Detect nested configuration classes even for empty outer classes

Issue: SPR-16839

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

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2002-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.annotation;
+
+import org.junit.Test;
+
+import org.springframework.beans.factory.support.RootBeanDefinition;
+
+/**
+ * @author Andy Wilkinson
+ */
+public class ImportVersusDirectRegistrationTests {
+
+	@Test
+	public void thingIsAvailableWhenOuterConfigurationIsRegisteredDirectly() {
+		try (AnnotationConfigApplicationContext directRegistration = new AnnotationConfigApplicationContext()) {
+			directRegistration.register(AccidentalLiteConfiguration.class);
+			directRegistration.refresh();
+			directRegistration.getBean(Thing.class);
+		}
+	}
+
+	@Test
+	public void thingIsAvailableWhenOuterConfigurationIsRegisteredWithClassName() {
+		try (AnnotationConfigApplicationContext directRegistration = new AnnotationConfigApplicationContext()) {
+			directRegistration.registerBeanDefinition("config",
+					new RootBeanDefinition(AccidentalLiteConfiguration.class.getName()));
+			directRegistration.refresh();
+			directRegistration.getBean(Thing.class);
+		}
+	}
+
+	@Test
+	public void thingIsAvailableWhenOuterConfigurationIsImported() {
+		try (AnnotationConfigApplicationContext viaImport = new AnnotationConfigApplicationContext()) {
+			viaImport.register(Importer.class);
+			viaImport.refresh();
+			viaImport.getBean(Thing.class);
+		}
+	}
+
+}
+
+
+@Import(AccidentalLiteConfiguration.class)
+class Importer {
+}
+
+
+class AccidentalLiteConfiguration {
+
+	@Configuration
+	class InnerConfiguration {
+
+		@Bean
+		public Thing thing() {
+			return new Thing();
+		}
+	}
+}
+
+
+class Thing {
+}