[backport] More aggressive reduction of type selection (fixes parboil…

…ed2)

This backports scala#14987 which fixes a regression introduced in 3.1.3-RC1 that
prevented parboiled2 from compiling and also fixes a performance issue affecting
parboiled2.

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1545,9 +1545,10 @@ object Types {
       @tailrec def loop(pre: Type): Type = pre.stripTypeVar match {
         case pre: RefinedType =>
           pre.refinedInfo match {
-            case TypeAlias(alias) =>
-              if (pre.refinedName ne name) loop(pre.parent) else alias
-            case _ => loop(pre.parent)
+            case tp: AliasingBounds =>
+              if (pre.refinedName ne name) loop(pre.parent) else tp.alias
+            case _ =>
+              loop(pre.parent)
           }
         case pre: RecType =>
           val candidate = pre.parent.lookupRefined(name)
@@ -4637,7 +4638,7 @@ object Types {
       myRepr.nn
     }
 
-    override def toString: String = s"Skolem($hashCode)"
+    override def toString: String = s"SkolemType($hashCode)"
   }
 
   /** A skolem type used to wrap the type of the qualifier of a selection.

tests/pos/13491.scala

@@ -87,7 +87,7 @@ object Rule {
 
   def rule[I <: HList, O <: HList](r: Rule[I, O]): Rule[I, O] = ???
 
-  implicit def valueMap[T, Out0 <: HList](m: Map[String, T])(implicit h: HListable[T] { type Out = Out0 }): RuleN[Out0] = ???
+  implicit def valueMap[T, Out0 <: HList](m: Map[String, T])(implicit h: HListable[T]): RuleN[h.Out] = ???
 }
 
 object Test {

tests/pos/i14903a.scala

@@ -0,0 +1,22 @@
+trait Wrapper[T] {
+    type Out
+  }
+
+  type Func[T] =
+    T match {
+      case String => Long
+      case Long   => Int
+      case Int    => Float
+      case Float => Double
+      case Double => Unit
+      case Unit => String
+    }
+
+  implicit def infer[A]: Wrapper[One[A]] { type Out = Func[A] } = ???
+
+  trait One[A] {
+    def use(implicit w: Wrapper[One[A]]): One[w.Out]
+  }
+
+  val x: One[Long] = null
+  val _ = x.use.use.use.use.use.use.use

tests/pos/i14903b.scala

@@ -0,0 +1,54 @@
+import annotation.unchecked.uncheckedVariance
+
+sealed trait HList
+sealed trait HNil extends HList
+case object HNil extends HNil
+case class ::[+H, +T <: HList](head: H, tail: T) extends HList
+
+type Concat[X <: HList, Y <: HList] <: HList = X match
+  case HNil   => Y
+  case h :: t => h :: Concat[t, Y]
+
+/**
+  * Decompose L into Prefix ++ Suffix if possible
+*/
+type StripSuffix[L <: HList, Suffix <: HList] <: Option[HList] = L match
+  case Suffix => Some[HNil]
+  case h :: t => StripSuffix[t, Suffix] match
+    case Some[x] => Some[h :: x]
+    case _ => None.type
+  case _      => None.type
+
+/**
+  * type-level implementation of this logic:
+  *   Out =
+  *     R                      if T has a tail of type L
+  *     (L dropRight T) ++ R   if L has a tail of type T
+*/
+sealed trait TailSwitch[L <: HList, T <: HList, R <: HList]:
+  type Out <: HList
+
+object TailSwitch:
+  type TS[L <: HList, T <: HList, R <: HList] <: HList =
+    StripSuffix[T, L] match
+      case Some[_] => R
+      case _ => StripSuffix[L, T] match
+        case Some[x] => Concat[x, R]
+
+  implicit def tailSwitch[L <: HList, T <: HList, R <: HList]: (TailSwitch[L, T, R] {
+    type Out = TS[L, T, R]
+  }) = new TailSwitch[L, T, R] { type Out = TS[L, T, R] }
+
+/**
+ * Rule popping I from stack and pushing back O
+*/
+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 Test:
+  def dot = new Rule[HNil, HNil] {}
+  def num = new Rule[HNil, Byte :: HNil] {}
+  def pattern = num ~ dot ~ num ~ dot ~ num ~ dot ~ num // error