a.swift

extension Array {
    func sortedWith<T: Comparable>(_ comparableTransform: (Array.Element) -> T) -> [Array.Element] {
        return sorted(by: { (a, b) -> Bool in
            return comparableTransform(a) > comparableTransform(b)
        })
    }
}

        let carts: [Card] = []

        carts = carts.sortedWith({ (cart) -> (Int, Int) in // generic parameter error here
            let c: (Int, Int) = (cart.rowIndex, cart.columnIndex)
            return c
        })