UnknownDecodable.swift

protocol UnknownDecodable: RawRepresentable, Decodable {
    static var unknown: Self { get }
}

extension UnknownDecodable where RawValue: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        self = Self.init(rawValue: try container.decode(RawValue.self)) ?? .unknown
    }
}

enum Foo: String, UnknownDecodable {
    case foo
    case unknown
}

struct Bar: Decodable {
    var foo: Foo
}

let data1 = Data(#"{"foo":"foo"}"#.utf8)
let bar1 = try JSONDecoder().decode(Bar.self, from: data1)
bar1.foo == .foo

let data2 = Data(#"{"foo":"bar"}"#.utf8)
let bar2 = try JSONDecoder().decode(Bar.self, from: data2)
bar2.foo == .unknown  // true