Playground.playground

import Foundation

public protocol Journey { }
class CommsJourney: Journey { }

typealias Completion = (Result<[Journey]?, Error>) -> Void
typealias CommsCompletion = (Result<[CommsJourney]?, Error>) -> Void

class Comms {
    func refreshTimes(completion: @escaping CommsCompletion) {
        
        let array = [CommsJourney()]
        completion(.success(array))
    }
}

/**
 Completion block returns `Journey` protocol if successful
 */
func refreshTimes(completion: @escaping Completion) {
    
    // Passing the previous completion block fails to compile as signatures are different
//    Comms().refreshTimes(completion: completion)
    
    Comms().refreshTimes { (result) in
        
        switch result {
        case .success(let results):
            // result is `CommsCompletion` yet returns perfectly fine
            completion(.success(results))
        case .failure(let error):
            completion(.failure(error))
        }
    }
}