Returning Self from a non-final class method

My Objective-C framework defines a protocol with a singleton getter:

@protocol ShareableProtocol
+ (nonnull instancetype)sharedInstance;
@end

A Swift class implements the protocol.

import SharingFramework

public class ShareableClass: NSObject, ShareableProtocol {
 
 public static singleton = ShareableClass()

 public static func sharedInstance() -> Self {
      return singleton // error
  }

}

The sharedInstance signature was generated by Swift 5. How might I return a concrete class instance, singleton, while satisfying the 'Self' return type? The compiler wants a dynamic return type (like Self) in order to also work from inherited classes, but I don't know how to return an explicit object (singleton) in this way. I've tried:

• "return singleton" fails with "Cannot convert return expression of type 'ShareableClass' to return type 'Self'. This is because of SR-695.
• "return singleton as ShareableProtocol" fails similarly: cannot convert type ShareableProtocol to type Self.
• Changing sharedInstance()'s return type to ShareableProtocol or ShareableClass causes the class to no longer conform to ShareableProtocol.
• Making the class final and returning my class type satisfies SR-695 and compiles, but feels forced because I don't necessarily want this class final. I suspect there is a more dynamic way, saying "I'm returning an instance of the current class, whatever that is."

public final class ShareableClass: NSObject, ShareableProtocol { // add 'final'
public static func sharedInstance() -> ShareableClass { // return concrete class

Thank you.