GenericDataSource.swift

import UIKit

protocol DataSource<CellType>: AnyObject {
    associatedtype CellType

    func createCellFor(index: Int, reusing cell: CellType?) -> CellType
}

class ContainerView<CellType> where CellType: UIView {

    weak var dataSource: (any DataSource)?

    func fetchCells() {
        // Member 'createCellFor' cannot be used on value of type 'any DataSource'; consider using a generic constraint instead
        let cell = dataSource?.createCellFor(index: 0, reusing: reuseCell())
    }

    func reuseCell() -> CellType {
        return CellType()
    }

}

class SomeClass: NSObject, DataSource {

    let thing = ContainerView<UICollectionViewCell>()

    override init() {
        super.init()

        self.thing.dataSource = self
    }

    func createCellFor(index: Int, reusing cell: CellType?) -> UICollectionViewCell {
        return UICollectionViewCell(frame: .zero)
    }


    typealias CellType = UICollectionViewCell

}

/// is the solution something like:

class ContainerView<CellType, DataSource> where CellType: UIView, DataSource: DataSource, DataSource.CellType == CellType {
  /* ... */
}