Clear __classname__ of duped class/module; ref #4027

src/kernel.c

@@ -282,7 +282,9 @@ init_copy(mrb_state *mrb, mrb_value dest, mrb_value obj)
     case MRB_TT_CLASS:
     case MRB_TT_MODULE:
       copy_class(mrb, dest, obj);
-      /* fall through */
+      mrb_iv_copy(mrb, dest, obj);
+      mrb_iv_remove(mrb, dest, mrb_intern_lit(mrb, "__classname__"));
+      break;
     case MRB_TT_OBJECT:
     case MRB_TT_SCLASS:
     case MRB_TT_HASH: