Type-safe CKRecord with dynamicMemberLookup

CKRecord+DynamicMemberLookup.swift

extension CKRecord {
    subscript<Root, Value: CKRecordValueProtocol>(dynamicMember keyPath: WritableKeyPath<Root, Value>) -> Value? {
        get {
            let key = NSExpression(forKeyPath: keyPath).keyPath
            return self[key]
        }
        set {
            let key = NSExpression(forKeyPath: keyPath).keyPath
            // Fatal error: Could not extract a String from KeyPath Swift.ReferenceWritableKeyPath
            self[key] = newValue
        }
    }
}

Usage.swift

@objc protocol MyRecord {
    var aaa: Bool { get set }
    var bbb: String { get set }
}

@dynamicMemberLookup
protocol MyRecordProtocol {
    subscript<Value: CKRecordValueProtocol>(dynamicMember keyPath: WritableKeyPath<MyRecord, Value>) -> Value? { get set }
}

extension CKRecord: MyRecordProtocol {
}

func usage() {
    var record = CKRecord(recordType: "Test") as MyRecordProtocol
    record.aaa = true
    // Xcode infers type as `Bool?`, but NSExpression occur fatal error.

    let value = record.aaa
    print(value)
}

This seems good type-safe if it's enable to extract string from keyPath.
See also other ways.
https://gist.github.com/Koze/7fb045148c4772c5c28acb3958192cd9
https://gist.github.com/Koze/f6afb934377717bf34fd285439e86281

Any ideas on how to fix this crash?

I'm trying to implement a similar utility to access NSDictionary with NSManagedObject properties, and getting the same error. Looks like dynamicMemberLookup provides some different kind of key path, not working with NSExpression for some reason.

@dtrofimov
This was a bug in Xcode, and has been fixed in Xcode 12.5.
See Xcode 12.5 release notes.
https://developer.apple.com/documentation/xcode-release-notes/xcode-12_5-release-notes

Fixed a crash that occurred when setting a value using a ReferenceWritableKeyPath that was upcast to a WritableKeyPath. (74191390) (FB8999603)

Now you can use the technique of this gist!