code: Type classEnter(JCTree tree, Env<AttrContext> env) {
          Env<AttrContext> prevEnv = this.env;
          try {
              this.env = env;
              annotate.blockAnnotations();
              tree.accept(this);
              return result;
          }  catch (CompletionFailure ex) {
              return chk.completionError(tree.pos(), ex);
          } finally {
              annotate.unblockAnnotations();
              this.env = prevEnv;
          }
      }
docs: Visitor method: enter all classes in given tree, catching any
  completion failure exceptions. Return the tree's type.
  
  @param tree    The tree to be visited.
  @param env     The environment visitor argument.