Fix potential class-cast exception.

See #3895

Author: mp911de

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/query/QueryRenderer.java

@@ -190,6 +190,10 @@ public static QueryRenderer expression(QueryTokenStream tokenStream) {
 			return EmptyQueryRenderer.INSTANCE;
 		}
 
+		if (!(tokenStream instanceof QueryRenderer)) {
+			tokenStream = QueryRenderer.from(tokenStream);
+		}
+
 		if (tokenStream.isExpression()) {
 			return (QueryRenderer) tokenStream;
 		}
@@ -207,6 +211,10 @@ public static QueryRenderer inline(QueryTokenStream tokenStream) {
 			return EmptyQueryRenderer.INSTANCE;
 		}
 
+		if (!(tokenStream instanceof QueryRenderer)) {
+			tokenStream = QueryRenderer.from(tokenStream);
+		}
+
 		if (!tokenStream.isExpression()) {
 			return (QueryRenderer) tokenStream;
 		}
@@ -341,6 +349,12 @@ public int size() {
 		public boolean isExpression() {
 			return !nested.isEmpty() && nested.get(nested.size() - 1).isExpression();
 		}
+
+		public Stream<QueryRenderer> renderers() {
+			return nested.stream()
+					.flatMap(renderer -> renderer instanceof CompositeRenderer ? ((CompositeRenderer) renderer).renderers()
+							: Stream.of(renderer));
+		}
 	}
 
 	/**

src/main/antora/modules/ROOT/pages/repositories/projections.adoc

@@ -32,9 +32,12 @@ Support for string-based queries covers both, JPQL queries(`@Query`) and native
 
 ==== JPQL Queries
 
-When using <<projections.dtos,Class-based projections>> with JPQL, you must use *constructor expressions* in your JPQL query, e.g. `SELECT new com.example.NamesOnly(u.firstname, u.lastname) from User u`.
+JPA's mechanism to return <<projections.dtos,Class-based projections>> using JPQL is *constructor expressions*.
+Therefore, your query must define a constructor expression such as `SELECT new com.example.NamesOnly(u.firstname, u.lastname) from User u`.
 (Note the usage of a FQDN for the DTO type!) This JPQL expression can be used in `@Query` annotations as well where you define any named queries.
-As a workaround you may use named queries with `ResultSetMapping` or the Hibernate-specific javadoc:{hibernatejavadocurl}org.hibernate.query.ResultListTransformer[]
+As a workaround you may use named queries with `ResultSetMapping` or the Hibernate-specific javadoc:{hibernatejavadocurl}org.hibernate.query.ResultListTransformer[].
+
+Spring Data JPA can aid with rewriting your query to a constructor expression if your query selects the primary entity or a list of select items.
 
 ===== DTO Projection JPQL Query Rewriting