Make rewritings of hk applications configurable

Beta-reduce only if `Config.simplifyApplications` is true.
Turning off beta-reduction revealed two problems which are
also fixed in this commit:

1. Bad treatement of higher-kinded argyments in cyclicity checking
2. Wrong variance for higher-kinded arguments in TypeAccumulator

src/dotty/tools/dotc/config/Config.scala

@@ -98,6 +98,13 @@ object Config {
    */
   final val splitProjections = false
 
+  /** If this flag is on, always rewrite an application `S[Ts]` where `S` is an alias for
+   *  `[Xs] -> U` to `[Xs := Ts]U`. If this flag is off, the rewriting is only done if `S` is a
+   *  reference to an instantiated parameter. Turning this flag on was observed to
+   *  give a ~6% speedup on the JUnit test suite.
+   */
+  final val simplifyApplications = true
+
   /** Initial size of superId table */
   final val InitialSuperIdsSize = 4096
 

src/dotty/tools/dotc/core/TypeApplications.scala

@@ -469,18 +469,21 @@ class TypeApplications(val self: Type) extends AnyVal {
       case dealiased: TypeLambda =>
         def tryReduce =
           if (!args.exists(_.isInstanceOf[TypeBounds])) {
-            val reduced = dealiased.instantiate(args)
-            if (dealiased eq stripped) reduced
-            else reduced match {
-              case AppliedType(tycon, args) if variancesConform(typParams, tycon.typeParams) =>
-                // Reducing is safe for type inference, as kind of type constructor does not change
-                //println(i"reduced: $reduced instead of ${HKApply(self, args)}")
-                reduced
+            val followAlias = stripped match {
+              case stripped: TypeRef =>
+                stripped.symbol.is(BaseTypeArg)
               case _ =>
-                // Reducing changes kind, keep hk application instead
-                //println(i"fallback: ${HKApply(self, args)} instead of $reduced")
-                HKApply(self, args)
+                Config.simplifyApplications && {
+                  dealiased.resType match {
+                    case AppliedType(tyconBody, _) =>
+                      variancesConform(typParams, tyconBody.typeParams)
+                        // Reducing is safe for type inference, as kind of type constructor does not change
+                    case _ => false
+                  }
+                }
             }
+            if ((dealiased eq stripped) || followAlias) dealiased.instantiate(args)
+            else HKApply(self, args)
           }
           else dealiased.resType match {
             case AppliedType(tycon, args1) if tycon.safeDealias ne tycon =>
@@ -665,11 +668,6 @@ class TypeApplications(val self: Type) extends AnyVal {
       }
   }
 
-  final def typeConstructor(implicit ctx: Context): Type = self.stripTypeVar match {
-    case AppliedType(tycon, _) => tycon
-    case self => self
-  }
-
   /** If this is the image of a type argument; recover the type argument,
    *  otherwise NoType.
    */

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

@@ -3422,8 +3422,9 @@ object Types {
         case tp: HKApply =>
           def mapArg(arg: Type, tparam: TypeParamInfo): Type = {
             val saved = variance
-            if (tparam.paramVariance < 0) variance = -variance
-            else if (tparam.paramVariance == 0) variance = 0
+            val pvariance = tparam.paramVariance
+            if (pvariance < 0) variance = -variance
+            else if (pvariance == 0) variance = 0
             try this(arg)
             finally variance = saved
           }
@@ -3629,7 +3630,23 @@ object Types {
         this(x, prefix)
 
       case tp @ HKApply(tycon, args) =>
-        foldOver(this(x, tycon), args)
+        def foldArgs(x: T, tparams: List[TypeParamInfo], args: List[Type]): T =
+          if (args.isEmpty) {
+            assert(tparams.isEmpty)
+            x
+          }
+          else {
+            val tparam = tparams.head
+            val saved = variance
+            val pvariance = tparam.paramVariance
+            if (pvariance < 0) variance = -variance
+            else if (pvariance == 0) variance = 0
+            val acc =
+              try this(x, args.head)
+              finally variance = saved
+            foldArgs(acc, tparams.tail, args.tail)
+          }
+        foldArgs(this(x, tycon), tp.typeParams, args)
 
       case tp: AndOrType =>
         this(this(x, tp.tp1), tp.tp2)

src/dotty/tools/dotc/typer/Checking.scala

@@ -204,6 +204,8 @@ object Checking {
         tp.derivedRefinedType(this(parent), name, this(rinfo, nestedCycleOK, nestedCycleOK))
       case tp: RecType =>
         tp.rebind(this(tp.parent))
+      case tp @ HKApply(tycon, args) =>
+        tp.derivedAppliedType(this(tycon), args.map(this(_, nestedCycleOK, nestedCycleOK)))
       case tp @ TypeRef(pre, name) =>
         try {
           // A prefix is interesting if it might contain (transitively) a reference