Why does this hang at line 13? (Xcode 6.2 6C86e)

List.swift

final class Box<T> {
    let unbox: T
    init(_ value: T) { self.unbox = value }
}

enum List<T> {
    case Empty
    case Cons(T, Box<List<T>>) // <- Converting this to Cons(Box<T>, Box<List<T>>) fixes it (thanks @CodeFi_)
}

println("START")

let b: Box<List<Int>> = Box(List.Empty) // Hangs here at runtime. Never gets to FINISHED

println("FINISH")

// Stack at hung point:
#0	0x00007fff9895c132 in __psynch_cvwait ()
#1	0x00007fff966f1ea0 in _pthread_cond_wait ()
#2	0x00007fff9a4a2c95 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) ()
#3	0x0000000100197dd2 in swift_getGenericMetadata ()
#4	0x0000000100197f75 in swift_getGenericMetadata1 ()
#5	0x0000000100179eeb in create_generic_metadata1 ()
#6	0x000000010019f549 in std::__1::__function::__func<swift::swift_getGenericMetadata::$_0, std::__1::allocator<swift::swift_getGenericMetadata::$_0>, (anonymous namespace)::GenericCacheEntry* ()>::operator()() ()
#7	0x0000000100197e37 in swift_getGenericMetadata ()
#8	0x0000000100197f75 in swift_getGenericMetadata1 ()
#9	0x000000010017c145 in type metadata accessor for List.List<Swift.Int> ()
#10	0x000000010017c0e0 in type metadata accessor for List.Box<List.List<Swift.Int>> ()
#11	0x0000000100179740 in top_level_code at /Users/rnapier/Documents/Talks/CocoaConf/Array To Zipper/List/List/main.swift:13
#12	0x0000000100179f6a in main ()
#13	0x00007fff974e65c9 in start ()