CKRecord+KeyPath.swift

extension CKRecord {
    public subscript<Root, Value: CKRecordValueProtocol>(keyPath keyPath: WritableKeyPath<Root, Value>) -> Value? {
        get {
            let key = NSExpression(forKeyPath: keyPath).keyPath
            return self[key]
        }
        set {
            let key = NSExpression(forKeyPath: keyPath).keyPath
            self[key] = newValue
        }
    }
}

Usage.swift

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

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

extension CKRecord: MyRecordProtocol {
}

func usage() {
    var record = CKRecord(recordType: "Test") as MyRecordProtocol
    record[keyPath: \.aaa] = true
    // Xcode doesn't show code completion, but shows error when use wrong keyPath and variable type.
  
    let value = record[keyPath: \.aaa]
    // Xcode infers type as `Bool?`

    print(value)
    // Optional(true)
}

I think this is better than other ways for current Xcode 11.3.1 and Swift 5.1.
See also other trial and error.
https://gist.github.com/Koze/2ab02e4f249ce00242f2948f186a3fda
https://gist.github.com/Koze/7fb045148c4772c5c28acb3958192cd9
https://gist.github.com/Koze/f6afb934377717bf34fd285439e86281