Avoid dealiasing on type application

When applying a type alias of a type lambda, keep the original
application instead of reducing. But reduce anyway if

 - the reduced type is an application where the type
   constructor has the same kind as the original type constructor, or
 - some of the arguments are wildcards.

Author: odersky

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

@@ -300,19 +300,21 @@ trait ConstraintHandling {
        *  to refer to such a lambda parameter because the lambda parameter is
        *  not visible where `A` is defined. Consequently, we need to
        *  approximate the bound so that the lambda parameter does not appear in it.
-       *  Test case in neg/i94-nada.scala. This test crashes with an illegal instance
-       *  error when the rest of the SI-2712 fix is applied but `pruneLambdaParams` is
+       *  If `tp` is an upper bound, we need to approximate with something smaller,
+       *  otherwise something larger.
+       *  Test case in pos/i94-nada.scala. This test crashes with an illegal instance
+       *  error in Test2 when the rest of the SI-2712 fix is applied but `pruneLambdaParams` is
        *  missing.
        */
       def pruneLambdaParams(tp: Type) =
-        if (comparingLambdas) {
+        if (comparingLambdas && param.binder.isInstanceOf[PolyType]) {
           val approx = new ApproximatingTypeMap {
             def apply(t: Type): Type = t match {
               case t @ PolyParam(tl: TypeLambda, n) =>
                 val effectiveVariance = if (fromBelow) -variance else variance
                 val bounds = tl.paramBounds(n)
-                if (effectiveVariance > 0) bounds.hi
-                else if (effectiveVariance < 0 ) bounds.lo
+                if (effectiveVariance > 0) bounds.lo
+                else if (effectiveVariance < 0) bounds.hi
                 else NoType
               case _ =>
                 mapOver(t)

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

@@ -491,7 +491,7 @@ object Symbols {
 
     // TypeParamInfo methods
     def isTypeParam(implicit ctx: Context) = denot.is(TypeParam)
-    def paramName(implicit ctx: Context): Name = name
+    def paramName(implicit ctx: Context) = name.asTypeName
     def paramBounds(implicit ctx: Context) = denot.info.bounds
     def paramBoundsAsSeenFrom(pre: Type)(implicit ctx: Context) = pre.memberInfo(this).bounds
     def paramBoundsOrCompleter(implicit ctx: Context): Type = denot.infoOrCompleter

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

@@ -338,10 +338,10 @@ class TypeApplications(val self: Type) extends AnyVal {
    *
    *  TODO: Handle parameterized lower bounds
    */
-  def LambdaAbstract(tparams: List[TypeParamInfo])(implicit ctx: Context): Type = {
+  def LambdaAbstract(tparams: List[Symbol])(implicit ctx: Context): Type = {
     def expand(tp: Type) =
       TypeLambda(
-        tpnme.syntheticLambdaParamNames(tparams.length), tparams.map(_.paramVariance))(
+        tpnme.syntheticLambdaParamNames(tparams.length), tparams.map(_.variance))(
           tl => tparams.map(tparam => tl.lifted(tparams, tparam.paramBounds).bounds),
           tl => tl.lifted(tparams, tp))
     assert(!isHK, self)
@@ -439,20 +439,13 @@ class TypeApplications(val self: Type) extends AnyVal {
     }
   }
 
