Mutating Generic Enum Issue

List.swift

enum List<Element> {
    case End
    indirect case Node(Element, next: List<Element>)
}

enum ListError: ErrorType {
    case EndCannotAppendValue
}

extension List {
    mutating func append(value: Element) throws -> List {
        guard case .Node(let element) = self else { throw ListError.EndCannotAppendValue }
        let next = List.Node(value, next: .End)
        // Error: Cannot convert value of type 'List<Element>' to expected argument type 'List<_>'
        self = List.Node(element, next: next)
        return next
    }
}

Bad news: append function can only return a immutable value, which can't be call in chain.

var head = List.Node(1, next: .End)
// error: cannot use mutating member on immutable value: function call returns immutable value
try! head.append(2).append(2)