Is that a bug? 🤔

type-inf.swift

// It seems like Swift cannot correctly infer the type on the *second element* of the expression 
func postOrder(_ node: Node?) -> [Int] {
  guard let node = node else { return [] }
  return [node.content] + postOrder(node.left) + postOrder(node.right) // ❌ ERROR: 'Int' is not convertible to [Int]
}

// If I specify than it works... 
func postOrder(_ node: Node?) -> [Int] {
  guard let node = node else { return [] }
  return [node.content] + postOrder(node.left) + (postOrder(node.right) as [Int]) // ✅ That works 
}