Behold, a unit test for Scanner::BookmarkScope (& scanner bookmarking).

src/parsing/scanner.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Features shared by parsing and pre-parsing scanners.
 
 #include "src/parsing/scanner.h"
 
 #include <stdint.h>
 
@@ skipping 1590 matching lines @@
   DCHECK(BookmarkHasBeenSet());  // Caller hasn't called SetBookmark.
 
   source_->ResetToBookmark();
   c0_ = bookmark_c0_;
   CopyToNextTokenDesc(&bookmark_current_);
   current_ = next_;
   CopyToNextTokenDesc(&bookmark_next_);
   bookmark_c0_ = kBookmarkWasApplied;
 }
 
-
-bool Scanner::BookmarkHasBeenSet() { return bookmark_c0_ >= 0; }
-
+bool Scanner::BookmarkHasBeenSet() {
+  return bookmark_c0_ != kNoBookmark && bookmark_c0_ != kBookmarkWasApplied;
+}
 
 bool Scanner::BookmarkHasBeenReset() {
   return bookmark_c0_ == kBookmarkWasApplied;
 }
 
 
 void Scanner::DropBookmark() { bookmark_c0_ = kNoBookmark; }
 
 void Scanner::CopyToNextTokenDesc(TokenDesc* from) {
   StartLiteral();
   StartRawLiteral();
   CopyTokenDesc(&next_, from);
   if (next_.literal_chars->length() == 0) next_.literal_chars = nullptr;
   if (next_.raw_literal_chars->length() == 0) next_.raw_literal_chars = nullptr;
 }
 
 void Scanner::CopyTokenDesc(TokenDesc* to, TokenDesc* from) {
   DCHECK_NOT_NULL(to);
   DCHECK_NOT_NULL(from);
   to->token = from->token;
   to->location = from->location;
   to->literal_chars->CopyFrom(from->literal_chars);
   to->raw_literal_chars->CopyFrom(from->raw_literal_chars);
 }
 
 
 
 }  // namespace internal
 }  // namespace v8