Fix #3518: Erase explicit Unit.box and Unit.unbox in erasure

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -479,6 +479,7 @@ object Erasure {
       val Apply(fun, args) = tree
       if (fun.symbol == defn.cbnArg)
         typedUnadapted(args.head, pt)
+      else if ((tree.symbol.name == nme.box || tree.symbol.name == nme.unbox) && tree.symbol.owner.asClass.companionClass == defn.UnitClass) ref(defn.BoxedUnit_UNIT)
       else typedExpr(fun, FunProto(args, pt, this)) match {
         case fun1: Apply => // arguments passed in prototype were already passed
           fun1

tests/run/i3518.check

@@ -0,0 +1,2 @@
+()
+()

tests/run/i3518.scala

@@ -0,0 +1,8 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    Unit.box(())
+    Unit.unbox(Unit.box(()))
+    println(Unit.box(()))
+    println(Unit.unbox(Unit.box(())))
+  }
+}