要求associatedtype在@convention（c）块中可表示

Question

我希望有一种像Swift 3一样的通用方法：

public protocol Callable {
    associatedtype In : CVarArg
    associatedtype Out : CVarArg
}

public struct IntCallable : Callable {
    public typealias In = Int
    public typealias Out = Double

    public typealias FunctionalBlock = @convention(c) (In) -> Out

    public func call(_ block: FunctionalBlock) { /* do stuff */ }
}

所以我希望它看起来更像这样：

public protocol Callable {
    associatedtype In : CVarArg
    associatedtype Out : CVarArg
    typealias FunctionalBlock = @convention(c) (In) -> Out
}

public struct IntCallable : Callable {
    public typealias In = Int
    public typealias Out = Double
}

public extension Callable {
    public func call(_ block: FunctionalBlock) { /* do stuff */ }
}

然而，我收到错误：

'(Self.In) -> Self.Out' is not representable in Objective-C, so it cannot be used with '@convention(c)'

我可以对In / Out关联类型进行约束，这将允许我声明FunctionalBlock的通用形式吗？没有@convention(c)它工作正常，但我需要它才能形成一个C函数调用。