mkStableAttributedRef.scala

scala> def id[A <: AnyRef](a: A): a.type = a
id: [A <: AnyRef](a: A)a.type

scala> val c = ""; id(c): c.type
c: String = ""
res0: c.type = ""

scala> val c = ""; { val temp = c; id(temp)} : c.type
<console>:11: error: type mismatch;
 found   : temp.type (with underlying type String)
 required: c.type
        { val temp = c; id(temp)} : c.type
                          ^

scala> val c = ""; { val temp: c.type = c; id(temp)} : c.type
c: String = ""
res2: c.type = ""