Allow fallback on subclass of ProblemDetail

Closes gh-30533

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

@@ -175,7 +175,7 @@ public abstract class AbstractMessageWriterResultHandler extends HandlerResultHa
 		}
 
 		// For ProblemDetail, fall back on RFC 7807 format
-		if (bestMediaType == null && elementType.toClass().equals(ProblemDetail.class)) {
+		if (bestMediaType == null && ProblemDetail.class.isAssignableFrom(elementType.toClass())) {
 			bestMediaType = selectMediaType(exchange, () -> getMediaTypesFor(elementType), this.problemMediaTypes);
 		}
 

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

@@ -139,9 +139,9 @@ public class ResponseBodyResultHandlerTests {
 	}
 
 	private void testProblemDetailMediaType(MockServerWebExchange exchange, MediaType expectedMediaType) {
-		ProblemDetail problemDetail = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
+		MyProblemDetail problemDetail = new MyProblemDetail(HttpStatus.BAD_REQUEST);
 
-		Method method = on(TestRestController.class).returning(ProblemDetail.class).resolveMethod();
+		Method method = on(TestRestController.class).returning(MyProblemDetail.class).resolveMethod();
 		HandlerResult result = getHandlerResult(new TestRestController(), problemDetail, method);
 
 		this.resultHandler.handleResult(exchange, result).block(Duration.ofSeconds(5));
@@ -196,7 +196,7 @@ public class ResponseBodyResultHandlerTests {
 			return null;
 		}
 
-		public ProblemDetail handleToProblemDetail() {
+		public MyProblemDetail handleToProblemDetail() {
 			return null;
 		}
 
@@ -217,4 +217,13 @@ public class ResponseBodyResultHandlerTests {
 		}
 	}
 
+
+	private static class MyProblemDetail extends ProblemDetail {
+
+		public MyProblemDetail(HttpStatus status) {
+			super(status.value());
+		}
+
+	}
+
 }

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/RequestResponseBodyMethodProcessorTests.java

@@ -1,4 +1,4 @@
-/*
+`/*
  * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -374,7 +374,7 @@ class RequestResponseBodyMethodProcessorTests {
 	}
 
 	private void testProblemDetailMediaType(String expectedContentType) throws Exception {
-		ProblemDetail problemDetail = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
+		MyProblemDetail problemDetail = new MyProblemDetail(HttpStatus.BAD_REQUEST);
 
 		this.servletRequest.setRequestURI("/path");
 
@@ -805,7 +805,7 @@ class RequestResponseBodyMethodProcessorTests {
 	}
 
 	@SuppressWarnings("ConstantConditions")
-	ProblemDetail handleAndReturnProblemDetail() {
+	MyProblemDetail handleAndReturnProblemDetail() {
 		return null;
 	}
 
@@ -821,6 +821,15 @@ class RequestResponseBodyMethodProcessorTests {
 	}
 
 
+	private static class MyProblemDetail extends ProblemDetail {
+
+		public MyProblemDetail(HttpStatus status) {
+			super(status.value());
+		}
+
+	}
+
+
 	private static abstract class MyParameterizedController<DTO extends Identifiable> {
 
 		@SuppressWarnings("unused")