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[SR-3616] Default implementation being called when specialized one should be picked #46201

swift-ci opened this issue Jan 12, 2017 · 2 comments


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@swift-ci swift-ci commented Jan 12, 2017

Previous ID SR-3616
Radar None
Original Reporter TadeasKriz (JIRA User)
Type Bug
Status Resolved
Resolution Duplicate

OS X El Capitan 10.11.6
Xcode 8.2 (8C38)
Swift 3 - default Xcode 8.2 toolchain

Additional Detail from JIRA
Votes 0
Labels Bug
Assignee None
Priority Medium

md5: 37d41da5c66e66345ea5bd0549666027


  • SR-103 Protocol Extension: function's implementation cannot be overridden by a subclass

Issue Description:

The following code has unexpected results.

protocol Component: class {
    func test() -> Int

extension Component {
    func test() -> Int {
        return -1

class BaseComponent: Component {
    let delegate = ComponentDelegate<BaseComponent>()

    init() {
        delegate.component = self

class ChildComponent: BaseComponent {
    func test() -> Int {
        return 10

final class ComponentDelegate<COMPONENT: Component> {
    weak var component: COMPONENT?

    func runTest() -> Int? {
        return component?.test()

let x = BaseComponent()
let y = ChildComponent()

x.test() // Returns -1
y.test() // Returns 10

x.delegate.runTest() // Returns -1
y.delegate.runTest() // Returns -1

Results from the delegated functions seem to be wrong and the default implementation is being used. A way to workaround this is to implement the `test` method in the `BaseComponent` class. Then all child classes work as well.

I tried changing the `delegate` property type to `ComponentDelegate<ChildComponent>` and then forcefully casting `self` in the init, but this produces the same results = calling the default implementation.

Is this a bug in Swift or intended behavior? If it's an intended behavior, it seems a bit confusing to me and might be unnecessary constraining.

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@belkadan belkadan commented Jan 12, 2017

Known issue!

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@swift-ci swift-ci commented Jan 12, 2017

Comment by Tadeas Kriz (JIRA)

Oh, thanks, I didn't find that issue.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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