Ensure DataBinder can bind constructor with a Map with simple values

This change ensures that DataBinder can bind constructor with a Map
parameter that has no nested properties, but just simple values like
a String: `someMap[0]=exampleString`.

Integration tests have been added to cover similar cases that use the
ServletRequestDataBinder.

Closes gh-34043

spring-test/src/test/java/org/springframework/test/web/servlet/samples/spr/ServletRequestDataBinderIntegrationTests.java

@@ -0,0 +1,138 @@
+/*
+ * 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.spr;
+
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.test.context.junit.jupiter.web.SpringJUnitWebConfig;
+import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.web.bind.annotation.ModelAttribute;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.context.WebApplicationContext;
+
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+import static org.springframework.test.web.servlet.setup.MockMvcBuilders.webAppContextSetup;
+
+@SpringJUnitWebConfig(ServletRequestDataBinderIntegrationTests.SpringWebKeyValueController.class)
+class ServletRequestDataBinderIntegrationTests {
+
+	@Test // gh-34043
+	void postMap(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/map")
+						.param("someMap[a]", "valueA")
+						.param("someMap[b]", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	@Test
+	void postArray(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/array")
+						.param("someArray[0]", "valueA")
+						.param("someArray[1]", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	@Disabled("see gh-34121")
+	@Test // gh-34121
+	void postArrayWithEmptyIndex(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/array")
+						.param("someArray[]", "valueA")
+						.param("someArray[]", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	@Test
+	void postArrayWithoutIndex(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/array")
+						.param("someArray", "valueA")
+						.param("someArray", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	@Test
+	void postList(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/list")
+						.param("someList[0]", "valueA")
+						.param("someList[1]", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	@Disabled("see gh-34121")
+	@Test // gh-34121
+	void postListWithEmptyIndex(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/list")
+						.param("someList[]", "valueA")
+						.param("someList[]", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	@Test
+	void postListWithoutIndex(WebApplicationContext wac) throws Exception {
+		MockMvc mockMvc = webAppContextSetup(wac).build();
+		mockMvc.perform(post("/list")
+						.param("someList", "valueA")
+						.param("someList", "valueB"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("valueB"));
+	}
+
+	record PayloadWithMap(Map<String, String> someMap) {}
+
+	record PayloadWithArray(String[] someArray) {}
+
+	record PayloadWithList(List<String> someList) {}
+
+	@RestController
+	@SuppressWarnings("unused")
+	static class SpringWebKeyValueController {
+
+		@PostMapping("/map")
+		String postMap(@ModelAttribute("payload") PayloadWithMap payload) {
+			return payload.someMap.get("b");
+		}
+
+		@PostMapping("/array")
+		String postArray(@ModelAttribute("payload") PayloadWithArray payload) {
+			return payload.someArray[1];
+		}
+
+		@PostMapping("/list")
+		String postList(@ModelAttribute("payload") PayloadWithList payload) {
+			return payload.someList.get(1);
+		}
+	}
+
+}