public func encode(to encoder: Encoder) throws {
...
        // current and autoupdatingCurrent are sentinel values. Calendar could theoretically not treat 'current' as a sentinel, but it is required for Locale (one of the properties of Calendar), so transitively we have to do the same here
        if self == Calendar.autoupdatingCurrent {
            try container.encode(Current.autoupdatingCurrent, forKey: .current)
        } else if self == Calendar.current {  // < --- encode as current here because they compare equal
            try container.encode(Current.current, forKey: .current)
        } else {
            try container.encode(Current.fixed, forKey: .current)
        }
    }

     public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        if let current = try container.decodeIfPresent(Current.self, forKey: .current) {
            switch current {
            case .autoupdatingCurrent:
                self = Calendar.autoupdatingCurrent
                return
            case .current:
                self = Calendar.current // <--- decode self as current here
                return
            case .fixed:
                // Fall through to identifier-based
                break
            }
        }