import Foundation

protocol Actor {
    var id: String { get }
}

protocol User: Actor {
    var email: String { get }
}

protocol Group: Actor {
    var name: String { get }
}

protocol Conversation {
    var actor: Actor { get }
}

enum StreamError: Error {
    case outOfBounds
}

struct ConversationStream {
    
    let conversations: [Conversation]
    
    func item(at: Int) throws -> Conversation {
        if at < 0 || at >= conversations.count {
            throw StreamError.outOfBounds
        } else {
            return self.conversations[at]
        }
    }
}

let stream = ConversationStream(conversations: [])
let usersToFetch = [Int]()


//let users: [String] = usersToFetch.compactMap({ (index) in //Uncomment this line to see the desired resul
let users = usersToFetch.compactMap({ (index) in
    guard let conversation = try? stream.item(at: index) else {
        return nil
    }
    let actor = conversation.actor as? User //This line fails with the error: failed to produce diagnostic for expression; please file a bug report
    return actor?.id
})