Fix #3518: Do not generate calls to BoxesRunTime for Unit box/unbox

Author: nicolasstucki

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -169,8 +169,10 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     s eq defn.newArrayMethod
   }
 
-  def isBox(sym: Symbol): Boolean = Erasure.Boxing.isBox(sym)
-  def isUnbox(sym: Symbol): Boolean = Erasure.Boxing.isUnbox(sym)
+  def isBox(sym: Symbol): Boolean = Erasure.Boxing.isBox(sym) && !isInUnit(sym)
+  def isUnbox(sym: Symbol): Boolean = Erasure.Boxing.isUnbox(sym) && !isInUnit(sym)
+
+  private def isInUnit(sym: Symbol) = sym.denot.owner.denot.linkedClass == defn.UnitClass
 
   val primitives: Primitives = new Primitives {
     val primitives = new DottyPrimitives(ctx)

tests/run/i3518.check

@@ -0,0 +1,4 @@
+()
+()
+A
+()

tests/run/i3518.scala

@@ -0,0 +1,12 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    Unit.box(())
+    Unit.unbox(Unit.box(()))
+    println(Unit.box(()))
+    println(Unit.unbox(Unit.box(())))
+    println(Unit.box({
+      println("A")
+      ()
+    }))
+  }
+}