[swift-users] Static type when expecting dynamic type

[Howard Lovatt via swift-users]

Hi All,

Anyone know what is going on here:

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

If I change `e` to:
    let e: (DefaultMutableReference<[Int]>, Int) -> Void = {
$0.value.append($1) }

It works fine.

IE the closure is using the static type of its first argument and not
dynamically dispatching.

Am I doing something wrong? Is there a way round where I can still use the
base class for `e`?


Thanks for any help in advance,

-- Howard.

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