返回符合常规约束的对象

我正在尝试创建一个 Builder 为了我的 ComplexObject :

import Foundation

class ComplexObject {
    // lots of stuff

    init<ObjectType, T>(_ closure: ((ObjectType) -> T)) {
        // lots of init/setup code
    }

    // other initializers with generics, constructed 
    // by other Builders than ConcreteBuilder<O> below
}

protocol BuilderType {
    associatedtype ObjectType
    func title(_: String) -> Self
    func build<T>(_ closure: ((ObjectType) -> T)) -> ComplexObject
}

struct Injected<O> {
    //...
}

extension ComplexObject {
    static func newBuilder<Builder: BuilderType, O>(someDependency: Injected<O>) -> Builder where Builder.ObjectType == O {
        // vvvv
        return ConcreteBuilder(someDependency: someDependency)
        // ^^^^
        // Cannot convert return expression of type 'ComplexObject.ConcreteBuilder<O>' to return type 'Builder'
    }

    struct ConcreteBuilder<O>: BuilderType {
        private let dependency: Injected<O>

        private var title: String

        init(someDependency: Injected<O>) {
            self.dependency = someDependency
        }

        func title(_ title: String) -> ConcreteBuilder<O> {
            var builder = self
            builder.title = title
            return builder
        }

        func build<T>(_ closure: ((O) -> T)) -> ComplexObject {
            return ComplexObject(closure)
        }
    }
}

swiftc 抱怨 return ConcreteBuilder(...) 线

Cannot convert return expression of type 'ComplexObject.ConcreteBuilder<O>' to return type 'Builder'

我也试过了

static func newBuilder<Builder: BuilderType>(someDependency: Injected<Builder.ObjectType>) -> Builder {
    return ConcreteBuilder(someDependency: someDependency)
}

同样的结果。我知道我可以直接暴露出来 ConcreteBuilder ,但我希望能够隐藏实现细节。我错过了什么?