func customCodableEnum() {

    struct SubJSON: Codable {
        let subJSONKey: String
    }

    enum MyEnum<T: Codable>: Codable {

        case myCase(key: String, value: T)
        case otherCase(Int)
        case unmanagedCase //Could throw a custom error instead

        init(from decoder: Decoder) throws {
            let values = try decoder.singleValueContainer()
            if let asInt = try? values.decode(Int.self) {
                self = .otherCase(asInt)
            } else {
                let asDict = try values.decode([String: T].self)
                if let first = asDict.keys.first {
                    self = .myCase(key: first, value: asDict[first]!)
                } else {
                    self = .unmanagedCase //Could throw a custom error instead
                }
            }
        }
    }

    do {
        let jsonStr = """
    [3,
     {"Key": {
            "subJSONKey": "SubJSONValue"
        }
     }
    ]
    """
        let json = Data(jsonStr.utf8)
        let enums = try JSONDecoder().decode([MyEnum<SubJSON>].self, from: json)
        enums.forEach {
            switch $0 {
            case .myCase(key: let key, value: let value):
                print("MyEnum.myCase(key: \(key),  value: \(value))")
            case .otherCase(let intValue):
                print("MyEnum.otherCase(\(intValue))")
            case .unmanagedCase:
                print("MyEnum.unmanagedCase")
            }
        }
    } catch {
        print("Error: \(error)")
    }
}

customCodableEnum()