Hi Lincoln,
This is a known issue with ‘super’ method calls:
https://bugs.swift.org/browse/SR-1736 <https://bugs.swift.org/browse/SR-1736>
A workaround is to cast the result of the supermethod call to Self explicitly:
let copied = super.copy() as! Self
Slava
> On Aug 10, 2017, at 8:16 PM, 吴君恺 via swift-users <swift-users@swift.org>
> wrote:
>
> Hi all,
>
> here is the scenario, I want to implement a Copying protocol which can be
> used homogeneously
>
> protocol Copying {
> func copy() -> Self
> }
>
> Because this protocol contains neither associated-type-requirements or
> self-requirements(requiring a method returning Self is different), it can
> surely be used homogeneously.
>
> let objects: [Copying] = ....
> for copyable in objects {
> .....
> }
>
> It will work if I make one of my base class conform to Copying
>
> class Shape: Copying {
> var color: UIColor?
>
> required init() {}
>
> func copy() -> Self {
> let copied = type(of: self).init()
> copied.color = color
> return copied
> }
> }
>
> The implementation of `copy` above is forced by compiler to avoid any
> explicit specification of type `Shape`, so that the returning value will have
> a dynamic type, which is just what I want. Here, the type of `copied` is
> `Self`
>
> However, if I try to make a subclass of Shape, I can't find a elegant way to
> implement this `copy` method in that subclass, the following code will not
> compile.
>
> class Circle: Shape {
> var radius: Float = 5
>
> func copy() -> Self {
> let copied = super.copy()
> copied.radius = radius // compilation error
> return copied
> }
> }
>
> The compiler will complain that `copied` has no property `radius`. It turns
> out that calling copy() on super will yield a value of type Shape, rather
> than Self.
>
> Swift now forbids explicit conversion to `Self` (I totally agree with this
> rule), and will automatically allow `Self` to be treated as a specific type
> in some circumstances. But for this case, I wonder whether this is the
> correct behavior or a bug? Why calling `super.copy()` not be able to get a
> `Self`?
>
> I did find a work-around for this problem afterwards, but this question
> really haunts me...
>
> Cheers,
> Lincoln.
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> swift-users@swift.org
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