Resolve repository method return types against the repository interface

src/main/java/org/springframework/data/repository/core/support/QueryExecutorMethodInterceptor.java

@@ -51,6 +51,7 @@
  * @author Christoph Strobl
  * @author John Blum
  * @author Johannes Englmeier
+ * @author Donghwan Kim
  */
 class QueryExecutorMethodInterceptor implements MethodInterceptor {
 
@@ -137,7 +138,7 @@ private void invokeListeners(RepositoryQuery query) {
 	public @Nullable Object invoke(MethodInvocation invocation) throws Throwable {
 
 		Method method = invocation.getMethod();
-		MethodParameter returnType = returnTypeMap.computeIfAbsent(method, it -> new MethodParameter(it, -1));
+		MethodParameter returnType = returnTypeMap.computeIfAbsent(method, this::createReturnTypeParameter);
 
 		QueryExecutionConverters.ExecutionAdapter executionAdapter = QueryExecutionConverters //
 				.getExecutionAdapter(returnType.getParameterType());
@@ -150,6 +151,16 @@ private void invokeListeners(RepositoryQuery query) {
 				.apply(() -> resultHandler.postProcessInvocationResult(doInvoke(invocation), returnType));
 	}
 
+	/**
+	 * Creates the {@link MethodParameter} describing the return type of the given {@link Method}. The repository
+	 * interface is registered as containing class so that a return type declared as type variable on a base interface
+	 * (e.g. {@code T findById(ID id)}) resolves against the concrete repository interface instead of the type variable's
+	 * bound.
+	 */
+	private MethodParameter createReturnTypeParameter(Method method) {
+		return new MethodParameter(method, -1).withContainingClass(repositoryInformation.getRepositoryInterface());
+	}
+
 	@SuppressWarnings("NullAway")
 	private @Nullable Object doInvoke(MethodInvocation invocation) throws Throwable {
 

src/test/java/org/springframework/data/repository/core/support/RepositoryFactorySupportUnitTests.java

@@ -87,6 +87,7 @@
  * @author Mark Paluch
  * @author Ariel Carrera
  * @author Johannes Englmeier
+ * @author Donghwan Kim
  */
 @ExtendWith(MockitoExtension.class)
 @MockitoSettings(strictness = Strictness.LENIENT)
@@ -458,6 +459,17 @@ void shouldAllowVoidMethods() {
 		verify(backingRepo).deleteAll();
 	}
 
+	@Test // GH-3507
+	void unwrapsOptionalForTypeVariableReturnTypeDeclaredOnBaseInterface() {
+
+		var user = new User();
+		when(backingRepo.findById(any())).thenReturn(Optional.of(user));
+
+		var repository = factory.getRepository(TypeVariableReturningRepository.class);
+
+		assertThat(repository.findById(1L)).isSameAs(user);
+	}
+
 	@Test // DATACMNS-1154
 	void considersRequiredKotlinParameter() {
 
@@ -635,6 +647,15 @@ interface CustomRepository extends ReadOnlyRepository<Object, Long> {
 
 	}
 
+	interface TypeVariableReturningBaseRepository<T, ID> extends Repository<T, ID> {
+
+		T findById(ID id);
+	}
+
+	interface TypeVariableReturningRepository extends TypeVariableReturningBaseRepository<User, Long> {
+
+	}
+
 	@RepositoryDefinition(domainClass = Object.class, idClass = Long.class)
 	interface AnnotatedRepository {