Improve support of Kotlin beans w/ primary and default ctors

This commit add the default constructor if available as
fallback after to the primary constructor.

Issue: SPR-16012

spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

@@ -367,7 +367,9 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 						candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
 					}
 					else if (kotlinPrimaryConstructor != null) {
-						candidateConstructors = new Constructor<?>[] {kotlinPrimaryConstructor};
+						candidateConstructors = (defaultConstructor != null ?
+								new Constructor<?>[] {kotlinPrimaryConstructor, defaultConstructor} :
+								new Constructor<?>[] {kotlinPrimaryConstructor});
 					}
 					else {
 						candidateConstructors = new Constructor<?>[0];

spring-beans/src/test/kotlin/org/springframework/beans/factory/annotation/KotlinAutowiredTests.kt

@@ -122,6 +122,36 @@ class KotlinAutowiredTests {
 		assertSame(colour, kb.injectedFromSecondaryConstructor)
 	}
 
+	@Test  // SPR-16012
+	fun `Fallback on the default constructor when no autowirable primary constructor is defined`() {
+		val bf = DefaultListableBeanFactory()
+		val bpp = AutowiredAnnotationBeanPostProcessor()
+		bpp.setBeanFactory(bf)
+		bf.addBeanPostProcessor(bpp)
+		val bd = RootBeanDefinition(KotlinBeanWithPrimaryAndDefaultConstructors::class.java)
+		bd.scope = RootBeanDefinition.SCOPE_PROTOTYPE
+		bf.registerBeanDefinition("bean", bd)
+
+		val kb = bf.getBean("bean", KotlinBeanWithPrimaryAndDefaultConstructors::class.java)
+		assertNotNull(kb.testBean)
+	}
+
+	@Test  // SPR-16012
+	fun `Instantiation via primary constructor when a default is defined`() {
+		val bf = DefaultListableBeanFactory()
+		val bpp = AutowiredAnnotationBeanPostProcessor()
+		bpp.setBeanFactory(bf)
+		bf.addBeanPostProcessor(bpp)
+		val bd = RootBeanDefinition(KotlinBeanWithPrimaryAndDefaultConstructors::class.java)
+		bd.scope = RootBeanDefinition.SCOPE_PROTOTYPE
+		bf.registerBeanDefinition("bean", bd)
+		val tb = TestBean()
+		bf.registerSingleton("testBean", tb)
+
+		val kb = bf.getBean("bean", KotlinBeanWithPrimaryAndDefaultConstructors::class.java)
+		assertEquals(tb, kb.testBean)
+	}
+
 
 	class KotlinBean(val injectedFromConstructor: TestBean?) {
 		
@@ -163,4 +193,9 @@ class KotlinAutowiredTests {
 		var injectedFromSecondaryConstructor: Colour? = null
 	}
 
+	@Suppress("unused")
+	class KotlinBeanWithPrimaryAndDefaultConstructors(val testBean: TestBean) {
+		constructor() : this(TestBean())
+	}
+
 }