Another pass at unboxed newtype in Scala

gistfile1.scala

type Tagged[T] = { type Tag = T }
type @@[T, U] = T with Tagged[U]

class MyStringOps(s : String) {
  def mySize = s.size
}

// Use of Any means we forget the underlying type
type MyString = Any @@ MyStringOps
def MyString(s : String) = s.asInstanceOf[MyString] 

// Implicit resolution is driven by the tag
implicit def myStringOps(ms : MyString) = new MyStringOps(ms.asInstanceOf[String])  
    
val s = "foo"
val ms = MyString(s)
    
ms eq s    // true, showing that we have an unboxed representation

val s2 : String = ms  // Does not compile
ms.size               // Does not compile
ms.mySize             // Compiles