Fix #3000: Do not flatten across packages

Author: nicolasstucki

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

@@ -27,7 +27,20 @@ class Flatten extends MiniPhaseTransform with SymTransformer { thisTransform =>
     else ref
   }
 
-  private val liftedDefs = new mutable.ListBuffer[Tree]
+  private var liftedDefs = new mutable.ListBuffer[Tree]
+
+  private lazy val liftedDefsQueue = mutable.Stack.empty.asInstanceOf[mutable.Stack[mutable.ListBuffer[Tree]]]
+
+  override def prepareForPackageDef(tree: PackageDef)(implicit ctx: Context) = {
+    liftedDefsQueue.push(liftedDefs)
+    liftedDefs = new mutable.ListBuffer[Tree]
+    this
+  }
+
+  override def transformPackageDef(tree: PackageDef)(implicit ctx: Context, info: TransformerInfo) = {
+    liftedDefs = liftedDefsQueue.pop()
+    tree
+  }
 
   private def liftIfNested(tree: Tree)(implicit ctx: Context, info: TransformerInfo) =
     if (ctx.owner is Package) tree

tests/pos/i3000.scala

@@ -0,0 +1,5 @@
+class Foo {
+  new Object { }
+}
+
+package bar { }