https://twitter.com/mpurland/status/707833907329114112 answer

company-clients.swift

protocol ClientType: Hashable {
    var fullName: String { get }
}

extension ClientType {
    var hashValue: Int {
        return fullName.hashValue
    }
}

func ==<C: ClientType>(lhs: C, rhs: C) -> Bool {
    return lhs.fullName == rhs.fullName
}

protocol CompanyType {
    typealias Client: ClientType
    var clients: Set<Client> { get }
}

struct Company<Client: ClientType> {
    private let _clients: () -> Set<Client>
    
    init(clients: () -> Set<Client>) {
        _clients = clients
    }
    
    var client: Set<Client> {
        return _clients()
    }
}

struct CustomClient: ClientType {
    let fullName: String
}

let microsoft = Company { return [ CustomClient(fullName: "Bill Gates") ] }
microsoft.client.forEach {
    print("\($0.fullName)")
}