Added initial implementation of check for supported function signatures

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/config/ContextFunctionCatalogAutoConfigurationTests.java

@@ -35,6 +35,7 @@ import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.beans.BeansException;
+import org.springframework.beans.factory.BeanCreationException;
 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.beans.factory.config.AbstractFactoryBean;
 import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
@@ -289,6 +290,11 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 						.isAssignableFrom(Mono.class);
 	}
 
+	@Test(expected = BeanCreationException.class)
+	public void monoToMonoNonVoidFunction() {
+		create(MonoToMonoNonVoidConfiguration.class);
+	}
+
 	@Test
 	public void messageFunction() {
 		create(MessageConfiguration.class);
@@ -867,6 +873,17 @@ public class ContextFunctionCatalogAutoConfigurationTests {
 
 	}
 
+	@EnableAutoConfiguration
+	@Configuration
+	protected static class MonoToMonoNonVoidConfiguration {
+
+		@Bean
+		public Function<Mono<String>, Mono<String>> function() {
+			return mono -> mono;
+		}
+
+	}
+
 	@EnableAutoConfiguration
 	@Configuration
 	protected static class MessageConfiguration {