Fix #505: Remove unnecessary boxing of synchronized block

`synchronized` is now treated as a magic method by the compiler and is no
longer erased (similarly to `asInstanceOf` and `isInstanceOf`). Backend
is updated to handle this new magic polymorphic method.

Author: allanrenucci

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

@@ -116,6 +116,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   val Throwable_Type: Type = defn.ThrowableType
   val Object_isInstanceOf: Symbol = defn.Any_isInstanceOf
   val Object_asInstanceOf: Symbol = defn.Any_asInstanceOf
+  val Object_synchronized: Symbol = defn.Object_synchronized
   val Object_equals: Symbol = defn.Any_equals
   val ArrayClass: Symbol = defn.ArrayClass
   val UnitClass: Symbol = defn.UnitClass

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

@@ -1074,7 +1074,7 @@ class Definitions {
   lazy val NoInitClasses: Set[Symbol] = NotRuntimeClasses + FunctionXXLClass
 
   def isPolymorphicAfterErasure(sym: Symbol): Boolean =
-     (sym eq Any_isInstanceOf) || (sym eq Any_asInstanceOf)
+     (sym eq Any_isInstanceOf) || (sym eq Any_asInstanceOf) || (sym eq Object_synchronized)
 
   def isTupleType(tp: Type)(implicit ctx: Context): Boolean = {
     val arity = tp.dealias.argInfos.length

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

@@ -168,11 +168,10 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
     val initSymbol = ctx.newSymbol(x.symbol.owner, initName, initFlags, MethodType(Nil, tpe), coord = x.pos)
     val rhs = x.rhs.changeOwnerAfter(x.symbol, initSymbol, this)
     val initialize = holderRef.select(lazyNme.initialize).appliedTo(rhs)
-    val initBody =
-      adaptToType(
-        holderRef.select(defn.Object_synchronized).appliedTo(
-          adaptToType(If(initialized, getValue, initialize), defn.ObjectType)),
-        tpe)
+    val initBody = holderRef
+      .select(defn.Object_synchronized)
+      .appliedToType(tpe)
+      .appliedTo(If(initialized, getValue, initialize).ensureConforms(tpe))
     val initTree = DefDef(initSymbol, initBody)
 
     // def x(): Int = if (x$lzy.initialized()) x$lzy.value() else x$lzycompute()

scala-backend

@@ -0,0 +1,15 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val a: Int = synchronized(1)
+    val b: Long = synchronized(1L)
+    val c: Boolean = synchronized(true)
+    val d: Float = synchronized(1f)
+    val e: Double = synchronized(1.0)
+    val f: Byte = synchronized(1.toByte)
+    val g: Char = synchronized('1')
+    val h: Short = synchronized(1.toShort)
+    val i: String = synchronized("Hello")
+    val j: List[Int] = synchronized(List(1))
+    synchronized(())
+  }
+}