Resolve base type in parameterized type if necessary

See gh-34086

Author: rPraml

spring-core/src/main/java/org/springframework/core/GenericTypeResolver.java

@@ -180,7 +180,7 @@ else if (genericType instanceof ParameterizedType parameterizedType) {
 								generics[i] = resolvedTypeArgument;
 							}
 							else {
-								generics[i] = ResolvableType.forType(typeArgument);
+								generics[i] = ResolvableType.forType(typeArgument).resolveType();
 							}
 						}
 						else if (typeArgument instanceof ParameterizedType) {

spring-test/src/test/java/org/springframework/test/web/servlet/samples/standalone/GenericReturnTests.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright 2002-2024 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.test.web.servlet.samples.standalone;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.MediaType;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+import static org.springframework.test.web.servlet.setup.MockMvcBuilders.standaloneSetup;
+
+/**
+ * Demonstrates, if returning a List>? extends SomeType< can be correctly serialized.
+ *
+ * @author Roland Praml
+ */
+public class GenericReturnTests {
+	@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
+	@JsonSubTypes({
+			@JsonSubTypes.Type(value = SubType1.class, name = "one"),
+			@JsonSubTypes.Type(value = SubType2.class, name = "two")
+	})
+	public static class BaseType {
+
+	}
+
+	public static class SubType1 extends BaseType {
+	}
+
+	public static class SubType2 extends BaseType {
+	}
+
+	@Test
+	public void genericReturnTest() throws Exception {
+
+		standaloneSetup(new Controller()).build()
+				.perform(get("/genericReturnList").accept(MediaType.APPLICATION_JSON))
+				.andExpect(status().isOk())
+				.andExpect(content().contentType("application/json"))
+				.andExpect(jsonPath("$[0].type").value("one"))
+				.andExpect(jsonPath("$[1].type").value("two"));
+	}
+
+	@RestController
+	@SuppressWarnings("unchecked")
+	public static class Controller {
+
+		@GetMapping(value = "/genericReturnList", produces = MediaType.APPLICATION_JSON_VALUE)
+		public <T extends BaseType> List<T> get() {
+			List<T> list = new ArrayList<>();
+			list.add((T) new SubType1());
+			list.add((T) new SubType2());
+			return list;
+		}
+
+	}
+}