Transform annotations only if defined in current run

There's no point transforming annotations that come from
classfiles. It's inefficient to do so and it's also risky
because it means we'd have to make sense of Scala-2 generated trees.

This should avoid the error in #1222.

Author: odersky

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

@@ -184,13 +184,15 @@ object TreeTransforms {
       val info1 = transformInfo(ref.info, ref.symbol)
 
       ref match {
-        case ref: SymDenotation =>
+        case ref: SymDenotation if ref.symbol.isDefinedInCurrentRun =>
+          // leave annotations read from class files alone
           val annotTrees = ref.annotations.map(_.tree)
           val annotTrees1 = annotTrees.mapConserve(annotationTransformer.macroTransform)
           val annots1 = if (annotTrees eq annotTrees1) ref.annotations else annotTrees1.map(new ConcreteAnnotation(_))
           if ((info1 eq ref.info) && (annots1 eq ref.annotations)) ref
           else ref.copySymDenotation(info = info1, annotations = annots1)
-        case _ => if (info1 eq ref.info) ref else ref.derivedSingleDenotation(ref.symbol, info1)
+        case _ =>
+          if (info1 eq ref.info) ref else ref.derivedSingleDenotation(ref.symbol, info1)
       }
     }
   }