Look at ResponseStatusException's cause for binding errors

Fixes gh-41984

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/reactive/error/DefaultErrorAttributes.java

@@ -122,6 +122,9 @@ else if (error instanceof MethodValidationResult methodValidationResult) {
 		else if (error instanceof ResponseStatusException responseStatusException) {
 			errorAttributes.put("message", responseStatusException.getReason());
 			exception = (responseStatusException.getCause() != null) ? responseStatusException.getCause() : error;
+			if (exception instanceof BindingResult bindingResult) {
+				errorAttributes.put("errors", bindingResult.getAllErrors());
+			}
 		}
 		else {
 			exception = error;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/reactive/error/DefaultErrorAttributesTests.java

@@ -250,6 +250,21 @@ void extractBindingResultErrors() throws Exception {
 		assertThat(attributes).containsEntry("errors", bindingResult.getAllErrors());
 	}
 
+	@Test
+	void extractBindingResultErrorsThatCausedAResponseStatusException() throws Exception {
+		Method method = getClass().getDeclaredMethod("method", String.class);
+		MethodParameter stringParam = new MethodParameter(method, 0);
+		BindingResult bindingResult = new MapBindingResult(Collections.singletonMap("a", "b"), "objectName");
+		bindingResult.addError(new ObjectError("c", "d"));
+		Exception ex = new WebExchangeBindException(stringParam, bindingResult);
+		MockServerHttpRequest request = MockServerHttpRequest.get("/test").build();
+		Map<String, Object> attributes = this.errorAttributes.getErrorAttributes(
+				buildServerRequest(request, new ResponseStatusException(HttpStatus.BAD_REQUEST, "Invalid", ex)),
+				ErrorAttributeOptions.of(Include.MESSAGE, Include.BINDING_ERRORS));
+		assertThat(attributes.get("message")).isEqualTo("Invalid");
+		assertThat(attributes).containsEntry("errors", bindingResult.getAllErrors());
+	}
+
 	@Test
 	void extractMethodValidationResultErrors() throws Exception {
 		Object target = "test";