i16018.scala

// Regression test for #16018: type inference with `_ <: T` wildcards in
// pattern matching over capture-converted scrutinees.
//
// Coverage axes for the positive matrix:
//   1. The mbovel minimization (single wildcard via List.map) — was fixed
//      since 3.2.0, kept here as a pin.
//   2. Pure-Scala collections — never trigger the capture-conversion path.
//   3. Identity / generic Scala helpers over the wildcard-bearing collection.
//
// The dual cases reached through Java-generic interop (java.lang.Iterable /
// java.util.List), which were the original akka regression, are in
// tests/pos/i16018b.scala.

object Test:

  class Box[T](val value: T)
  class Container[+S, +M]
  class SubContainer[+S, +M] extends Container[S, M]

  // ---- 1. mbovel's minimized case from #16018 ------------------------
  def f1[T](l: List[Box[? <: T]]): List[T] = l.map(_.value)

  // 1b. Variant: returning the wildcard-bearing element preserved.
  def f1b[T](l: List[Box[? <: T]]): List[Box[? <: T]] = l.map(identity)

  // ---- 2. Pure-Scala scrutinee — never goes through capture conversion -
  def f2[M](xs: scala.collection.immutable.Seq[Container[Any, ? <: M]])
      : scala.collection.immutable.Seq[Container[Any, M]] =
    xs.collect {
      case g: SubContainer[Any, M] @unchecked => g
      case other                              => other
    }

  // 2b. Variant: `.map` instead of `.collect`.
  def f2b[M](xs: scala.collection.immutable.Seq[Container[Any, ? <: M]])
      : scala.collection.immutable.Seq[Container[Any, M]] =
    xs.map {
      case g: SubContainer[Any, M] @unchecked => g
      case other                              => other
    }

  // 2c. Variant: List instead of Seq.
  def f2c[M](xs: List[Container[Any, ? <: M]])
      : List[Container[Any, M]] =
    xs.collect {
      case g: SubContainer[Any, M] @unchecked => g
      case other                              => other
    }

  // ---- 3. Generic helper that stays pure-Scala does not skolemize ----
  def idSeq[T](it: Iterable[T]): scala.collection.immutable.Seq[T] = ???

  def f3[M](xs: List[Container[Any, ? <: M]])
      : scala.collection.immutable.Seq[Container[Any, M]] =
    idSeq(xs).collect {
      case g: SubContainer[Any, M] @unchecked => g
      case other                              => other
    }