Only remove synthetic companion objects

If the object was explicitly written, it might be
referenced, even if it is empty.

Author: odersky

src/dotty/tools/dotc/core/Flags.scala

@@ -606,6 +606,7 @@ object Flags {
   final val AbstractFinal = allOf(Abstract, Final)
   final val AbstractSealed = allOf(Abstract, Sealed)
   final val SyntheticArtifact = allOf(Synthetic, Artifact)
+  final val SyntheticModule = allOf(Synthetic, Module)
   final val SyntheticTermParam = allOf(Synthetic, TermParam)
   final val SyntheticTypeParam = allOf(Synthetic, TypeParam)
   final val SyntheticCase = allOf(Synthetic, Case)

src/dotty/tools/dotc/transform/DropEmptyCompanions.scala

@@ -26,10 +26,12 @@ class DropEmptyCompanions extends MiniPhase { thisTransform =>
 
     /** Is `tree` an empty companion object? */
     private def isEmptyCompanion(tree: Tree)(implicit ctx: Context) = tree match {
-      case TypeDef(_, impl: Template) =>
-        tree.symbol.is(Module) &&
+      case TypeDef(_, impl: Template) if
+        tree.symbol.is(SyntheticModule) &&
         tree.symbol.companionClass.exists &&
-        impl.body.forall(_.symbol.isPrimaryConstructor)
+        impl.body.forall(_.symbol.isPrimaryConstructor) =>
+        //println(i"removing ${tree.symbol}")
+        true
       case _ =>
         false
     }