New Swift 5 warnings for Objective-C enums: how to get rid of them?

As of Xcode 10.2, when using enums I've defined in Objective-C, but in a Swift 5 switch statement, I get the following warning, even if I've exhausted all the possible enum values.

Switch covers known cases, but 'MyObjectiveCEnumName' may have additional 
unknown values

Xcode is telling me I should fix this by

Handle unknown values using "@unknown default"

Why is this happening and what can I do about it?

Example

Objective-C enum

typedef NS_ENUM(NSUInteger, CardColor) {
  CardColorBlack,
  CardColorRed
};

Swift 5 switch statement

var cardColor: CardColor = .black

switch (cardColor) {
case .black:
  print("black")
case .red:
  print("red")
}

Solution

TL;DR

If you want Objective-C enums to be treated just like Swift ones, you now need to declare them using a different macro, NS_CLOSED_ENUM, versus the old NS_ENUM. Changing this will make the warning disappear.

The example above would become

typedef NS_CLOSED_ENUM(NSUInteger, CardColor) {
  CardColorBlack,
  CardColorRed
};

Deets

From the Swift 5 release notes:

In Swift 5 mode, switches over enumerations that are declared in Objective-C or that come from system frameworks are required to handle unknown cases—cases that might be added in the future, or that may be defined privately in an Objective-C implementation file. Formally, Objective-C allows storing any value in an enumeration as long as it fits in the underlying type. These unknown cases can be handled by using the new @unknown default case, which still provides warnings if any known cases are omitted from the switch. They can also be handled using a normal default case.

If you’ve defined your own enumeration in Objective-C and you don’t need clients to handle unknown cases, you can use the NS_CLOSED_ENUM macro instead of NS_ENUM. The Swift compiler recognizes this and doesn’t require switches to have a default case.