xxxx-Swift.h No type or protocol named 'ASWebAuthenticationPresentationContextProviding'

I'm trying to implement an ASWebAuthenticationSession interaction. I've created all the bits correctly, as far as I can tell, including implementing the

Code Block    
func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
    return self.view.window ?? ASPresentationAnchor()
  }

function and including the appropriate protocol in the class definition:

Code Block 
class CatalogTableViewController: UIViewController, UISplitViewControllerDelegate, URLSessionDelegate,
  UITableViewDelegate, UITableViewDataSource, ASWebAuthenticationPresentationContextProviding {

When I try to build, I'm getting the following error:

xxxx-Swift.h:357:59: No type or protocol named 'ASWebAuthenticationPresentationContextProviding'

I'm very confused, because I have the appropriate import that should include the protocol:

Code Block 
import AuthenticationServices

I've been beating my head against this now, for a couple of days. Any help or guidance would be appreciated. This is all Swift code, no ObjC.

Steve