import language.experimental.captureChecking

trait Source[+T]:
  def poll(): T

trait List[+T]:
  def foreach[R](op: T => R): Unit

def foo[T](sources: List[Source[T]^]): Unit =
  sources.foreach(_.poll())

-- Error: issues/mapsource.scala:10:10 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
10 |  sources.foreach(_.poll())
   |  ^^^^^^^^^^^^^^^
   |  Reach capability cap and universal capability cap cannot both
   |  appear in the type [R](op: box Source[T]^{sources*} => R): Unit of this expression