protocol ItemModel: Hashable {}

protocol SectionModel: Hashable {
    associatedtype ItemCollection: Collection where ItemCollection.Element: ItemModel
    var elements: ItemCollection { get }
}

final class BlocksViewController<SectionBlock: SectionModel>: UIViewController {
    typealias ItemCollectionElement = SectionBlock.ItemCollection.Element
    
    private let initalBlocks: [SectionBlock]

    private lazy var dataSource: UICollectionViewDiffableDataSource<SectionBlock, ItemCollectionElement> = {
        return UICollectionViewDiffableDataSource(collectionView: UICollectionView(), cellProvider: { _, _, _ in
            return nil
        })
    }()
    
    func applyInitialData() {
        let snapshot = NSDiffableDataSourceSnapshot<SectionBlock, ItemCollectionElement>()
        snapshot.appendSections(initalBlocks)

        for section in initalBlocks {
            let items = section.elements
            snapshot.appendItems(items, toSection: section) //ERROR: Cannot convert value of type 'SectionBlock.ItemCollection' to expected argument type '[_]'
        }

        dataSource.apply(snapshot, animatingDifferences: false)
    }

    init(initalBlocks: [SectionBlock]) {
        self.initalBlocks = initalBlocks
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}