-  /** Encode
+  /** The type representing
    *
    *     T[U1, ..., Un]
    *
    *  where
    *  @param  self   = `T`
    *  @param  args   = `U1,...,Un`
-   *  performing the following simplifications
-   *
-   *  1. If `T` is an eta expansion `[X1,..,Xn] -> C[X1,...,Xn]` of class `C` compute
-   *     `C[U1, ..., Un]` instead.
-   *  2. If `T` is some other type lambda `[X1,...,Xn] -> S` none of the arguments
-   *     `U1,...,Un` is a wildcard, compute `[X1:=U1, ..., Xn:=Un]S` instead.
-   *  3. If `T` is a polytype, instantiate it to `U1,...,Un`.
    */
   final def appliedTo(args: List[Type])(implicit ctx: Context): Type = /*>|>*/ track("appliedTo") /*<|<*/ {
     val typParams = self.typeParams
@@ -469,30 +462,52 @@ class TypeApplications(val self: Type) extends AnyVal {
         }
       case nil => t
     }
+    val stripped = self.stripTypeVar
+    val dealiased = stripped.safeDealias
     if (args.isEmpty || ctx.erasedTypes) self
-    else self.stripTypeVar.safeDealias match {
-      case self: TypeLambda =>
-        if (!args.exists(_.isInstanceOf[TypeBounds])) self.instantiate(args)
-        else {
-          val reducer = new Reducer(self, args)
-          val reduced = reducer(self.resType)
-          if (reducer.allReplaced) reduced
-          else HKApply(self, args)
-        }
-      case self: PolyType =>
-        self.instantiate(args)
-      case self: AndOrType =>
-        self.derivedAndOrType(self.tp1.appliedTo(args), self.tp2.appliedTo(args))
-      case self: TypeAlias =>
-        self.derivedTypeAlias(self.alias.appliedTo(args))
-      case self: TypeBounds =>
-        self.derivedTypeBounds(self.lo, self.hi.appliedTo(args))
-      case self: LazyRef =>
-        LazyRef(() => self.ref.appliedTo(args))
-      case self: WildcardType =>
-        self
-      case self: TypeRef if self.symbol == defn.NothingClass =>
-        self
+    else dealiased match {
+      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
+              case _ =>
+                // Reducing changes kind, keep hk application instead
+                //println(i"fallback: ${HKApply(self, args)} instead of $reduced")
+                HKApply(self, args)
+            }
+          }
+          else dealiased.resType match {
+            case AppliedType(tycon, args1) if tycon.safeDealias ne tycon =>
+              dealiased
+                .derivedTypeLambda(resType = tycon.safeDealias.appliedTo(args1))
+                .appliedTo(args)
+            case _ =>
+              val reducer = new Reducer(dealiased, args)
+              val reduced = reducer(dealiased.resType)
+              if (reducer.allReplaced) reduced
+              else HKApply(dealiased, args)
+          }
+        tryReduce
+      case dealiased: PolyType =>
+        dealiased.instantiate(args)
+      case dealiased: AndOrType =>
+        dealiased.derivedAndOrType(dealiased.tp1.appliedTo(args), dealiased.tp2.appliedTo(args))
+      case dealiased: TypeAlias =>
+        dealiased.derivedTypeAlias(dealiased.alias.appliedTo(args))
+      case dealiased: TypeBounds =>
+        dealiased.derivedTypeBounds(dealiased.lo, dealiased.hi.appliedTo(args))
+      case dealiased: LazyRef =>
+        LazyRef(() => dealiased.ref.appliedTo(args))
+      case dealiased: WildcardType =>
+        dealiased
+      case dealiased: TypeRef if dealiased.symbol == defn.NothingClass =>
+        dealiased
       case _ if typParams.isEmpty || typParams.head.isInstanceOf[LambdaParam] =>
         HKApply(self, args)
       case dealiased =>

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

@@ -628,9 +628,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           val tparams1 = tparams1a.drop(lengthDiff)
           variancesConform(tparams1, tparams) && {
             if (lengthDiff > 0)
-              tycon1b = tycon1a
-                .appliedTo(args1.take(lengthDiff) ++ tparams1.map(_.paramRef))
-                .LambdaAbstract(tparams1)
+              tycon1b = TypeLambda(tparams1.map(_.paramName), tparams1.map(_.paramVariance))(
+                tl => tparams1.map(tparam => tl.lifted(tparams, tparam.paramBounds).bounds),
+                tl => tycon1a.appliedTo(args1.take(lengthDiff) ++
+                        tparams1.indices.toList.map(PolyParam(tl, _))))
             (ctx.mode.is(Mode.TypevarsMissContext) ||
               tryInstantiate(tycon2, tycon1b.ensureHK)) &&
               isSubType(tp1, tycon1b.appliedTo(args2))

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

@@ -1,6 +1,6 @@
 package dotty.tools.dotc.core
 
-import Names.Name
+import Names.TypeName
 import Contexts.Context
 import Types.{Type, TypeBounds}
 
@@ -15,22 +15,22 @@ trait TypeParamInfo {
   def isTypeParam(implicit ctx: Context): Boolean
 
   /** The name of the type parameter */
-  def paramName(implicit ctx: Context): Name
+  def paramName(implicit ctx: Context): TypeName
 
   /** The info of the type parameter */
   def paramBounds(implicit ctx: Context): TypeBounds
 
   /** The info of the type parameter as seen from a prefix type.
    *  For type parameter symbols, this is the `memberInfo` as seen from `prefix`.
-   *  For type lambda parameters, it's the same as `paramBounds` as 
+   *  For type lambda parameters, it's the same as `paramBounds` as
    *  `asSeenFrom` has already been applied to the whole type lambda.
    */
   def paramBoundsAsSeenFrom(pre: Type)(implicit ctx: Context): TypeBounds
-  
+
   /** The parameter bounds, or the completer if the type parameter
    *  is an as-yet uncompleted symbol.
    */
-  def paramBoundsOrCompleter(implicit ctx: Context): Type 
+  def paramBoundsOrCompleter(implicit ctx: Context): Type
 
