Generics_Swift_4.swift

protocol CustomComparator {
    associatedtype CustomType
    static func compare(_ lhs: CustomType, _ rhs: CustomType) -> Bool
}

protocol InternalTypeProtocol {
    associatedtype CustomType: CustomComparator
    var property: CustomType? { get set }
}

class GenericClass<InternalType> where InternalType: InternalTypeProtocol {
    let instance: InternalType? = nil

    func doSomething() {
        guard let property = instance?.property else { return }
        print(InternalType.CustomType.compare(property, property)) 
    // Cannot invoke 'compare' with an argument list of type '(InternalType.CustomType, InternalType.CustomType)'
    }
}

class SomeClass {

    let helper = GenericClass<InternalType>()

    enum InternalEnum: String, CustomComparator {
        typealias CustomType = InternalEnum
        case myCase

        static func compare(_ lhs: InternalEnum,_ rhs: InternalEnum) -> Bool {
            return true
        }
    }

    class InternalType: InternalTypeProtocol {
        var property: InternalEnum?
    }
}