Swift generic extension with self casting

Your call to dequeueReusable never requires any particular type for T, so the most general is selected. The type you want, however, is Self (the type of the current subclass).

The natural (but slightly wrong) way to write this would be:

class func dequeueReusable(_ collectionView: UICollectionView, 
                           for indexPath: IndexPath) -> Self {
    return collectionView.dequeueReusableCell(withReuseIdentifier: self.reuseID,
                                                              for: indexPath) as! Self
}

I honestly don't know why this doesn't work. Self can't be used in the as! Self construct. It can, however, be tricked into working with a generic wrapper:

func cast<T>(_ value: Any) -> T { return value as! T }

With that, you get the working version:

class func dequeueReusable(_ collectionView: UICollectionView,
                           for indexPath: IndexPath) -> Self {
    return cast(collectionView.dequeueReusableCell(withReuseIdentifier: self.reuseID, 
                                                   for: indexPath))
}