Support abstract @⁠Configuration classes without @⁠Bean methods again

Historically, @⁠Configuration classes that did not declare @⁠Bean
methods were allowed to be abstract. However, the changes made in
76a6b9ea79 introduced a regression that prevents such classes from
being abstract, resulting in a BeanInstantiationException. This change
in behavior is caused by the fact that such a @⁠Configuration class is
no longer replaced by a concrete subclass created dynamically by CGLIB.

This commit restores support for abstract @⁠Configuration classes
without @⁠Bean methods by modifying the "no enhancement required" check
in ConfigurationClassParser.

See gh-34486
Closes gh-34663

spring-context/src/main/java/org/springframework/context/annotation/ConfigurationClassParser.java

@@ -179,8 +179,9 @@ class ConfigurationClassParser {
 				}
 
 				// Downgrade to lite (no enhancement) in case of no instance-level @Bean methods.
-				if (!configClass.hasNonStaticBeanMethods() && ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(
-						bd.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE))) {
+				if (!configClass.getMetadata().isAbstract() && !configClass.hasNonStaticBeanMethods() &&
+						ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(
+								bd.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE))) {
 					bd.setAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE,
 							ConfigurationClassUtils.CONFIGURATION_CLASS_LITE);
 				}

spring-context/src/test/java/org/springframework/context/annotation/ConfigurationClassPostProcessorTests.java

@@ -129,6 +129,22 @@ class ConfigurationClassPostProcessorTests {
 		assertThat(beanFactory.getDependentBeans("config")).contains("bar");
 	}
 
+	@Test  // gh-34663
+	void enhancementIsPresentForAbstractConfigClassWithoutBeanMethods() {
+		beanFactory.registerBeanDefinition("config", new RootBeanDefinition(AbstractConfigWithoutBeanMethods.class));
+		ConfigurationClassPostProcessor pp = new ConfigurationClassPostProcessor();
+		pp.postProcessBeanFactory(beanFactory);
+		RootBeanDefinition beanDefinition = (RootBeanDefinition) beanFactory.getBeanDefinition("config");
+		assertThat(beanDefinition.hasBeanClass()).isTrue();
+		assertThat(beanDefinition.getBeanClass().getName()).contains(ClassUtils.CGLIB_CLASS_SEPARATOR);
+		Foo foo = beanFactory.getBean("foo", Foo.class);
+		Bar bar = beanFactory.getBean("bar", Bar.class);
+		assertThat(bar.foo).isSameAs(foo);
+		assertThat(beanFactory.getDependentBeans("foo")).contains("bar");
+		String[] dependentsOfSingletonBeanConfig = beanFactory.getDependentBeans(SingletonBeanConfig.class.getName());
+		assertThat(dependentsOfSingletonBeanConfig).containsOnly("foo", "bar");
+	}
+
 	@Test
 	void enhancementIsNotPresentForProxyBeanMethodsFlagSetToFalse() {
 		beanFactory.registerBeanDefinition("config", new RootBeanDefinition(NonEnhancedSingletonBeanConfig.class));
@@ -181,7 +197,7 @@ class ConfigurationClassPostProcessorTests {
 		assertThat(bar.foo).isNotSameAs(foo);
 	}
 
-	@Test
+	@Test  // gh-34486
 	void enhancementIsNotPresentWithEmptyConfig() {
 		beanFactory.registerBeanDefinition("config", new RootBeanDefinition(EmptyConfig.class));
 		ConfigurationClassPostProcessor pp = new ConfigurationClassPostProcessor();
@@ -1195,6 +1211,12 @@ class ConfigurationClassPostProcessorTests {
 		}
 	}
 
+	@Configuration
+	@Import(SingletonBeanConfig.class)
+	abstract static class AbstractConfigWithoutBeanMethods {
+		// This class intentionally does NOT declare @Bean methods.
+	}
+
 	@Configuration
 	static final class EmptyConfig {
 	}