Component used: Room 1.0.0
Version used:
Devices/Android versions reproduced on: Android API 26

- Sample project is here: https://github.com/rogerhu/android-rest-client-template/blob/room_fail

- Kotlin annotation processor throws this error: "e: [kapt] An exception occurred: java.lang.IllegalArgumentException: ? extends com.codepath.apps.restclienttemplate.models.Tweetcannot be converted to an Element".

The particular line in question is here:

https://github.com/rogerhu/android-rest-client-template/blob/room_fail/app/src/main/java/com/codepath/apps/restclienttemplate/models/UserDao.java#L12

@Query("SELECT * FROM User WHERE userId = :id")
UserTweetsFail byId(long id); // change to UserTweetsWorks to see this pass

Where UserTweetsFail.kt is:

class UserTweetsFail {
@Embedded var user: User? = null
@Relation(parentColumn = "userId", entityColumn = "id", entity=Tweet::class)
var tweets: List<Tweet> = listOf()

}

If I change to use UserTweetWorks to use the Java version

UserTweetWorks.java:

public class UserTweetsWorks {
@Embedded
User user;

@Relation(parentColumn = "userId", entityColumn = "id", entity=Tweet.class)
List<Tweet> tweets;
}

The difference seems to be that a Kotlin collection of tweets is essentially:

@Relation(parentColumn = "userId", entityColumn = "id", entity=Tweet.class)
List<? extends Tweet> tweets;

This seems to break the Auto library, since the list is now a subtype of Tweet.

I'm confused why this didn't show up in the integration tests in AOSP -- there seems to be an exact class (https://github.com/aosp-mirror/platform_frameworks_support/blob/master/room/integration-tests/kotlintestapp/src/androidTest/java/android/arch/persistence/room/integration/kotlintestapp/vo/PublisherWithBooks.kt) that tests this behavior.