public struct Observable<T> {}
public enum Geo { public struct Rect {} }

public protocol MapController: class {
  associatedtype PlaceType
  associatedtype OptionsType

  var places: Observable<[PlaceType]> { get }

  var region: Geo.Rect { get set }
  var options: OptionsType { get set }
  func setRegion(_ newRegion: Geo.Rect, options: OptionsType)
}

protocol MapControllerWrapper: MapController {
  associatedtype Wrapped: MapController
  associatedtype OptionsType = Wrapped.OptionsType

  var base: Wrapped { get }
}

// This default implementation seems to work. Note it is declared 'public'.

extension MapControllerWrapper {
  public var region: Geo.Rect {
    get { return base.region }
    set { base.region = newValue }
  }
}

// This one doesn't.
// "Cannot declare a public var in an extension with internal requirements".

extension MapControllerWrapper where OptionsType == Wrapped.OptionsType {
  public var options: OptionsType {
    get { return base.options }
    set { base.options = newValue }
  }
  public func setRegion(_ newRegion: Geo.Rect, options: OptionsType) {
    base.setRegion(newRegion, options: options)
  }
}