Enum initialized with a non-existent rawValue does not fail and return nil

Question

I have the following code in a playground (Xcode 9.0.1):

import MapKit

enum Test: UInt {
    case first
    case second
    case third
}

let test = Test(rawValue: 4) as Any
print(test)           // nil

let type = MKMapType(rawValue: 999)
print(type == nil)    // false
print(type!.rawValue) // 999

MKMapType is defined as

enum MKMapType : UInt

As the maximum value of a MKMapType is 5, I expect the initializer of the enum to fail and return nil. Instead it returns 999. Am I missing some ObjC/Swift bridging here or could this be a bug?

Answer 1

I filed a bug with Apple and this is the reply I received:

"Engineering has determined that this issue behaves as intended based on the following information:

Because C enums may have values added in future releases, or even have "private case" values used by the framework that are not included in the headers, there's no way to check whether a value provided in Swift is actually valid or invalid. Therefore, init(rawValue:) is obliged to produce a value just as a C cast would. There are discussions in the Swift Open Source project on how to improve this situation in later versions of Swift, but the initializer for MKMapType still won't return nil."

Thanks to Apple Engineering for this explanation.