Scala assignment and assigned class update

Having explored macros and reflection (2.10., 2.11.) still unable to find a solution for

case class C(var tag: Option[String] = None) 

so that

val myTagValue = C()
assert ( myTagValue.tag == Some("myTagValue") )

To ask for any hints to approach this, perhaps with macros/reflection that update instance C() at compile time; any other approaches equally valid.

Many Thanks.

Note related (unanswered) posts with similar enquiries,

• Scala quasiquotes and class instance reified


• Scala macro for assigned value label in apply method