DefaultListableBeanFactory checks for pre-converted Optional wrappers

Issue: SPR-17607

spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -1549,7 +1549,8 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 						super.resolveCandidate(beanName, requiredType, beanFactory));
 			}
 		};
-		return Optional.ofNullable(doResolveDependency(descriptorToUse, beanName, null, null));
+		Object result = doResolveDependency(descriptorToUse, beanName, null, null);
+		return (result instanceof Optional ? (Optional<?>) result : Optional.ofNullable(result));
 	}
 
 
@@ -1630,7 +1631,7 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 
 
 	/**
-	 * A dependency descriptor marker for multiple elements.
+	 * A dependency descriptor for a multi-element declaration with nested elements.
 	 */
 	private static class MultiElementDescriptor extends NestedDependencyDescriptor {