Added initial implementation of check for supported function signatures

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/FunctionRegistration.java

@@ -147,6 +147,7 @@ public class FunctionRegistration<T> implements BeanNameAware {
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
 	public <S> FunctionRegistration<S> wrap() {
+		this.isFunctionSignatureSupported();
 		FunctionRegistration<S> result;
 		if (this.type == null) {
 			result = (FunctionRegistration<S>) this;
@@ -192,4 +193,12 @@ public class FunctionRegistration<T> implements BeanNameAware {
 		}
 	}
 
+	private void isFunctionSignatureSupported() {
+		if (type != null) {
+			Assert.isTrue(!(Mono.class.isAssignableFrom(this.type.getOutputWrapper())
+					&& Mono.class.isAssignableFrom(this.type.getInputWrapper())),
+					"Function<Mono, Mono> is not supported.");
+		}
+	}
+
 }

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 {