1

I'm unsure if the below code is a compiler bug or a limitation of Swift generics. I'm trying to instantiate a generic type which is constrained to be a protocol (which includes the init), which is constrained be a concrete UIView subclass.

Xcode autocomplete finds the protocol init method on ViewType, but then emits a compiler error (note I have also tried ViewType.init(model:)). Based on the constraints, it seems unambiguous to me that let view: UIView = ViewType(model: model) should succeed. Should I raise a bug for this?

protocol ListModel {
}

protocol ListView where Self: UIView {
    init(model: ListModel)
}

class ListController<ViewType>: UIViewController where ViewType: ListView {

    func update(listModels: [ListModel]) {
        let views = listModels.map { (model: ListModel) -> UIView in
            let view: UIView = ViewType(model: model) // error
            return view
        }
        /** ... add UIViews as subviews */
    }
}

Here's the console error:

error: non-nominal type 'ViewType' does not support explicit initialization
            let view: UIView = ViewType(model: model)
                               ^~~~~~~~~~~~~~~~~~~~~~
2

This got stuck behind the askimet filter for a while, if anyone has any recommendations on opening bug here let me know.

3

It looks like Objective-C interop is relevant to the problem, because it compiles fine if we make everything in pure Swift:

class MyView {}
class MyController {}

protocol ListModel {}
protocol ListView where Self: MyView {
    init(model: ListModel)
}

class ListController<ViewType: ListView> : MyController {
    func update(listModels: [ListModel]) {
        let views = listModels.map { (model: ListModel) -> MyView in
            let view: MyView = ViewType(model: model)
            return view
        }
    }
}

But if we make MyView inherit from NSObject, then we get the error “Non-nominal type 'ViewType' does not support explicit initialization”.

4

I agree that this looks sensible. Please file a bug!