enum doesn't conform to expected type 'JSONDecodable'

[okla]

Following code causes error:

enum Numbers: String {

  case one, two, three
}

// In argument type 'Numbers.Type', 'Numbers' does not conform to expected type 'JSONDecodable'
let number = try? json.decode(at: 0, type: Numbers.self)

But if I add

extension Numbers: JSONDecodable {}

the error's gone.

Shouldn't Numbers adopt JSONDecodable implicitly?

Swift 3, Xcode 8

[zwaldowski]

This is fully intended behavior. We can't make your types conform to JSONDecodable for you; we can only provide default implementations for your implementation.

[okla]

Ok, thanks!