ConstReferenceTypeAccess.swift

class Po<Element> {
  var element: Element?
  
  init(element: Element) {
    self.element = element
  }
}

protocol ConstPo {
  associatedtype Element
  var element: Element? { get }
}

extension Po: ConstPo {}

func processPo<X: ConstPo>(po: X, process: (X.Element) -> Void) {
  process(po.element!)
  // po.element = nil // Compile error!
}

let po = Po(element: 42)

processPo(po: po) { (intValue: Int) in
  print(intValue)  
}