trait TC[X]
object TC {
  given [X: scala.deriving.Mirror.ProductOf]: TC[X] = ???
}

trait Base[T] {
  given TC[T] = scala.compiletime.deferred
}

case class P()
object A extends Base[P]

No class, trait or object is defined in the compilation unit.
The incremental compiler cannot record the dependency information in such case.
Some errors like unused import referring to a non-existent class might not be reported.

1 warning found