ColumnSelection.swift

import Foundation

enum ColumnSelection<Item> where Item: Hashable {
    
    case viewAll
    case selection(item: Item)
    
}

extension ColumnSelection: Hashable {
    
    // MARK: - Hashable
    
    var hashValue: Int {
        switch self {
        case .viewAll: return 0
        case .selection(let item): return item.hashValue + 1
        }
    }
    
    // MARK: - Equatable
    
    static func ==(lhs: ColumnSelection<Item>, rhs: ColumnSelection<Item>) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }

}

Gross. I thought I was being clever with the item.hashValue + 1 in line 17 to avoid a hash collision with .viewAll. However, hashValue is an Int (not UInt) therefore I could still collide with a hash value of -1.