optional: Sequence where Wrapped: Sequence {
    public typealias Element = Wrapped.Element
    public typealias Iterator = AnyIterator<Element>

    public func makeIterator() -> AnyIterator<Wrapped.Element> {
        switch self {
        case let value?: return AnyIterator(value.makeIterator())
        case nil: return AnyIterator({ return nil })
        }
    }
}