Fix HttpServiceMethod support for suspending functions

This commit fixes nested type handling for suspending
functions in HttpServiceMethod.

Closes gh-30266

spring-web/src/test/kotlin/org/springframework/web/service/invoker/KotlinHttpServiceMethodTests.kt

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2002-2023 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.web.service.invoker
+
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.runBlocking
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.springframework.core.ParameterizedTypeReference
+import org.springframework.http.HttpStatus
+import org.springframework.http.ResponseEntity
+import org.springframework.lang.Nullable
+import org.springframework.web.service.annotation.GetExchange
+
+/**
+ * Kotlin tests for [HttpServiceMethod].
+ *
+ * @author Sebastien Deleuze
+ */
+class KotlinHttpServiceMethodTests {
+
+	private val client = TestHttpClientAdapter()
+	private val proxyFactory = HttpServiceProxyFactory.builder(client).build()
+
+	@Test
+	fun coroutinesService(): Unit = runBlocking {
+		val service = proxyFactory.createClient(CoroutinesService::class.java)
+
+		val stringBody = service.stringBody()
+		assertThat(stringBody).isEqualTo("requestToBody")
+		verifyClientInvocation("requestToBody", object : ParameterizedTypeReference<String>() {})
+
+		service.listBody()
+		verifyClientInvocation("requestToBody", object : ParameterizedTypeReference<MutableList<String>>() {})
+
+		val flowBody = service.flowBody()
+		assertThat(flowBody.toList()).containsExactly("request", "To", "Body", "Flux")
+		verifyClientInvocation("requestToBodyFlux", object : ParameterizedTypeReference<String>() {})
+
+		val stringEntity = service.stringEntity()
+		assertThat(stringEntity).isEqualTo(ResponseEntity.ok<String>("requestToEntity"))
+		verifyClientInvocation("requestToEntity", object : ParameterizedTypeReference<String>() {})
+
+		service.listEntity()
+		verifyClientInvocation("requestToEntity", object : ParameterizedTypeReference<MutableList<String>>() {})
+
+		val flowEntity = service.flowEntity()
+		assertThat(flowEntity.statusCode).isEqualTo(HttpStatus.OK)
+		assertThat(flowEntity.body!!.toList()).containsExactly("request", "To", "Entity", "Flux")
+		verifyClientInvocation("requestToEntityFlux", object : ParameterizedTypeReference<String>() {})
+	}
+
+	private fun verifyClientInvocation(methodName: String, @Nullable expectedBodyType: ParameterizedTypeReference<*>) {
+		assertThat(client.invokedMethodName).isEqualTo(methodName)
+		assertThat(client.bodyType).isEqualTo(expectedBodyType)
+	}
+
+	private interface CoroutinesService {
+
+		@GetExchange
+		suspend fun stringBody(): String
+
+		@GetExchange
+		suspend fun listBody(): MutableList<String>
+
+		@GetExchange
+		fun flowBody(): Flow<String>
+
+		@GetExchange
+		suspend fun stringEntity(): ResponseEntity<String>
+
+		@GetExchange
+		suspend fun listEntity(): ResponseEntity<MutableList<String>>
+
+		@GetExchange
+		fun flowEntity(): ResponseEntity<Flow<String>>
+	}
+
+}