Refactor GenericConversionService

Refactor internal workings of GenericConversionService in order to
better support future enhancements. This commit should not affect
existing behavior.

Issue: SPR-9927

Author: philwebb

spring-core/src/main/java/org/springframework/core/convert/TypeDescriptor.java

@@ -23,6 +23,7 @@
 import java.util.Map;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 
@@ -249,6 +250,23 @@ public TypeDescriptor narrow(Object value) {
 				this.mapKeyTypeDescriptor, this.mapValueTypeDescriptor, this.annotations);
 	}
 
+	/**
+	 * Cast this {@link TypeDescriptor} to a superclass or implemented interface
+	 * preserving annotations and nested type context.
+	 *
+	 * @param superType the super type to cast to (can be {@code null}
+	 * @return a new TypeDescriptor for the up-cast type
+	 * @throws IllegalArgumentException if this type is not assignable to the super-type
+	 */
+	public TypeDescriptor upcast(Class<?> superType) {
+		if (superType == null) {
+			return null;
+		}
+		Assert.isAssignable(superType, getType());
+		return new TypeDescriptor(superType, this.elementTypeDescriptor,
+				this.mapKeyTypeDescriptor, this.mapValueTypeDescriptor, this.annotations);
+	}
+
 	/**
 	 * Returns the name of this type: the fully qualified class name.
 	 */