Recursive structures in Swift without Box classes

Problem.swift

struct Node<T> { // error: recursive value type 'Node<T>' is not allowed
  var child: Node?
  var value: T
  
  init(value: T, child: Node? = nil) {
    self.value = value
    self.child = child
  }
}

Solution.swift

struct Node<T> {
  private var _child: [Node]?
  var child: Node? {
    set {
      _child = newValue.map{[$0]}
    }
    get {
      return _child?.first 
    }
  }
  var value: T
  
  init(value: T, child: Node? = nil) {
    _child = child.map{ [$0] }
    self.value = value
  }
}

// Usage:
var root = Node(value: "a")
let child = Node(value: "b", child: Node(value: "c"))
root.child = child