Swift 4.2, Xcode 10.2 nil coalescing operator warning

Question

On Xcode 10.1, the following statement wasn't having any issues, but after updating the Xcode to 10.2 compiler is generating warning for the statement.

return [
    "sublevels": (self.sublevels?.array ?? [Sublevel]()) as NSObject
]

sublevels is NSOrderedSet, and the warning generated is as follows:

Left side of nil coalescing operator '??' has non-optional type '[Any]?', so the right side is never used

But if I break the single statement as follows, the warning disappears.

let sublevels = self.sublevels?.array ?? [Sublevel]()
return [
    "sublevels": sublevels as NSObject
]

Please, will anyone explain - what is the issue with the first statement ?

A (minimal) *self-contained* compiling example would be helpful. — Martin R, Apr 08 '19 at 07:43
I think the error message should be reported as a bug. `[Any]?` definitely is an Optional. But you need to supply some `[Any]` as the left hand side of `??` is of type `[Any]?`. What if you replace `?? [Sublevel]()` to `?? []` ? — OOPer, Apr 08 '19 at 08:01
I wouldn't say it's a bug, regarding the [SE-0054](https://github.com/apple/swift-evolution/blob/master/proposals/0054-abolish-iuo.md) rules. `Any` is optional, as long as `Any` has no type basically. — Bram, Apr 08 '19 at 08:06
@Craz1k0ek, SE-0054 is describing about ImplicitlyUnwrappedOptional, and `[Any]?` aka `Optional>` is a normal Optional and nothing described in SE-0054 applies to normal Optional. Are you mistaking with some other proposal? — OOPer, Apr 08 '19 at 08:45
@OOPer ah yeah, you’re right. Was a bit quick. Better would be to link to [type casting](https://docs.swift.org/swift-book/LanguageGuide/TypeCasting.html) — Bram, Apr 08 '19 at 09:09

Bram · Accepted Answer · 2019-04-08T11:14:11.513

As mentioned by OOPer, the solution is to provide a default Any value as right hand side of the operator, in this case an [Any], because the NSOrderedSet has no specific type bound to it. The solution is:

return [
    "sublevels": (self.sublevels?.array ?? []) as NSObject
]

For more info on this matter I suggest you take a look at type casting. At the bottom of the page there is an explanation about casting the Any type.

Swift 4.2, Xcode 10.2 nil coalescing operator warning

1 Answers1