import Foundation

struct TimeZoneField : Decodable {
    let timeZone: TimeZone
    
    private enum CodingKeys : String, CodingKey {
        case timeZone
    }
    
    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        do {
            timeZone = try container.decode(TimeZone.self, forKey: .timeZone)
        } catch DecodingError.typeMismatch(_, _) {
            let identifier = try container.decode(String.self, forKey: .timeZone)
            timeZone = TimeZone(identifier: identifier)!
        }
    }
}

struct X : Decodable {
    private enum CodingKeys : String, CodingKey {
        case a, b
    }
    
    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let a = try container.decode(TimeZoneField.self, forKey: .a)

        // Works
        let b = try container.decode(Double.self, forKey: .b)
        
        // Does not work:
//        let container2 = try decoder.container(keyedBy: CodingKeys.self)
//        let b = try container2.decode(Double.self, forKey: .b)
        
        print("a: \(a)")
        print("b: \(b)")
    }
}

let json = "{ \"a\": { \"timeZone\": \"Australia/Sydney\" }, \"b\": 34.2 }".data(using: .utf8)!
let x = try JSONDecoder().decode(X.self, from: json)