GH-1260 Additional fix to Kotlin type resolution for generics

This is specifically relevant to the way Kotlin represents types. For example List<Message> resolves to List <? extends Message> which becomes the WildCard unlike in Java

Resolves #1260

Author: olegz

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

@@ -68,6 +68,7 @@
 import org.springframework.messaging.Message;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
 
@@ -166,10 +167,13 @@ public static Type getGenericType(Type type) {
 	 * @return instance of {@link Class} as raw representation of the provided {@link Type}
 	 */
 	public static Class<?> getRawType(Type type) {
-//		((WildcardType) type).getUpperBounds();
-//		Class cazz = ResolvableType.forType(type).getRawClass();
 		if (type instanceof WildcardType) {
-			return Object.class;
+			Type[] upperbounds = ((WildcardType) type).getUpperBounds();
+			/*
+			 * Kotlin may have something like this <? extends Message> which is technically a whildcard yet it has upper/lower types.
+			 * See GH-1260
+			 */
+			return ObjectUtils.isEmpty(upperbounds) ? Object.class : getRawType(upperbounds[0]);
 		}
 		return ResolvableType.forType(type).getRawClass();
 	}
@@ -392,7 +396,12 @@ public static Type getInputType(Type functionType) {
 				resolvableInputType = resolvableFunctionType.as(Function.class);
 			}
 			else {
-				resolvableInputType = resolvableFunctionType.as(Consumer.class);
+				if (KotlinDetector.isKotlinPresent() && Function1.class.isAssignableFrom(getRawType(functionType))) { // Kotlin
+					return ResolvableType.forType(getImmediateGenericType(functionType, 1)).getType();
+				}
+				else {
+					resolvableInputType = resolvableFunctionType.as(Consumer.class);
+				}
 			}
 			if (resolvableInputType.getType() instanceof ParameterizedType) {
 				return resolvableInputType.getGeneric(0).getType();
@@ -477,7 +486,12 @@ public static Type getOutputType(Type functionType) {
 			resolvableOutputType = resolvableFunctionType.as(Function.class);
 		}
 		else {
-			resolvableOutputType = resolvableFunctionType.as(Supplier.class);
+			if (KotlinDetector.isKotlinPresent() && Function1.class.isAssignableFrom(getRawType(functionType))) { // Kotlin
+				return ResolvableType.forType(getImmediateGenericType(functionType, 1)).getType();
+			}
+			else {
+				resolvableOutputType = resolvableFunctionType.as(Supplier.class);
+			}
 		}
 
 		Type outputType;
@@ -629,6 +643,7 @@ private static void assertSupportedTypes(Type type) {
 		Class<?> candidateType = (Class<?>) type;
 
 		Assert.isTrue(Supplier.class.isAssignableFrom(candidateType)
+						|| (KotlinDetector.isKotlinPresent() && (Function0.class.isAssignableFrom(candidateType) || Function1.class.isAssignableFrom(candidateType)))
 						|| Function.class.isAssignableFrom(candidateType)
 						|| Consumer.class.isAssignableFrom(candidateType)
 						|| FunctionRegistration.class.isAssignableFrom(candidateType)

spring-cloud-function-kotlin/src/test/java/org/springframework/cloud/function/kotlin/KotlinTypeDiscoveryTests.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2019-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.function.kotlin;
+
+
+
+import java.lang.reflect.Type;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.cloud.function.context.catalog.FunctionTypeUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class KotlinTypeDiscoveryTests {
+
+	@Test
+	public void testOutputInputTypes() {
+		Type functionType = FunctionTypeUtils.discoverFunctionTypeFromClass(KotlinComponentMessageFunction.class);
+		Type outputType = FunctionTypeUtils.getOutputType(functionType);
+		assertThat(FunctionTypeUtils.isMessage(outputType)).isTrue();
+
+		Type inputType = FunctionTypeUtils.getInputType(functionType);
+		assertThat(FunctionTypeUtils.isMessage(inputType)).isTrue();
+	}
+}

spring-cloud-function-kotlin/src/test/kotlin/org/springframework/cloud/function/kotlin/KotlinComponentMessageFunction.kt

@@ -0,0 +1,15 @@
+package org.springframework.cloud.function.kotlin
+
+import org.springframework.messaging.Message
+import org.springframework.messaging.MessageHeaders
+import org.springframework.messaging.support.MessageBuilder
+import org.springframework.stereotype.Component
+
+import java.util.function.Function
+
+@Component
+class KotlinComponentMessageFunction : (List<Message<Char>>) -> List<Message<Char>> {
+	override fun invoke(input: List<Message<Char>>): List<Message<Char>> {
+		return input
+	}
+}