Component used: room-runtime Version used: 2.5.1 Devices/Android versions reproduced on: N/A

After upgrading a project from Room 2.4.2 to 2.5.1, I noticed a new compilation failure when setting a QueryCallback on our Room database. Since this upgrade moved us to a Room version written in Kotlin for the first time, I have a theory about the cause. I've also added a minimal repro project where the first and second commits compare room 2.4.2 and 2.5.1. My theory is below.

We had been using a lambda for our callback, along the lines of:

builder.setQueryCallback({ sqlQuery, bindArgs ->
    Log.d("Room DB", "SQL Query: $sqlQuery SQL Args: $bindArgs")
  },
  Executors.newSingleThreadExecutor())

This worked, presumably, because the compiler performed a single-abstract-method (SAM) conversion and treated the lambda expression as an implementation of QueryCallback.onQuery.

Following the Room upgrade to Kotlin for its own code, I believe this conversion no longer occurs. Kotlin supports SAM conversions for Kotlin interfaces, but they must be declared as fun interface Foo rather than just interface Foo. As of 2.5.2, QueryCallback is just an interface.

This isn't blocking, strictly. We can instead construct an object implementing QueryCallback in place of the lambda:

object : QueryCallback {
  override fun onQuery(sqlQuery: String, bindArgs: List<Any?>) {
    Log.d("Room DB", "SQL Query: $sqlQuery SQL Args: $bindArgs")
  }
}

But in addition to being more verbose, I found I had to add a -Xjvm-default=enable compiler flag to our project so that the interface implementation would compile. Ideally we'd be able to continue relying on SAM conversion and write a lambda instead.