Handle generic return type in Spring Data JPA repositories

Fixes: #31774

Author: geoand

extensions/spring-data-jpa/deployment/src/main/java/io/quarkus/spring/data/deployment/generate/DerivedMethodsAdder.java

@@ -20,10 +20,12 @@
 import org.jboss.jandex.IndexView;
 import org.jboss.jandex.MethodInfo;
 import org.jboss.jandex.Type;
+import org.jboss.jandex.TypeVariable;
 import org.springframework.data.domain.Pageable;
 import org.springframework.data.domain.Sort;
 
 import io.quarkus.deployment.bean.JavaBeanUtil;
+import io.quarkus.deployment.util.JandexUtil;
 import io.quarkus.gizmo.ClassCreator;
 import io.quarkus.gizmo.ClassOutput;
 import io.quarkus.gizmo.FieldDescriptor;
@@ -169,7 +171,8 @@ public void add(ClassCreator classCreator, FieldDescriptor entityClassFieldDescr
                             methodCreator.readInstanceField(entityClassFieldDescriptor, methodCreator.getThis()),
                             methodCreator.load(finalQuery), sort, paramsArray);
 
-                    Type resultType = verifyQueryResultType(method.returnType(), index);
+                    Type resultType = extractResultType(repositoryClassInfo, method);
+
                     DotName customResultTypeName = resultType.name();
 
                     if (customResultTypeName.equals(entityClassInfo.name())
@@ -287,6 +290,25 @@ public void add(ClassCreator classCreator, FieldDescriptor entityClassFieldDescr
         }
     }
 
+    private Type extractResultType(ClassInfo repositoryClassInfo, MethodInfo method) {
+        Type resultType = verifyQueryResultType(method.returnType(), index);
+        if (resultType.kind() == Type.Kind.TYPE_VARIABLE) {
+            // we can handle the generic result type case where interface only declares one generic type that is the same as the method result type uses (TODO: look into enhancing)
+            // this is accomplished by resolving the generic type from the interface we are actually implementing
+            TypeVariable resultTypeVariable = resultType.asTypeVariable();
+            List<TypeVariable> interfaceTypeVariables = method.declaringClass().typeParameters();
+            if (interfaceTypeVariables.size() == 1 && interfaceTypeVariables.get(0)
+                    .equals(resultTypeVariable)) {
+                List<Type> resolveTypeParameters = JandexUtil.resolveTypeParameters(repositoryClassInfo.name(),
+                        method.declaringClass().name(), index);
+                if (resolveTypeParameters.size() == 1) {
+                    return resolveTypeParameters.get(0);
+                }
+            }
+        }
+        return resultType;
+    }
+
     private void addAllMethodOfIntermediateRepository(DotName interfaceDotName, Set<MethodInfo> result) {
         if (GenerationUtil.isIntermediateRepository(interfaceDotName, index)) {
             ClassInfo classInfo = index.getClassByName(interfaceDotName);

integration-tests/spring-data-jpa/src/main/java/io/quarkus/it/spring/data/jpa/IntermediatePostRepository.java

@@ -3,10 +3,13 @@
 import java.time.ZonedDateTime;
 import java.util.List;
 
+import org.springframework.data.repository.NoRepositoryBean;
+
 /**
  * Used to ensure that entity relationships work correctly
  */
-public interface IntermediatePostRepository extends BypassHolderRepository<Post, Long> {
+@NoRepositoryBean
+public interface IntermediatePostRepository<T extends ByPassHolder> extends BypassHolderRepository<T, Long> {
 
-    List<Post> findByPostedBefore(ZonedDateTime zdt);
+    List<T> findByPostedBefore(ZonedDateTime zdt);
 }

integration-tests/spring-data-jpa/src/main/java/io/quarkus/it/spring/data/jpa/PostRepository.java

@@ -5,7 +5,7 @@
 /**
  * Used to ensure that entity relationships work correctly
  */
-public interface PostRepository extends IntermediatePostRepository {
+public interface PostRepository extends IntermediatePostRepository<Post> {
 
     List<Post> findAllByOrganization(String organization);