cc @jckarter

Comment by Nobuo Saito (JIRA)

I found problems obvious by Conditional Conformance.
In Swift4.1 (Xcode 9.3 b2), that happen.

let x = Int?.none
let y = Int??.none

x == y // false

let a = [x]
let b = [y]

a == b // true

That last example is definitely a bit surprising, but I think it's the same problem that you've noticed in the main issue. The == operator is effectively behaving as an as cast for x and for a: it takes two arguments of the same type and so it needs to make sure both sides match (for the first one, both sides have type Int??, and for the second, both sides have type [Int??]).

Here's a different version of the same problem:

func sameType<T>(_ x: T, _ y: T) {
    print(x, y, T.self)
}

let x = Int?.none
let y = Int??.none
sameType(x, y)

let a: [Int?] = [x]
let b: [Int??] = [y]
sameType(a, b)

output:

Optional(nil) nil Optional<Optional<Int>>
[nil] [nil] Array<Optional<Optional<Int>>>

It seems like the [T?] to [T??] conversions are being implemented as

array.map { $0 { .some($0) } }
// instead of
array.map { .some($0) }

#33561 should fix this for the non-optimized case (where the cast is being handled by the runtime). There may still be issues with optimized casts, though.

I believe this is fully resolved now. Please let me know if you see any further issues in this area.