class AClass {}
protocol AProto { var value: Int { get } }

class Box<T> { 
  private var _boxed: T
  init(_ b: T) { _boxed = b } 
}
extension Box where T: AClass { 
  var boxed: T { 
    get { return _boxed }
    set { _boxed = newValue } 
  } 
}

class One: AClass, AProto { var value: Int { return 1 } }
class Two: AClass, AProto { var value: Int { return 2 } }

let a: Box<AClass & AProto> = Box(One())
print(a.boxed) // error: 'AClass & AProto' is not convertible to 'AnyObject'
a.boxed = Two()
print(a.boxed)

let b: AClass & AProto = One()
print(b is AClass) // 'true'