使用通用协议作为接口

我想创建一个 UseCase 协议,使用通用协议。然后我想为所有实现创建一个接口,以便为测试创建mock。

struct Product {}

protocol UseCase {
    associatedtype ReturnType
    associatedtype Param
    func execute(_ params: Param, completion: ((ReturnType) -> Void))
}


protocol FetchProductsUseCase: UseCase {
    associatedtype ReturnType = [Product]
    associatedtype Param = Void
}


struct FetchProductsUseCaseImpl: FetchProductsUseCase {
    func execute(_ params: Param , completion: ((ReturnType) -> Void)) {
        completion([])
    }
}

//ERROR: Protocol 'FetchProductsUseCase' can only be used as a generic constraint because it has Self or associated type requirements
var useCase: FetchProductsUseCase!

有人能帮我修一下吗?

我搜索了SOF,找到了很多关于泛型的主题,但是没有一个对我的案例有帮助。