Fix match_phrase_prefix query with single term on _all field (#20471)

* Fix match_phrase_prefix query with single term on _all field

This change fixes the match_phrase_prefix query when a single term is queried on the _all field.
It builds a prefix query instead of an AllTermQuery which would not match any prefix.

Fixes #20470

* Add missing change

core/src/main/java/org/elasticsearch/common/lucene/all/AllTermQuery.java

@@ -64,6 +64,10 @@ public final class AllTermQuery extends Query {
         this.term = term;
     }
 
+    public Term getTerm() {
+        return term;
+    }
+
     @Override
     public boolean equals(Object obj) {
         if (sameClassAs(obj) == false) {

core/src/main/java/org/elasticsearch/index/search/MatchQuery.java

@@ -37,6 +37,7 @@ import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.common.lucene.all.AllTermQuery;
 import org.elasticsearch.common.lucene.search.MultiPhrasePrefixQuery;
 import org.elasticsearch.common.lucene.search.Queries;
 import org.elasticsearch.common.unit.Fuzziness;
@@ -324,6 +325,9 @@ public class MatchQuery {
             } else if (query instanceof TermQuery) {
                 prefixQuery.add(((TermQuery) query).getTerm());
                 return prefixQuery;
+            } else if (query instanceof AllTermQuery) {
+                prefixQuery.add(((AllTermQuery) query).getTerm());
+                return prefixQuery;
             }
             return query;
         }

core/src/test/java/org/elasticsearch/index/search/MultiMatchQueryTests.java

@@ -22,15 +22,16 @@ package org.elasticsearch.index.search;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.queries.BlendedTermQuery;
 import org.apache.lucene.search.BooleanClause;
+import org.apache.lucene.search.BooleanClause.Occur;
 import org.apache.lucene.search.BooleanQuery;
 import org.apache.lucene.search.BoostQuery;
 import org.apache.lucene.search.DisjunctionMaxQuery;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
-import org.apache.lucene.search.BooleanClause.Occur;
 import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.common.compress.CompressedXContent;
+import org.elasticsearch.common.lucene.search.MultiPhrasePrefixQuery;
 import org.elasticsearch.index.IndexService;
 import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.index.mapper.MapperService;
@@ -45,6 +46,8 @@ import java.io.IOException;
 import java.util.Arrays;
 
 import static org.elasticsearch.index.query.QueryBuilders.multiMatchQuery;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
 
 public class MultiMatchQueryTests extends ESSingleNodeTestCase {
 
@@ -154,4 +157,13 @@ public class MultiMatchQueryTests extends ESSingleNodeTestCase {
         Query actual = MultiMatchQuery.blendTerm(new BytesRef("baz"), null, 1f, new FieldAndFieldType(ft1, 2), new FieldAndFieldType(ft2, 3));
         assertEquals(expected, actual);
     }
+
+    public void testMultiMatchPrefixWithAllField() throws IOException {
+        QueryShardContext queryShardContext = indexService.newQueryShardContext();
+        queryShardContext.setAllowUnmappedFields(true);
+        Query parsedQuery =
+            multiMatchQuery("foo").field("_all").type(MultiMatchQueryBuilder.Type.PHRASE_PREFIX).toQuery(queryShardContext);
+        assertThat(parsedQuery, instanceOf(MultiPhrasePrefixQuery.class));
+        assertThat(parsedQuery.toString(), equalTo("_all:\"foo*\""));
+    }
 }