Avoid erasing side effects of argumet of Unit.{box|unbox}

Author: nicolasstucki

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

@@ -477,9 +477,8 @@ object Erasure {
 	 */
     override def typedApply(tree: untpd.Apply, pt: Type)(implicit ctx: Context): Tree = {
       val Apply(fun, args) = tree
-      if (fun.symbol == defn.cbnArg)
+      if (fun.symbol == defn.cbnArg || (tree.symbol.name == nme.box || tree.symbol.name == nme.unbox) && tree.symbol.owner.asClass.companionClass == defn.UnitClass)
         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

@@ -1,2 +1,4 @@
 ()
 ()
+A
+()

tests/run/i3518.scala

@@ -4,5 +4,9 @@ object Test {
     Unit.unbox(Unit.box(()))
     println(Unit.box(()))
     println(Unit.unbox(Unit.box(())))
+    println(Unit.box({
+      println("A")
+      ()
+    }))
   }
 }