   /** The variance of the type parameter */
   def paramVariance(implicit ctx: Context): Int

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

@@ -117,6 +117,7 @@ object Types {
           case _ =>  this1.symbol eq sym
         }
       case this1: RefinedOrRecType => this1.parent.isRef(sym)
+      case this1: HKApply => this1.superType.isRef(sym)
       case _ => false
     }
 
@@ -857,6 +858,10 @@ object Types {
         tp.derivedAnnotatedType(tp.tpe.dealias, tp.annot)
       case tp: LazyRef =>
         tp.ref.dealias
+      case app @ HKApply(tycon, args) =>
+        val tycon1 = tycon.dealias
+        if (tycon1 ne tycon) app.superType.dealias
+        else this
       case _ => this
     }
 
@@ -2586,7 +2591,7 @@ object Types {
     lazy val typeParams: List[LambdaParam] =
       paramNames.indices.toList.map(new LambdaParam(this, _))
 
-    def derivedTypeLambda(paramNames: List[TypeName], paramBounds: List[TypeBounds], resType: Type)(implicit ctx: Context): Type =
+    def derivedTypeLambda(paramNames: List[TypeName] = paramNames, paramBounds: List[TypeBounds] = paramBounds, resType: Type)(implicit ctx: Context): Type =
       resType match {
         case resType @ TypeAlias(alias) =>
           resType.derivedTypeAlias(duplicate(paramNames, paramBounds, alias))
@@ -2640,12 +2645,21 @@ object Types {
   abstract case class HKApply(tycon: Type, args: List[Type])
   extends CachedProxyType with ValueType {
 
+    private var validSuper: Period = Nowhere
+    private var cachedSuper: Type = _
+
     override def underlying(implicit ctx: Context): Type = tycon
 
-    override def superType(implicit ctx: Context): Type = tycon match {
-      case tp: TypeLambda => defn.AnyType
-      case tp: TypeProxy => tp.superType.applyIfParameterized(args)
-      case _ => defn.AnyType
+    override def superType(implicit ctx: Context): Type = {
+      if (ctx.period != validSuper) {
+        cachedSuper = tycon match {
+          case tp: TypeLambda => defn.AnyType
+          case tp: TypeProxy => tp.superType.applyIfParameterized(args)
+          case _ => defn.AnyType
+        }
+        validSuper = ctx.period
+      }
+      cachedSuper
     }
 /*
    def lowerBound(implicit ctx: Context): Type = tycon.stripTypeVar match {
@@ -2760,7 +2774,11 @@ object Types {
       else bounds(paramNum)
     }
     // no customized hashCode/equals needed because cycle is broken in PolyType
-    override def toString = s"PolyParam($paramName)"
+    override def toString =
+      try s"PolyParam($paramName)"
+      catch {
+        case ex: IndexOutOfBoundsException => s"PolyParam(<bad index: $paramNum>)"
+      }
 
     override def computeHash = doHash(paramNum, binder.identityHash)
 

tests/neg/boundspropagation.scala

@@ -40,5 +40,5 @@ object test4 {
 }
 
 class Test5 {
-"": ({ type U = this.type })#U                        // error // error
+"": ({ type U = this.type })#U                        // error
 }

tests/neg/i94-nada.scala

@@ -35,7 +35,7 @@ trait Test2 {
   case class Left[A,B](x: A) extends Either[A,B] with Monad[A]
   case class Right[A,B](x: B) extends Either[A,B] with Monad[B]
   def flatMap[X,Y,M[X]](m: M[X], f: X => M[Y]): M[Y]
-  println(flatMap(Right(1), {x: Int => Right(x)}))
+  println(flatMap(Left(1), {x: Int => Left(x)}))
 }
 trait Test3 {
   def flatMap[X,Y,M[X]](m: M[X], f: X => M[Y]): M[Y]

tests/pos/i94-nada.scala

@@ -0,0 +1,12 @@
+package test
+
+object Tags {
+  type Tagged[A, T] = {type Tag = T; type Self = A}
+
+  type @@[T, Tag] = Tagged[T, Tag]
+
+  trait Disjunction
+
+  def meh[M[_], A](ma: M[A]): M[A] = ma
+  meh(null: Int @@ Disjunction)//.asInstanceOf[Int @@ Disjunction])
+}