Flexible StandardQueryParser: the (...)@N min-should-match operator throws ClassCastException when the group does not build a BooleanQuery, e.g. (the fox)@1 with a stop-word analyzer or the boosted group (fox dog)^2@1

Finding LFV-732Area: queryparser / flexible: standard query builders (MinShouldMatchNodeBuilder for the @N min-should-match operator, composed with the grammar's group boost and the single-child Boolean optimization)Priority: lowImpact: wrong errorNovelty: new

Finding page for LFV-732

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

What it shows

You parse (the fox)@1 ('at least one of the, fox') with the flexible StandardQueryParser and an analyzer that drops the stopword 'the', or the boosted group (fox dog)^2@1; the class javadoc documents @N for disjunction groups and boosts on grouped clauses, and parse() declares QueryNodeException for failures, so you expect a query (at least one of the remaining clauses, boosted as written). Instead parse() throws java.lang.ClassCastException ('TermQuery / BoostQuery cannot be cast to BooleanQuery') from MinShouldMatchNodeBuilder.build, which casts the group's query to BooleanQuery without a check: after the stopword is removed the one-clause group is collapsed to a TermQuery, and the grammar puts the boost inside the min-should-match node, so the group's query is a BoostQuery. Groups holding a single term, phrase, prefix or other non-Boolean query, such as (fox)@1 or ("quick fox")@1, fail the same way, while (fox dog)@1 works.

The test shows that StandardQueryParser with a stop-word analyzer cannot parse the documented min-should-match group "(the fox)@1".

Expected vs actual

Expected: parse() returns a query (at least 1 of {fox}); actual: ClassCastException "TermQuery cannot be cast to BooleanQuery" from MinShouldMatchNodeBuilder.java:40 (-ea and -da).

Files

Run it

# 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/facet/build/libs/lucene-facet-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:$LUCENE/lucene/queryparser/build/libs/lucene-queryparser-$V.jar
CP=$CP:$LUCENE/lucene/sandbox/build/libs/lucene-sandbox-$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 73200001
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=73200001 \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.queryparser.flexible.standard.TestMinShouldMatchStopwordGroup

The module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.

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

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