Improve PhantomDeclErasure postcondition check.

Author: nicolasstucki

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

@@ -30,14 +30,20 @@ class PhantomDeclErasure extends MiniPhaseTransform with InfoTransformer {
    */
   override def checkPostCondition(tree: tpd.Tree)(implicit ctx: Context): Unit = {
     def assertNotPhantom(tpe: Type): Unit =
-      assert(!tpe.derivesFrom(defn.PhantomAnyClass), "All phantom type values should be erased in " + tree)
+      assert(!tpe.derivesFrom(defn.PhantomAnyClass), "All phantom type declarations should be erased in " + tree)
 
     tree match {
       case _: TypeTree             =>
       case ValDef(_, tpt, _)       => assertNotPhantom(tpt.typeOpt)
       case DefDef(_, _, _, tpt, _) => assertNotPhantom(tpt.typeOpt)
       case tree: Trees.TypeDef[_]  =>
-//        tree.symbol.asClass.classInfo.decls // TODO check decls
+        tree.symbol match {
+          case sym: ClassSymbol =>
+            assertNotPhantom(sym.info)
+            assert(!sym.classInfo.decls.exists(sym2 => isPhantomMethodType(sym2.info)),
+                "All phantom type declarations should be erased in " + sym.classInfo)
+          case _ =>
+        }
       case _                       =>
     }