class MyClass: NSManagedObject {
    @NSManaged var stringValue: String?
}

let keyPath = \MyClass.stringValue

print(NSExpression(forKeyPath: keyPath).keyPath) // "stringValue"
print(keyPath._kvcKeyPathString) // "stringValue"
myObject.observe(keyPath) { object, change in
    // ...
}

@dynamicMemberLookup Expression<O: NSManagedObject> {

    subscript<V>(dynamicMember member: KeyPath<O, V>) -> String {
        return NSExpression(forKeyPath: member).keyPath // crash here
        // return member._kvcKeyPathString // also crashes
    }
}
let expression = Expression<MyClass>()
print(expression.stringValue) // crash triggered here