//: Closure picks up static type not dynamic
class MutableReference<T> {
    init() {
        guard type(of: self) != MutableReference.self else {
            fatalError("MutableReference is an abstract class; create a derivative of MutableReference")
        }
    }
    var value: T {
        get { fatalError("Calculated property value getter should be overridden") }
        set { fatalError("Calculated property value setter should be overridden") }
    }
}

class DefaultMutableReference<T>: MutableReference<T> {
    private var _value: T
    override var value: T {
        get { return _value }
        set { _value = newValue }
    }
    init(_ value: T) { _value = value }
}

let er: (MutableReference<[Int]>, Int) -> Void = { $0.value.append($1) }
let ok: (MutableReference<[Int]>) -> [Int] = { $0.value }
let dm = DefaultMutableReference([2])
ok(dm) // [2]
er(dm, 2) // fatal error: Calculated property value getter should be overridden
ok(dm) // Expect [2, 2]