Support name attribute in @ModelAttribute in WebFlux

Prior to this commit, the `name` attribute in @ModelAttribute was not
supported when using WebFlux. This is because MethodParameter was used
instead of SynthesizingMethodParameter when retrieving the
@ModelAttribute annotation. In other words, @AliasFor was not honored
because the annotation was not synthesized. Consequently, only the
`value` attribute was supported in WebFlux when specifying a custom name
via @ModelAttribute.

This commit fixes this by using SynthesizingMethodParameter to retrieve
the @ModelAttribute annotation.

Closes gh-28423

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ErrorsMethodArgumentResolver.java

@@ -22,6 +22,7 @@
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
+import org.springframework.core.annotation.SynthesizingMethodParameter;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.BindingResult;
@@ -38,6 +39,7 @@
  * model attribute in the method signature.
  *
  * @author Rossen Stoyanchev
+ * @author Sam Brannen
  * @since 5.0
  */
 public class ErrorsMethodArgumentResolver extends HandlerMethodArgumentResolverSupport {
@@ -79,7 +81,7 @@ private Object getErrors(MethodParameter parameter, BindingContext context) {
 				"Errors argument must be declared immediately after a model attribute argument");
 
 		int index = parameter.getParameterIndex() - 1;
-		MethodParameter attributeParam = MethodParameter.forExecutable(parameter.getExecutable(), index);
+		MethodParameter attributeParam = SynthesizingMethodParameter.forExecutable(parameter.getExecutable(), index);
 		ReactiveAdapter adapter = getAdapterRegistry().getAdapter(attributeParam.getParameterType());
 
 		Assert.state(adapter == null, "An @ModelAttribute and an Errors/BindingResult argument " +
@@ -88,8 +90,8 @@ private Object getErrors(MethodParameter parameter, BindingContext context) {
 				"handle a WebExchangeBindException error signal through the async type.");
 
 		ModelAttribute ann = attributeParam.getParameterAnnotation(ModelAttribute.class);
-		String name = (ann != null && StringUtils.hasText(ann.value()) ?
-				ann.value() : Conventions.getVariableNameForParameter(attributeParam));
+		String name = (ann != null && StringUtils.hasText(ann.name()) ? ann.name() :
+				Conventions.getVariableNameForParameter(attributeParam));
 		Object errors = context.getModel().asMap().get(BindingResult.MODEL_KEY_PREFIX + name);
 
 		Assert.state(errors != null, () -> "An Errors/BindingResult argument is expected " +

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ErrorsMethodArgumentResolverTests.java

@@ -180,7 +180,7 @@ void handle(
 
 	@SuppressWarnings("unused")
 	void handleWithCustomModelAttributeName(
-			@ModelAttribute("custom") Foo foo,
+			@ModelAttribute(name = "custom") Foo foo,
 			Errors errors,
 			@ModelAttribute Mono<Foo> fooMono,
 			BindingResult bindingResult,