Swift generics link error

generic-link-error.swift

// xcrun swift generic-link-error.swift
// produces:
// LLVM ERROR: Program used external function '_OBJC_METACLASS_$__TtC4main1A' which could not be resolved!

import Cocoa

class A<X> : NSObject {
    override class func keyPathsForValuesAffectingValueForKey(key: String) -> Set<String> {
        return super.keyPathsForValuesAffectingValueForKey(key)
    }
}
class B : A<Int> {}

let b = B()
print("b = \(b)")