Swift compiler segmentation fault with generic recursive function - ios

Question

I've run into an interesting issue with the Swift compiler, which seems to be caused by a very simple generic function. I've got a workaround, but I'd really like to understand what the underlying issue is.
In my app I have a requirement to fade in some UICollectionViewCells in a given order, sequentially, with a slight overlap between the animations.
I've implemented this using the methods below. The revealCells method takes an array of cells. If the collection is empty, it simply returns. Otherwise it animates the fade-in on the first cell in the array, and then waits a certain amount of time before making a recursive call to itself, passing all the cells except the one it just animated.
Code below:
func revealCells(cells:[UICollectionViewCell],animationTime:NSTimeInterval,overlap:Double) {
if cells.count > 0 {
let firstCell = cells.first
UIView.animateWithDuration(0.3, animations: { () -> Void in
firstCell?.alpha = 1.0
let timeMinusOverlap = animationTime - overlap
let delayTime = dispatch_time(DISPATCH_TIME_NOW,Int64(timeMinusOverlap * Double(NSEC_PER_SEC)))
dispatch_after(delayTime, dispatch_get_main_queue(), { () -> Void in
self.revealCells(self.cdr(cells),animationTime: animationTime,overlap: overlap)
})
})
}
}
}
//Returns the collection passed into the function with its first element removed
//i.e the traditional LISP cdr function
func cdr<T>(input:[T]) -> [T] {
var rVal = [T]()
if input.count == 1 {
rVal.append(input.first!)
} else {
rVal = [T](input[1...input.count-1])
}
return rVal
}
All this works fine in the simulator. But when I try to archive the build, the swift compiler crashes with the message Swift Compiler Error Command failed due to signal: Segmentation fault 11. My setup is Xcode 6.3.1 (iOSSDK 8.3), and my min deployment target is 8.3.
Fixing the problem is trivial - if I just replace the code inside the dispatch_after with:
let newCells = [UICollectionViewCell](cells[1...cells.count-1])
self.revealCells(newCells,animationTime: animationTime,overlap: overlap)
the problem goes away. So it seems to be something to do with the generic function, or possibly something block related.
Stack trace is:
0 swift 0x000000010105ba18 llvm::sys::PrintStackTrace(__sFILE*) + 40
1 swift 0x000000010105bef4 SignalHandler(int) + 452
2 libsystem_platform.dylib 0x00007fff8725ef1a _sigtramp + 26
3 libsystem_platform.dylib 0x000000000000000f _sigtramp + 2027557135
4 swift 0x00000001021a98cb llvm::AsmPrinter::EmitFunctionBody() + 4379
5 swift 0x000000010116c84c llvm::ARMAsmPrinter::runOnMachineFunction(llvm::MachineFunction&) + 220
6 swift 0x0000000100d81d13 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 99
7 swift 0x00000001024d5edf llvm::FPPassManager::runOnFunction(llvm::Function&) + 495
8 swift 0x00000001024d60cb llvm::FPPassManager::runOnModule(llvm::Module&) + 43
9 swift 0x00000001024d658f llvm::legacy::PassManagerImpl::run(llvm::Module&) + 975
10 swift 0x0000000100a09b41 performIRGeneration(swift::IRGenOptions&, swift::Module*, swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, swift::SourceFile*, unsigned int) + 4369
11 swift 0x0000000100a09cb3 swift::performIRGeneration(swift::IRGenOptions&, swift::SourceFile&, swift::SILModule*, llvm::StringRef, llvm::LLVMContext&, unsigned int) + 51
12 swift 0x0000000100945687 frontend_main(llvm::ArrayRef<char const*>, char const*, void*) + 6647
13 swift 0x0000000100943ae6 main + 1814
14 libdyld.dylib 0x00007fff8a46b5c9 start + 1
(a long list of program arguments, which are mostly the names of file being compiled)
1. Running pass 'Function Pass Manager' on module '/Users/tolleyr/Library/Developer/Xcode/DerivedData/ParseTestQuiz-fgnfjkxxlyqfnrfrfntgtsjnrcfv/Build/Intermediates/ArchiveIntermediates/ParseTestQuiz/IntermediateBuildFilesPath/ParseTestQuiz.build/Release-iphoneos/ParseTestQuiz.build/Objects-normal/armv7/QuizQuestionViewController.o'.
2. Running pass 'ARM Assembly / Object Emitter' on function '#_TFC13ParseTestQuiz26QuizQuestionViewController13viewDidAppearfS0_FSbT_'
(the last part enabled me to figure out what code was causing the problem). The command being run was CompileSwift normal armv7
I'm going to file a radar for this, since the compiler itself is crashing , but thought I'd post it here in case anyone has an idea of what might be going on, or has run into the same issue.