Scala quasiquotes and class instance reified

Let

case class C(var tag: String = "")

and a tree from quasiquoting val a = C()

val x = q" val a = C() "
x: reflect.runtime.universe.ValDef = val a = C()

Given that

x.name.toString
String = a

how to set a.tag = x.name.toString so that

assert ( a.tag = "a" )