Fix scala#13491: Add regression test

tests/pos/13491.scala

@@ -0,0 +1,97 @@
+import scala.annotation.unchecked.uncheckedVariance
+
+import scala.language.implicitConversions
+
+sealed trait HList extends Product with Serializable
+final case class ::[+H, +T <: HList](head: H, tail: T) extends HList
+
+sealed trait HNil extends HList
+case object HNil extends HNil
+
+trait HListable[T] {
+  type Out <: HList
+}
+
+object HListable {
+  type HL0[T] <: HList = T match {
+    case Unit     => HNil
+    case HNil     => HNil
+    case ::[a, b] => ::[a, b]
+    case _        => T :: HNil
+  }
+
+  implicit def calc[T]: HListable[T] { type Out = HL0[T] } = ???
+}
+
+sealed trait TailSwitch[L <: HList, T <: HList, R <: HList] {
+  type Out <: HList
+}
+object TailSwitch {
+  type Reverse0[Acc <: HList, L <: HList] <: HList = L match {
+    case HNil     => Acc
+    case ::[h, t] => Reverse0[h :: Acc, t]
+  }
+
+  type Reverse1[L <: HList] <: HList = L match {
+    case HNil     => HNil
+    case ::[h, t] => Reverse0[h :: HNil, t]
+  }
+
+  type Prepend0[A <: HList, B <: HList] <: HList = A match {
+    case HNil     => B
+    case ::[h, t] => ::[h, Prepend0[t, B]]
+  }
+
+  // type-level implementation of this algorithm:
+  //   @tailrec def rec(L, LI, T, TI, R, RI) =
+  //     if (TI <: L) R
+  //     else if (LI <: T) RI.reverse ::: R
+  //     else if (LI <: HNil) rec(L, HNil, T, TI.tail, R, RI)
+  //     else if (TI <: HNil) rec(L, LI.tail, T, HNil, R, LI.head :: RI)
+  //     else rec(L, LI.tail, T, TI.tail, R, LI.head :: RI)
+  //   rec(L, L, T, T, R, HNil)
+  type TailSwitch0[L <: HList, LI <: HList, T <: HList, TI <: HList, R <: HList, RI <: HList] <: HList = TI match {
+    case L => R
+    case _ =>
+    LI match {
+      case T => Prepend0[Reverse1[RI], R]
+      case HNil =>
+      TI match {
+        case ::[_, t] => TailSwitch0[L, HNil, T, t, R, RI]
+      }
+      case ::[h, t] =>
+      TI match {
+        case HNil      => TailSwitch0[L, t, T, HNil, R, h :: RI]
+        case ::[_, tt] => TailSwitch0[L, t, T, tt, R, h :: RI]
+      }
+    }
+  }
+
+  type Aux[L <: HList, LI <: HList, T <: HList, TI <: HList, R <: HList, RI <: HList, Out <: HList] =
+    TailSwitch[L, T, R] { type Out = TailSwitch0[L, L, T, T, R, HNil] }
+
+  implicit def tailSwitch[L <: HList, T <: HList, R <: HList]
+  : TailSwitch[L, T, R] { type Out = TailSwitch0[L, L, T, T, R, HNil] } = ???
+}
+
+sealed class Rule[-I <: HList, +O <: HList] {
+  def ~[I2 <: HList, O2 <: HList](that: Rule[I2, O2])(implicit
+                                                      i: TailSwitch[I2, O@uncheckedVariance, I@uncheckedVariance],
+                                                      o: TailSwitch[O@uncheckedVariance, I2, O2]
+  ): Rule[i.Out, o.Out] = ???
+
+}
+object Rule {
+  type Rule0 = Rule[HNil, HNil]
+  type RuleN[+L <: HList]   = Rule[HNil, L]
+
+  def rule[I <: HList, O <: HList](r: Rule[I, O]): Rule[I, O] = ???
+  implicit def valueMap[T](m: Map[String, T])(implicit h: HListable[T]): RuleN[h.Out] = ???
+}
+
+object Test {
+  import Rule._
+  val colors: Map[String, Int] = Map("red" -> 1, "green" -> 2, "blue" -> 3)
+  def EOI: Rule0= ???
+  val r = rule(colors ~ EOI)
+}