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Summary:
The use of optional enums as parameters to closures in Swift code (such as for completion handlers) crashes the Swift compiler with a segmentation fault

Steps to Reproduce:
Build the attached CompilerCrasherExperiment with Xcode 6.1.1. It's an iOS project with an EnumThing.swift file:
enum EnumThing {
    case Foo,
    Bar
}

func doSomething (completion: (thing: EnumThing?) -> Void) {
    completion (thing: .Foo)
}

which is then called from ViewController.viewDidLoad() as follows:

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        doSomething { (thing: EnumThing?) -> Void in
            let msg = thing == .Foo ? "Foo" : "Bar"
        }
    }



Expected Results:
When run, this should do nothing (although a breakpoint would allow you to see that the msg variable is being set in doSomething()).

Actual Results:
Crashes the Xcode build. Swift Compiler Error: Command failed due to signal: Segmentation fault: 11

(too much text to paste - will attach as CompileSwift-crash-log.txt)

Version:
Mac OS X 10.10.1 (14B25), Xcode Version 6.1.1 (6A2008a)

Notes:
I have found that using non-optional enums as parameters eliminates crashes like this.

I've had similar swiftc crashes passing structs as optionals, but don't have a buglet put together for that.

Configuration:
MacBook Pro (Retina, 15-inch, Late 2013)

Attachments:
'CompilerCrasherExperiment.zip' and 'CompileSwift-crash-log.txt' were successfully uploaded.