WIP https://github.com/scala/scala/compare/2.12.x...dwijnand:wip/7685

data.scala

class Thingy(val unThingy: String) extends AnyVal
class Rocket(val unRocket: Thingy) extends AnyVal

func.scala

object Func {
  def thingyAAA = new Thingy("AAA")

  def rocketAAA = new Rocket(thingyAAA)
  def rocketBBB = new Rocket(new Thingy("BBB"))
}

run1.scala

object Test extends App { println(Func.thingyAAA) }

run2.scala

object Test extends App { println(Func.rocketAAA) }

run3.scala

object Test extends App { println(Func.rocketBBB) }

w.1.before.scala

$ qscala sandbox/data.scala && qscalac -Xprint:erasure,posterasure sandbox/func.scala && { qscala sandbox/run1.scala; qscala sandbox/run2.scala; qscala sandbox/run3.scala; }
[[syntax trees at end of                   erasure]] // func.scala
package <empty> {
  object Func extends Object {
    def <init>(): Func.type = {
      Func.super.<init>();
      ()
    };
    def thingyAAA(): ErasedValueType(class Thingy, String) = new Thingy("AAA".$asInstanceOf[String]()).unThingy().$asInstanceOf[ErasedValueType(class Thingy, String)]();
    def rocketAAA(): ErasedValueType(class Rocket, String) = new Thingy(Func.this.thingyAAA().$asInstanceOf[String]()).$asInstanceOf[String]();
    def rocketBBB(): ErasedValueType(class Rocket, String) = new Rocket("BBB".$asInstanceOf[ErasedValueType(class Thingy, String)]()).unRocket().$asInstanceOf[ErasedValueType(class Rocket, String)]()
  }
}

[[syntax trees at end of               posterasure]] // func.scala
package <empty> {
  object Func extends Object {
    def <init>(): Func.type = {
      Func.super.<init>();
      ()
    };
    def thingyAAA(): String = "AAA";
    def rocketAAA(): String = new Thingy(Func.this.thingyAAA()).$asInstanceOf[String]();
    def rocketBBB(): String = "BBB"
  }
}

Thingy(AAA)
java.lang.ClassCastException: Thingy cannot be cast to java.lang.String
	at Func$.rocketAAA(func.scala:4)
	at Main$.delayedEndpoint$Main$1(run2.scala:1)
  ...
Rocket(Thingy(BBB))

w.2.after.scala

$ qscala sandbox/data.scala && qscalac -Xprint:erasure,posterasure sandbox/func.scala && { qscala sandbox/run1.scala; qscala sandbox/run2.scala; qscala sandbox/run3.scala; }
[[syntax trees at end of                   erasure]] // func.scala
package <empty> {
  object Func extends Object {
    def <init>(): Func.type = {
      Func.super.<init>();
      ()
    };
    def thingyAAA(): ErasedValueType(class Thingy, String) = new Thingy("AAA".$asInstanceOf[String]()).unThingy().$asInstanceOf[ErasedValueType(class Thingy, String)]();
    def rocketAAA(): ErasedValueType(class Rocket, ErasedValueType(class Thingy, String)) = Func.this.thingyAAA();
    def rocketBBB(): ErasedValueType(class Rocket, ErasedValueType(class Thingy, String)) = "BBB".$asInstanceOf[Thingy]().unThingy().$asInstanceOf[ErasedValueType(class Thingy, String)]()
  }
}

[[syntax trees at end of               posterasure]] // func.scala
package <empty> {
  object Func extends Object {
    def <init>(): Func.type = {
      Func.super.<init>();
      ()
    };
    def thingyAAA(): String = "AAA";
    def rocketAAA(): String = Func.this.thingyAAA();
    def rocketBBB(): String = "BBB".$asInstanceOf[Thingy]().unThingy()
  }
}

Thingy(AAA)
Rocket(Thingy(AAA))
java.lang.ClassCastException: java.lang.String cannot be cast to Thingy
	at Func$.rocketBBB(func.scala:5)
	at Main$.delayedEndpoint$Main$1(run3.scala:1)
	...

z.diff

--- /s/before.txt	2016-06-17 20:29:12.000000000 +0200
+++ /s/after.txt	2016-06-17 20:29:12.000000000 +0200
@@ -6,8 +6,8 @@
       ()
     };
     def thingyAAA(): ErasedValueType(class Thingy, String) = new Thingy("AAA".$asInstanceOf[String]()).unThingy().$asInstanceOf[ErasedValueType(class Thingy, String)]();
-    def rocketAAA(): ErasedValueType(class Rocket, String) = new Thingy(Func.this.thingyAAA().$asInstanceOf[String]()).$asInstanceOf[String]();
-    def rocketBBB(): ErasedValueType(class Rocket, String) = new Rocket("BBB".$asInstanceOf[ErasedValueType(class Thingy, String)]()).unRocket().$asInstanceOf[ErasedValueType(class Rocket, String)]()
+    def rocketAAA(): ErasedValueType(class Rocket, ErasedValueType(class Thingy, String)) = Func.this.thingyAAA();
+    def rocketBBB(): ErasedValueType(class Rocket, ErasedValueType(class Thingy, String)) = "BBB".$asInstanceOf[Thingy]().unThingy().$asInstanceOf[ErasedValueType(class Thingy, String)]()
   }
 }

@@ -19,7 +19,7 @@
       ()
     };
     def thingyAAA(): String = "AAA";
-    def rocketAAA(): String = new Thingy(Func.this.thingyAAA()).$asInstanceOf[String]();
-    def rocketBBB(): String = "BBB"
+    def rocketAAA(): String = Func.this.thingyAAA();
+    def rocketBBB(): String = "BBB".$asInstanceOf[Thingy]().unThingy()
   }
 }