Handle parameters of Kotlin extension methods correctly

The EXTENSION_RECEIVER parameter of Kotlin's extension
methods appear as normal method parameters to Java and
thus require a name. The synthetic name "$receiver" is
used here, as it is not a valid Kotlin identifier,
but valid in Java.

Issue: SPR-16119

spring-core/src/main/java/org/springframework/core/KotlinReflectionParameterNameDiscoverer.java

@@ -75,11 +75,15 @@ public class KotlinReflectionParameterNameDiscoverer implements ParameterNameDis
 	private String[] getParameterNames(List<KParameter> parameters) {
 		List<KParameter> filteredParameters = parameters
 				.stream()
-				.filter(p -> KParameter.Kind.VALUE.equals(p.getKind()))
+				// Extension receivers of extension methods must be included as they appear as normal method parameters in Java
+				.filter(p -> KParameter.Kind.VALUE.equals(p.getKind()) || KParameter.Kind.EXTENSION_RECEIVER.equals(p.getKind()))
 				.collect(Collectors.toList());
 		String[] parameterNames = new String[filteredParameters.size()];
 		for (int i = 0; i < filteredParameters.size(); i++) {
-			String name = filteredParameters.get(i).getName();
+			KParameter parameter = filteredParameters.get(i);
+			// extension receivers are not explicitly named, but require a name for Java interoperability
+			// $receiver is not a valid Kotlin identifier, but valid in Java, so it can be used here
+			String name = KParameter.Kind.EXTENSION_RECEIVER.equals(parameter.getKind())  ? "$receiver" : parameter.getName();
 			if (name == null) {
 				return null;
 			}

spring-core/src/test/kotlin/org/springframework/core/KotlinReflectionParameterNameDiscovererTests.kt

@@ -42,6 +42,13 @@ class KotlinReflectionParameterNameDiscovererTests {
 		assertThat(actualParams, `is`(arrayOf("message")))
 	}
 
+	@Test
+	fun getParameterNamesOnExtensionMethod() {
+		val method = ReflectionUtils.findMethod(UtilityClass::class.java, "identity", String::class.java)!!
+		val actualParams = parameterNameDiscoverer.getParameterNames(method)!!
+		assertThat(actualParams, `is`(arrayOf("\$receiver")))
+	}
+
 	interface MessageService {
 		fun sendMessage(message: String)
 	}
@@ -49,4 +56,8 @@ class KotlinReflectionParameterNameDiscovererTests {
 	class MessageServiceImpl {
 		fun sendMessage(message: String) = message
 	}
+
+	class UtilityClass {
+		fun String.identity() = this
+	}
 }