Fix BeanUtils#instantiateClass w/ Kotlin + noarg constructor

Issue: SPR-15851

Author: sdeleuze

spring-beans/src/main/java/org/springframework/beans/BeanUtils.java

@@ -734,12 +734,11 @@ public static <T> Constructor<T> findPrimaryConstructor(Class<T> clazz) {
 		 * Instantiate a Kotlin class using the provided constructor.
 		 * @param ctor the constructor of the Kotlin class to instantiate
 		 * @param args the constructor arguments to apply (use null for unspecified parameter if needed)
-		 * @throws BeanInstantiationException if no primary constructor can be found
 		 */
-		public static <T> T instantiateClass(Constructor<T> ctor, Object... args) {
+		public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws IllegalAccessException, InvocationTargetException, InstantiationException {
 			KFunction<T> kotlinConstructor = ReflectJvmMapping.getKotlinFunction(ctor);
 			if (kotlinConstructor == null) {
-				throw new BeanInstantiationException(ctor.getDeclaringClass(), "No corresponding Kotlin constructor found");
+				return ctor.newInstance(args);
 			}
 			List<KParameter> parameters = kotlinConstructor.getParameters();
 			Map<KParameter, Object> argParameters = new HashMap<>(parameters.size());

spring-beans/src/test/kotlin/org/springframework/beans/BeanUtilsKotlinTests.kt

@@ -29,37 +29,46 @@ class BeanUtilsKotlinTests {
 	@Test
 	fun `Instantiate immutable class`() {
 		val constructor = BeanUtils.findPrimaryConstructor(Foo::class.java)!!
-		val foo = BeanUtils.instantiateClass(constructor, "bar", 3)
-		assertEquals("bar", foo.param1)
+		val foo = BeanUtils.instantiateClass(constructor, "a", 3)
+		assertEquals("a", foo.param1)
 		assertEquals(3, foo.param2)
 	}
 
 	@Test
 	fun `Instantiate immutable class with optional parameter and all parameters specified`() {
 		val constructor = BeanUtils.findPrimaryConstructor(Bar::class.java)!!
-		val bar = BeanUtils.instantiateClass(constructor, "baz", 8)
-		assertEquals("baz", bar.param1)
+		val bar = BeanUtils.instantiateClass(constructor, "a", 8)
+		assertEquals("a", bar.param1)
 		assertEquals(8, bar.param2)
 	}
 
 	@Test
 	fun `Instantiate immutable class with optional parameter and only mandatory parameters specified by position`() {
 		val constructor = BeanUtils.findPrimaryConstructor(Bar::class.java)!!
-		val bar = BeanUtils.instantiateClass(constructor, "baz")
-		assertEquals("baz", bar.param1)
+		val bar = BeanUtils.instantiateClass(constructor, "a")
+		assertEquals("a", bar.param1)
 		assertEquals(12, bar.param2)
 	}
 
 	@Test
-	fun `Instantiate immutable class with optional parameter specified with null value`() {
+		fun `Instantiate immutable class with optional parameter specified with null value`() {
 		val constructor = BeanUtils.findPrimaryConstructor(Bar::class.java)!!
-		val bar = BeanUtils.instantiateClass(constructor, "baz", null)
-		assertEquals("baz", bar.param1)
+		val bar = BeanUtils.instantiateClass(constructor, "a", null)
+		assertEquals("a", bar.param1)
 		assertEquals(12, bar.param2)
 	}
 
+	@Test  // SPR-15851
+	fun `Instantiate mutable class with declared constructor and default values for all parameters`() {
+		val baz = BeanUtils.instantiateClass(Baz::class.java.getDeclaredConstructor())
+		assertEquals("a", baz.param1)
+		assertEquals(12, baz.param2)
+	}
+
 	class Foo(val param1: String, val param2: Int)
 
 	class Bar(val param1: String, val param2: Int = 12)
-	
+
+	class Baz(var param1: String = "a", var param2: Int = 12)
+
 }