Swift linker error when accessing @NSManaged properties of a final class

问题

I'm having issues in a larger project, and boiled it down to this simple code. For demo purposes, I have created a new Swift project with the following in a Model.swift file:

import Foundation
import CoreData

class A: NSManagedObject {
    @NSManaged var foo: String
}

final class B: A {
    @NSManaged var bar: String
}

func testB(obj: B) -> String {
    return "\(obj.foo) \(obj.bar)"
}

This compiles, but the linker complains about accessing bar:

Undefined symbols for architecture armv7:
  "__TFC21TestFinalManagedClass1Bg3barSS", referenced from:
      __TF21TestFinalManagedClass5testBFCS_1BSS in Model.o
ld: symbol(s) not found for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)

The combination of an @NSManaged var in a final class seems to be what causes the error. (The class inheritance from A is included to demonstrate that the foo property is accessible on an object of type B, but not the bar property.)

I am using Xcode Version 6.1 (6A1052d).

Is there some rule hidden in the documentation about marking an NSManagedObject subclass as final? Or is this just a bug in the compiler/linker?