Fixed hang in typechecker.

Another page in the storied history of "check the normalized type, then
act on the unnormalized type", in this case leading to a tight loop of
foreverness. Closes SI-5156, review by moors.

Author: paulp

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -305,7 +305,7 @@ trait Implicits {
         case NoPrefix =>
           0
         case SingleType(pre, sym) =>
-          if (sym.isPackage) 0 else complexity(tp.widen)
+          if (sym.isPackage) 0 else complexity(tp.normalize.widen)
         case TypeRef(pre, sym, args) =>
           complexity(pre) + sum(args map complexity) + 1
         case RefinedType(parents, _) =>

test/files/pos/t5156.scala

@@ -0,0 +1,21 @@
+sealed trait HList
+final case class HCons[H, T <: HList](head : H, tail : T) extends HList 
+case object HNil extends HList 
+
+object HList {
+  type ::[H, T <: HList] = HCons[H, T]
+  type HNil = HNil.type
+
+  implicit def hlistOps[L <: HList](l : L) = new {
+    def ::[H](h : H) : H :: L = HCons(h, l)
+    def last(implicit last : Last[L]) {}
+  }
+
+  class Last[L <: HList]
+  implicit def hsingleLast[H] = new Last[H :: HNil]
+  implicit def hlistLast[H, T <: HList](implicit lt : Last[T]) = new Last[H :: T]
+
+  type III = Int :: Int :: Int :: HNil
+  val iii : III = 0 :: 0 :: 0 :: HNil
+  val l = iii.last
+}