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

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) && sym.denot.owner != defn.UnitModuleClass
+  def isUnbox(sym: Symbol): Boolean =
+    Erasure.Boxing.isUnbox(sym) && sym.denot.owner != defn.UnitModuleClass
 
   val primitives: Primitives = new Primitives {
     val primitives = new DottyPrimitives(ctx)

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -396,6 +396,7 @@ class Definitions {
 
   lazy val UnitType: TypeRef = valueTypeRef("scala.Unit", BoxedUnitType, java.lang.Void.TYPE, UnitEnc, nme.specializedTypeNames.Void)
   def UnitClass(implicit ctx: Context) = UnitType.symbol.asClass
+  def UnitModuleClass(implicit ctx: Context) = UnitType.symbol.asClass.linkedClass
   lazy val BooleanType = valueTypeRef("scala.Boolean", BoxedBooleanType, java.lang.Boolean.TYPE, BooleanEnc, nme.specializedTypeNames.Boolean)
   def BooleanClass(implicit ctx: Context) = BooleanType.symbol.asClass
     lazy val Boolean_notR   = BooleanClass.requiredMethodRef(nme.UNARY_!)

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")
+      ()
+    }))
+  }
+}