Honor @⁠Fallback semantics for Test Bean Overrides

Closes gh-33924

spring-test/src/main/java/org/springframework/test/context/bean/override/BeanOverrideBeanFactoryPostProcessor.java

@@ -331,6 +331,12 @@ private Set<String> getExistingBeanNamesByType(ConfigurableListableBeanFactory b
 		return beanNames;
 	}
 
+	/**
+	 * Determine the primary candidate in the given set of bean names.
+	 * <p>Honors both <em>primary</em> and <em>fallback</em> semantics.
+	 * @return the name of the primary candidate, or {@code null} if none found
+	 * @see org.springframework.beans.factory.support.DefaultListableBeanFactory#determinePrimaryCandidate(Map, Class)
+	 */
 	@Nullable
 	private static String determinePrimaryCandidate(
 			ConfigurableListableBeanFactory beanFactory, Set<String> candidateBeanNames, Class<?> beanType) {
@@ -340,6 +346,7 @@ private static String determinePrimaryCandidate(
 		}
 
 		String primaryBeanName = null;
+		// First pass: identify unique primary candidate
 		for (String candidateBeanName : candidateBeanNames) {
 			if (beanFactory.containsBeanDefinition(candidateBeanName)) {
 				BeanDefinition beanDefinition = beanFactory.getBeanDefinition(candidateBeanName);
@@ -352,6 +359,21 @@ private static String determinePrimaryCandidate(
 				}
 			}
 		}
+		// Second pass: identify unique non-fallback candidate
+		if (primaryBeanName == null) {
+			for (String candidateBeanName : candidateBeanNames) {
+				if (beanFactory.containsBeanDefinition(candidateBeanName)) {
+					BeanDefinition beanDefinition = beanFactory.getBeanDefinition(candidateBeanName);
+					if (!beanDefinition.isFallback()) {
+						if (primaryBeanName != null) {
+							// More than one non-fallback bean found among candidates.
+							return null;
+						}
+						primaryBeanName = candidateBeanName;
+					}
+				}
+			}
+		}
 		return primaryBeanName;
 	}
 

spring-test/src/test/java/org/springframework/test/context/bean/override/convention/TestBeanWithMultipleExistingBeansAndOneNonFallbackIntegrationTests.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2002-2024 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
+ *
+ *      https://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.test.context.bean.override.convention;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Fallback;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.bean.override.example.ExampleService;
+import org.springframework.test.context.junit.jupiter.SpringExtension;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Verifies that {@link TestBean @TestBean} can be used to override a bean by-type
+ * when there are multiple candidates and only one that is not a fallback.
+ *
+ * @author Sam Brannen
+ * @since 6.2.1
+ */
+@ExtendWith(SpringExtension.class)
+@DirtiesContext
+class TestBeanWithMultipleExistingBeansAndOneNonFallbackIntegrationTests {
+
+	@TestBean
+	ExampleService service;
+
+	@Autowired
+	List<ExampleService> services;
+
+
+	static ExampleService service() {
+		return () -> "overridden";
+	}
+
+
+	@Test
+	void test() {
+		assertThat(service.greeting()).isEqualTo("overridden");
+		assertThat(services).extracting(ExampleService::greeting)
+				.containsExactlyInAnyOrder("overridden", "two", "three");
+	}
+
+
+	@Configuration(proxyBeanMethods = false)
+	static class Config {
+
+		@Bean
+		ExampleService one() {
+			return () -> "one";
+		}
+
+		@Bean
+		@Fallback
+		ExampleService two() {
+			return () -> "two";
+		}
+
+		@Bean
+		@Fallback
+		ExampleService three() {
+			return () -> "three";
+		}
+	}
+
+}