Leverage Java reflection for Kotlin enums

As discussed in KT-25165, from a Kotlin POV enum constructors
have no parameter, this is an "implementation detail"
required for running on the JVM, so it seems relevant to skip
Kotlin reflection in that case and just delegate to Java
reflection.

Issue: SPR-16931

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

@@ -58,7 +58,7 @@ public class KotlinReflectionParameterNameDiscoverer implements ParameterNameDis
 	@Override
 	@Nullable
 	public String[] getParameterNames(Constructor<?> ctor) {
-		if (!KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
+		if (ctor.getDeclaringClass().isEnum() || !KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
 			return null;
 		}
 

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

@@ -0,0 +1,36 @@
+/*
+ * 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.core
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class KotlinDefaultParameterNameDiscovererTests {
+
+	private val parameterNameDiscoverer = DefaultParameterNameDiscoverer()
+
+	enum class MyEnum {
+		ONE, TWO
+	}
+
+	@Test  // SPR-16931
+	fun getParameterNamesOnEnum() {
+		val constructor = MyEnum::class.java.declaredConstructors[0]
+		val actualParams = parameterNameDiscoverer.getParameterNames(constructor)
+		assertEquals(2, actualParams!!.size)
+	}
+}