Reject suspend function with Call return type (#4013)

Author: Goooler

retrofit/kotlin-test/src/test/java/retrofit2/KotlinSuspendTest.kt

@@ -56,6 +56,8 @@ class KotlinSuspendTest {
         @Path("b") b: String,
         @Path("c") c: String
     ): String
+
+    @GET("/") suspend fun bodyWithCallType(): Call<String>
   }
 
   @Test fun body() {
@@ -353,6 +355,25 @@ class KotlinSuspendTest {
     }
   }
 
+  @Test fun rejectCallReturnTypeWhenUsingSuspend() {
+    val retrofit = Retrofit.Builder()
+        .baseUrl(server.url("/"))
+        .addConverterFactory(ToStringConverterFactory())
+        .build()
+    val example = retrofit.create(Service::class.java)
+
+    try {
+      runBlocking { example.bodyWithCallType() }
+      fail()
+    } catch (e: IllegalArgumentException) {
+      assertThat(e).hasMessage(
+          "Suspend functions should not return Call, as they already execute asynchronously.\n" +
+            "Change its return type to class java.lang.String\n" +
+            "    for method Service.bodyWithCallType"
+      )
+    }
+  }
+
   @Suppress("EXPERIMENTAL_OVERRIDE")
   private object DirectUnconfinedDispatcher : CoroutineDispatcher() {
     override fun isDispatchNeeded(context: CoroutineContext): Boolean = false

retrofit/src/main/java/retrofit2/HttpServiceMethod.java

@@ -53,6 +53,13 @@ static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotatio
         responseType = Utils.getParameterUpperBound(0, (ParameterizedType) responseType);
         continuationWantsResponse = true;
       } else {
+        if (getRawType(responseType) == Call.class) {
+          throw methodError(
+              method,
+              "Suspend functions should not return Call, as they already execute asynchronously.\n" +
+                "Change its return type to %s", Utils.getParameterUpperBound(0, (ParameterizedType) responseType));
+        }
+
         continuationIsUnit = Utils.isUnit(responseType);
         // TODO figure out if type is nullable or not
         // Metadata metadata = method.getDeclaringClass().getAnnotation(Metadata.class)