OptionalContradiction.swift

class Animal {}
class Cat: Animal {}

// Rule A
// Cat is subtype of Animal
do {
    let cat: Cat = Cat()
    let animal: Animal = cat
}

// Rule B
// Int is subtype of Int?
do {
    let int: Int = 0
    let intOptional: Int? = int // Int?.some(0)
}

// Rule C
// Optional has covariance
do {
    let catOptional: Cat? = Cat?.none
    let animalOptional: Animal? = catOptional // Animal?.none
}

// Question
// What happen replace `Cat : Animal` to `Int : Int?`
do {
    let intOptional: Int? = Int?.none
    let intOptionalOptional: Int?? = intOptional // Rule B: Int??.some(.none), Rule C: Int??.none
}