Process additional locations when non-default location configured

Fixes gh-20745

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/ConfigFileApplicationListener.java

@@ -622,12 +622,14 @@ public class ConfigFileApplicationListener implements EnvironmentPostProcessor,
 		}
 
 		private Set<String> getSearchLocations() {
+			Set<String> locations = getSearchLocations(CONFIG_ADDITIONAL_LOCATION_PROPERTY);
 			if (this.environment.containsProperty(CONFIG_LOCATION_PROPERTY)) {
-				return getSearchLocations(CONFIG_LOCATION_PROPERTY);
+				locations.addAll(getSearchLocations(CONFIG_LOCATION_PROPERTY));
+			}
+			else {
+				locations.addAll(
+						asResolvedSet(ConfigFileApplicationListener.this.searchLocations, DEFAULT_SEARCH_LOCATIONS));
 			}
-			Set<String> locations = getSearchLocations(CONFIG_ADDITIONAL_LOCATION_PROPERTY);
-			locations.addAll(
-					asResolvedSet(ConfigFileApplicationListener.this.searchLocations, DEFAULT_SEARCH_LOCATIONS));
 			return locations;
 		}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/config/ConfigFileApplicationListenerTests.java

@@ -913,6 +913,16 @@ class ConfigFileApplicationListenerTests {
 		assertThat(this.environment.getProperty("value")).isEqualTo("1234");
 	}
 
+	@Test
+	void additionalLocationWhenLocationConfiguredShouldTakesPrecedenceOverConfiguredLocation() {
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"spring.config.location=classpath:some.properties",
+				"spring.config.additional-location=classpath:override.properties");
+		this.initializer.postProcessEnvironment(this.environment, this.application);
+		assertThat(this.environment.getProperty("foo")).isEqualTo("bar");
+		assertThat(this.environment.getProperty("value")).isNull();
+	}
+
 	@Test
 	void locationReplaceDefaultLocation() {
 		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,