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

src/parsing/scanner.cc

Index: src/parsing/scanner.cc
diff --git a/src/parsing/scanner.cc b/src/parsing/scanner.cc
index be4100a0971688dea6165b9bf77cabe8a190185b..ea0b58cdf72ed0e7f39a630fd4e2cee736e01897 100644
--- a/src/parsing/scanner.cc
+++ b/src/parsing/scanner.cc
@@ -1608,9 +1608,9 @@ void Scanner::ResetToBookmark() {
   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;