DependencyDescriptor supports TypeDescriptor resolution for fields

This allows for proper nested type conversion in @Value Optional fields analogous to method parameters, through a new TypeDescriptor-based method in the TypeConverter SPI. As an additional and less involved measure that is worth backporting, DefaultListableBeanFactory defensively checks for pre-converted Optional wrappers.

Issue: SPR-17607

spring-context/src/main/java/org/springframework/validation/DataBinder.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.
@@ -41,6 +41,7 @@ import org.springframework.beans.TypeConverter;
 import org.springframework.beans.TypeMismatchException;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.format.Formatter;
 import org.springframework.format.support.FormatterPropertyEditorAdapter;
 import org.springframework.lang.Nullable;
@@ -700,6 +701,14 @@ public class DataBinder implements PropertyEditorRegistry, TypeConverter {
 		return getTypeConverter().convertIfNecessary(value, requiredType, field);
 	}
 
+	@Nullable
+	@Override
+	public <T> T convertIfNecessary(@Nullable Object value, @Nullable Class<T> requiredType,
+			@Nullable TypeDescriptor typeDescriptor) throws TypeMismatchException {
+
+		return getTypeConverter().convertIfNecessary(value, requiredType, typeDescriptor);
+	}
+
 
 	/**
 	 * Bind the given property values to this binder's target.