I have renamed types in your code to make it easier to read:

protocol Proto { }

struct A: Proto { }
struct B: Proto { }

var optA: A? = A()
var optB: B? = B()

if let optA = optA {
  let arr: [Proto] = [optA, optB].compactMap { $0 }
  print(arr)
}

I think there are two problems here:

1. You have a mixed array i.e a non-optional value and an optional value mixed.

2. The type of the intermediate array is [Any] (since you have a mixed array)

If you force-unwrap optB, then you'll see the real problem, which is that the closure's return type is Proto? and the argument is of type Any, and so Any cannot be converted to Proto?.

To fix your problem, you should cast the intermediate array before invoking compactMap on it:

let arr: [Proto] = ([optA, optB] as [Proto?]).compactMap({$0})

cc @xedin

Yeah, the problem here is that type-checker doesn't try to join types up to protocols so it would attempt element types or `Any`. The problem that `compactMap` disconnects result type from element type (signature is something like `((S.Element) -> Result) -> [Result]`, if it didn't, we'd be able to infer `Proto` for the element type.

@swift-ci create