UnifiedHighlighter and MatchRegionRetriever throw IllegalArgumentException on a MultiReader whose indexes differ in any field's schema, even only in norms or in an unrelated field, because FieldInfos.getMergedFieldInfos applies the single-index consistency check across the indexes

Finding LFV-602Area: highlighter / uhighlight + matchhighlight: default offset-source choice on composite readers (core index FieldInfos.getMergedFieldInfos)Priority: mediumImpact: crashNovelty: known outside Lucene

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

What it shows

You build two indexes in which a field is declared slightly differently, for example 'body' with norms in one and without norms in the other (both with offsets), open them together as new MultiReader(r1, r2), search body:alpha and get both hits. You expect UnifiedHighlighter.highlight("body", query, topDocs) to return both snippets ('alpha beta', 'alpha gamma'), as it does when both indexes use the same options or when each index is highlighted on its own. Instead it throws IllegalArgumentException 'cannot change field "body" from omitNorms=false to inconsistent omitNorms=true' (and 'cannot change field "tag" ...' when only an unrelated field differs), because the default offset-source choice builds FieldInfos.getMergedFieldInfos over the whole MultiReader, which applies the per-index field-consistency check across the two indexes; MatchRegionRetriever's default strategies fail the same way.

Expected vs actual

Expected: You expect UnifiedHighlighter.highlight("body", query, topDocs) to return both snippets ('alpha beta', 'alpha gamma'), as it does when both indexes use the same options or when each index is highlighted on its own.
Actual: It throws IllegalArgumentException 'cannot change field "body" from omitNorms=false to inconsistent omitNorms=true' (and 'cannot change field "tag" ...' when only an unrelated field differs), because the default offset-source choice builds FieldInfos.getMergedFieldInfos over the whole MultiReader, which applies the per-index field-consistency check across the two indexes; MatchRegionRetriever's default strategies fail the same way.

Files

src/org/apache/lucene/search/uhighlight/TestHighlightMultiReaderMixedFieldOptions.java (JUnit test)
src/repro/HighlightMultiReaderMixedOptionsMain.java (main() class)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main a9779ef3c193 (2026-10-26): reproduced.
How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual); a main() class that exits non-zero, with an exception or AssertionError, is the finding reproducing.
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.uhighlight.TestHighlightMultiReaderMixedFieldOptions

# run 2: assertions on, seed DEADBEEF, default flags
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" repro.HighlightMultiReaderMixedOptionsMain

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.