4.3.x when using spring cloud functions no longer returns 404 and exceptions forwarded to /error result in a AopConfigException

Describe the bug
When upgrading from spring cloud gateway 4.1.9 to 4.3.4 there was a few issues we encountered. These seem to be a result of adding support for spring-cloud-functions.

• When this is enabled it and it tries to find a route based on the value of spring.cloud.function.definition, it fails with an assertion exception. This causes it to throw an IllegalArgumentException, which gets bubbled up and turned in a 400. This check for the function happens in the RoutingFunction class on line 236. What we were expecting to see was a 404 as the route does not exist.
• Another issue we noticed was that when there is an exception and the error is being forwards to /error we get an AopConfigException. This causes the resulting response to be a500.

AopConfigException Stack Trace

jakarta.servlet.ServletException: Request processing failed: org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$StaticView: Common causes of this problem include using a final class or a non-visible class
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1022) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564) ~[tomcat-embed-core-10.1.54.jar:6.0]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.54.jar:6.0]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-6.2.18.jar:6.2.18]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.2.18.jar:6.2.18]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:101) ~[spring-web-6.2.18.jar:6.2.18]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:101) ~[spring-web-6.2.18.jar:6.2.18]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:138) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:610) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:394) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:321) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:266) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:374) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:206) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.StandardHostValve.throwable(StandardHostValve.java:283) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:147) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:83) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:72) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:903) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1797) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:973) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:491) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:63) ~[tomcat-embed-core-10.1.54.jar:10.1.54]
	at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
Caused by: org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$StaticView: Common causes of this problem include using a final class or a non-visible class
	at org.springframework.aop.framework.CglibAopProxy.buildProxy(CglibAopProxy.java:238) ~[spring-aop-6.2.18.jar:6.2.18]
	at org.springframework.aop.framework.CglibAopProxy.getProxy(CglibAopProxy.java:163) ~[spring-aop-6.2.18.jar:6.2.18]
	at org.springframework.aop.framework.ProxyFactory.getProxy(ProxyFactory.java:97) ~[spring-aop-6.2.18.jar:6.2.18]
	at org.springframework.cloud.function.context.catalog.BeanFactoryAwareFunctionRegistry.proxyTarget(BeanFactoryAwareFunctionRegistry.java:299) ~[spring-cloud-function-context-4.3.2.jar:4.3.2]
	at org.springframework.cloud.function.context.catalog.BeanFactoryAwareFunctionRegistry.lookup(BeanFactoryAwareFunctionRegistry.java:172) ~[spring-cloud-function-context-4.3.2.jar:4.3.2]
	at org.springframework.cloud.function.context.FunctionCatalog.lookup(FunctionCatalog.java:84) ~[spring-cloud-function-context-4.3.2.jar:4.3.2]
	at org.springframework.cloud.gateway.server.mvc.handler.HandlerFunctions.lambda$fn$3(HandlerFunctions.java:76) ~[spring-cloud-gateway-server-mvc-4.3.4.jar:4.3.4]
	at org.springframework.web.servlet.function.HandlerFilterFunction.lambda$ofRequestProcessor$3(HandlerFilterFunction.java:83) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at org.springframework.web.servlet.function.HandlerFilterFunction.lambda$apply$2(HandlerFilterFunction.java:70) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at org.springframework.web.servlet.function.support.HandlerFunctionAdapter.handle(HandlerFunctionAdapter.java:108) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1089) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:979) ~[spring-webmvc-6.2.18.jar:6.2.18]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014) ~[spring-webmvc-6.2.18.jar:6.2.18]
	... 36 common frames omitted
Caused by: java.lang.IllegalArgumentException: Cannot subclass final class org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$StaticView
	at org.springframework.cglib.proxy.Enhancer.generateClass(Enhancer.java:653) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.transform.TransformingClassGenerator.generateClass(TransformingClassGenerator.java:35) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:26) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.ClassLoaderAwareGeneratorStrategy.generate(ClassLoaderAwareGeneratorStrategy.java:58) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.AbstractClassGenerator.generate(AbstractClassGenerator.java:370) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.proxy.Enhancer.generate(Enhancer.java:575) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData.lambda$new$1(AbstractClassGenerator.java:107) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.internal.LoadingCache.lambda$createEntry$1(LoadingCache.java:52) ~[spring-core-6.2.18.jar:6.2.18]
	at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:317) ~[na:na]
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java) ~[na:na]
	at org.springframework.cglib.core.internal.LoadingCache.createEntry(LoadingCache.java:57) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.internal.LoadingCache.get(LoadingCache.java:34) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData.get(AbstractClassGenerator.java:129) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:321) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.proxy.Enhancer.createHelper(Enhancer.java:562) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.cglib.proxy.Enhancer.createClass(Enhancer.java:407) ~[spring-core-6.2.18.jar:6.2.18]
	at org.springframework.aop.framework.ObjenesisCglibAopProxy.createProxyClassAndInstance(ObjenesisCglibAopProxy.java:62) ~[spring-aop-6.2.18.jar:6.2.18]
	at org.springframework.aop.framework.CglibAopProxy.buildProxy(CglibAopProxy.java:229) ~[spring-aop-6.2.18.jar:6.2.18]
	... 48 common frames omitted

Both these issues go away when you set spring.cloud.gateway.function.enabled: false

Sample
I've uploaded an example project to a repo which replicates this issue.

https://github.com/jazdeol1/GatewayExample

The test which currently fails checks for an unmatched route gets a 400 back instead of a 404.

For the AopConfigException you can run the last test and look at the console logs.

Start with RoutingFunction around line 236 and HandlerFunctions.java, using the uploaded GatewayExample repository and its failing unmatched-route test. Reproduce both cases with spring.cloud.gateway.function.enabled enabled: an unmatched route should return 404 rather than 400, and forwarding to /error should not produce the reported AopConfigException.