More aggressive reduction of type selection (fixes parboiled2)

Previously, when reducing `a.T` we checked if the type of `a` was a subtype of
`RefinedType(.., T, TypeAlias(...))`, now we extend this check to handle
refinements where the `info` is a `TypeBounds` where both bounds are equal.

This solves two big issues at once:
- We can restore tests/pos/13491.scala to its original form from before #13780.
  The check for abstract types introduced by #13780 for soundness reasons is no
  longer hit because the type selection is reduced before we get to that point.
  This is important because parboiled2 relies on this and is therefore currently
  broken on 3.1.3-RC1 and main (sirthias/parboiled2#365).
- This fixes #14903 (slow compilation issue affecting parboiled2) without
  caching skolems (as in the alternative fix #14909). Again, this is due to the
  type containing skolem being reducible to a simpler type and therefore cacheable.

Author: smarter

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

@@ -1548,9 +1548,10 @@ 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 TypeBounds(lo, hi) if lo eq hi =>
+              if (pre.refinedName ne name) loop(pre.parent) else lo
+            case _ =>
+              loop(pre.parent)
           }
         case pre: RecType =>
           val candidate = pre.parent.lookupRefined(name)
@@ -4640,7 +4641,7 @@ 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