Classic Highlighter counts the unused positions before a MultiPhraseQuery's first position as gaps, so a phrase that starts at position 1 or later (a leading stop word plus a synonym) is highlighted with extra slop and non-matching pairs get marked

Finding LFV-829Area: highlighter / original Highlighter, QueryScorer and WeightedSpanTermExtractor (MultiPhraseQuery-to-span conversion)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-829

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You search for the quoted phrase "the dogs bark" with an analyzer that removes the stop word 'the' and adds the synonym dog for dogs (QueryParser and QueryBuilder build a MultiPhraseQuery [dogs|dog] at position 1, bark at position 2, slop 0), and highlight the text 'dogs often bark but my dogs bark' with Highlighter and QueryScorer, expecting 'dogs often bark but my dogs bark' (Weight#matches reports only positions 5-6, and the same query starting at position 0, or a PhraseQuery at positions 1 and 2, is highlighted that way). Instead you get 'dogs often bark but my dogs bark', with no error. WeightedSpanTermExtractor counts every empty slot from index 0 up to the last position as a position gap, including the unused slot 0 below the first position, so the phrase becomes an ordered SpanNearQuery with slop 1 instead of 0 (one extra unit per leading slot, also for sloppy phrases), while its PhraseQuery branch measures gaps from the first position.

The test shows that the classic Highlighter with QueryScorer adds one unit of slop for each unused position before a MultiPhraseQuery's first position, so [dogs|dog]@1 bark@2 (slop 0, what QueryBuilder builds for "the dogs bark" with a stop word and a synonym) also marks the non-adjacent "dogs often bark".

Expected vs actual

Expected dogs often bark but my dogs bark; actual dogs often bark but my dogs bark, with -ea and -da.

Files

src/org/apache/lucene/search/highlight/TestMultiPhraseLeadingPositionHighlight.java (JUnit test)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/highlighter/build/libs/lucene-highlighter-$V.jar
CP=$CP:$LUCENE/lucene/memory/build/libs/lucene-memory-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.search.highlight.TestMultiPhraseLeadingPositionHighlight

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.