Flattened hierarchical config

Config.swift

private protocol MutableConfigProtocol: ConfigProtocol {
    var key1: String { get set }
    var key2: Int { get set }
}

private protocol ConfigLayerProtocol {
    var key1: String? { get }
    var key2: Int? { get }
}

private struct FallbackConfig: MutableConfigProtocol {
    var key1 = ""
    var key2 = 0
}

private struct ConfigLayer: ConfigLayerProtocol {
    var key1: String?
    var key2: Int?
}

private struct Config: ConfigProtocol {
    public let key1: String
    public let key2: Int
}

private func ||(lhs: MutableConfigProtocol, rhs: ConfigLayerProtocol) -> MutableConfigProtocol {
    var merged = lhs

    if let value = rhs.key1 { merged.key1 = value }
    if let value = rhs.key2 { merged.key2 = value }

    return merged
}

public protocol ConfigProtocol {
    var key1: String { get }
    var key2: Int { get }
}

public class ConfigCoordinator {
    public static var config: ConfigProtocol {
        let base = FallbackConfig()

        var serverConfig = ConfigLayer()
        serverConfig.key1 = "fromServer"

        var userDefaultsConfig = ConfigLayer()
        userDefaultsConfig.key2 = 23

        return base || serverConfig || userDefaultsConfig
    }

}