'dispatch_once_t' is unavailable in Swift: Use lazily initialized globals instead

I am having trouble with `dispatch_once_t` when migrating to Swift 3.

According to [Apple's migration guide](

[To see links please register here]

):

> The free function dispatch_once is no longer available in Swift. In
> Swift, you can use lazily initialized globals or static properties and
> get the same thread-safety and called-once guarantees as dispatch_once
> provided. Example:
>
> `let myGlobal = { … global contains initialization in a call to a closure … }()`

> `_ = myGlobal // using myGlobal will invoke the initialization code only the first time it is used.`

So I wanted to migrate this code. So it was before migration:

class var sharedInstance: CarsConfigurator
{
struct Static {
static var instance: CarsConfigurator?
static var token: dispatch_once_t = 0
}

dispatch_once(&Static.token) {
Static.instance = CarsConfigurator()
}

return Static.instance!
}

After the migration, following the Apple's guidelines (manual migration), the code looks like this:

class var sharedInstance: CarsConfigurator
{
struct Static {
static var instance: CarsConfigurator?
static var token = {0}()
}

_ = Static.token

return Static.instance!
}

But when I run this I get the following error when accessing `return Static.instance!`:

> fatal error: unexpectedly found nil while unwrapping an Optional value

I see from this error that the `instance` member is `nil`, but why is it? Is it something wrong with my migration?

That code was overly verbose even though it was valid in Swift 2. In Swift 3, Apple forces you to use lazy initialization through closure:

class CarsConfigurator {
static let sharedInstance: CarsConfigurator = { CarsConfigurator() }()
}