SpanWithinQuery's Weight#matches reports the contained (little) span's positions but the enclosing (big) span's offsets and sub-matches, so Matches-based highlighting (the default UnifiedHighlighter, MatchRegionRetriever) marks the enclosing terms and a field with offsets highlights differently from the same text indexed with positions only

Finding LFV-266Area: queries / span queries Matches API (SpanWeight#matches takes offsets and sub-matches from ContainSpans#collect for SpanWithinQuery; seen through the UnifiedHighlighter and MatchRegionRetriever)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-266

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

What it shows

You index 'ab ac bc' in a field with offsets and ask Weight#matches for SpanWithinQuery(big = SpanNear([ab, bc], slop 1, in order), little = ac); you expect one match, 'ac' at position 1 with offsets [3,5) and sub-matches only inside it, which is what the equivalent Intervals.containedBy(ac, ordered(ab, bc)) reports. Instead the match says positions [1,1] but offsets [0,8) and lists ab@0 and bc@2 as sub-matches, because the offsets and sub-matches come from Spans#collect, which for SpanWithinQuery also collects the enclosing span. So the default UnifiedHighlighter marks 'ab ac bc' instead of 'ab ac bc', and MatchRegionRetriever returns [0,8) on a field with offsets but [3,5) on the same text indexed with positions only; hits and scores are unaffected.

The test shows that Weight#matches of SpanWithinQuery(SpanNear([ab, bc], 1, ordered), ac) on "ab ac bc" reports the contained span's position (ac@1) but the enclosing span's offsets.

Expected vs actual

Expected startOffset 3 (the match is "ac", offsets [3,5)); actual 0, the start of the enclosing "ab" (expected:<3> but was:<0>).

Files

src/org/apache/lucene/queries/spans/TestSpanWithinMatchesOffsets.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/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.queries.spans.TestSpanWithinMatchesOffsets

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.