Functions that use Deferred as return type should have a name with suffix Async. Otherwise, it's recommended to turn a function into a suspend one and unwrap Deferred inside it.

Example:

  fun calcEverything(): Deferred<Int> {
      return CompletableDeferred(42)
  }

After the fix applied (add the Async suffix):

  fun calcEverythingAsync(): Deferred<Int> {
      return CompletableDeferred(42)
  }

After the fix applied (turn the function into a suspend one):

  suspend fun calcEverything(): Int {
      return CompletableDeferred(42).await()
  }