OptionSet that is also an NSObject seems to be broken

[igor-makarov]

Description

When defining an OptionSet that inherits from NSObject, the contains(_:) method seems to be broken.

It's possible that other functionality is also broken. There are no compiler warnings or errors about this.

Reproduction

public class OptionA: NSObject, OptionSet {
    public let rawValue: Int32
    public static let alpha: OptionA = .init(rawValue: 1 << 0)
    public static let beta: OptionA = .init(rawValue: 1 << 1)
    public required init(rawValue: Int32) {
        self.rawValue = rawValue
    }
    // this seems to resolve it, but what else is wrong?
    // public func contains(_ member: OptionA) -> Bool {
    //     rawValue & member.rawValue == member.rawValue
    // }
}

print(OptionA.alpha.contains(.alpha)) // prints false

Expected behavior

The contains(_:) method should be implemented correctly, similar to cases where the OptionSet is not an NSObject.

I don't know whether other OptionSet methods are affected.

Environment

swift-driver version: 1.87.3 Apple Swift version 5.9.2 (swiftlang-5.9.2.2.56 clang-1500.1.0.2.5)
Target: arm64-apple-macosx14.0

Additional information

No response

[benrimmington]

I suspect that your subclass needs to override isEqual(_:) and hash. For example:

public override func isEqual(_ object: Any?) -> Bool {
  (object as? Self)?.rawValue == rawValue
}

public override var hash: Int {
  rawValue.hashValue
}

The default implementations from NSObject only use the memory address of the instance.

[igor-makarov]

It looks like the docs only say "For your type to automatically receive default implementations for set-related operations, the rawValue property must be of a type that conforms to the FixedWidthInteger protocol, such as Int or UInt8".

Doesn't seem to be anything about Equatable...

[benrimmington]

OptionSet inherits from SetAlgebra, which inherits from Equatable.

The documentation probably assumes that your option set will be a struct, for which the compiler will synthesize an Equatable conformance (SE-0185).

[igor-makarov]

@benrimmington You're right, there are 2 occurrences where SetAlgebra is relying on Equatable:

  // 1
  public func isSubset(of other: Self) -> Bool {
    return self.intersection(other) == self
  }
  // 2
  public var isEmpty: Bool {
    return self == Self()
  }

Both of these are broken if the OptionSet is a subclass of NSObject, unless -[isEqual:] is overridden.

This NSObject auto-conformance is a footgun that I've sadly triggered far too many times to count.