Merge pull request #1000 from dotty-staging/fix-#241

Fix #241

14 files changed, 724 insertions, 588 deletions

diff --git a/src/dotty/tools/dotc/core/ConstraintHandling.scala b/src/dotty/tools/dotc/core/ConstraintHandling.scala
index 577b958c8..ff7afe99a 100644
--- a/src/dotty/tools/dotc/core/ConstraintHandling.scala
+++ b/src/dotty/tools/dotc/core/ConstraintHandling.scala
@@ -232,6 +232,16 @@ trait ConstraintHandling {
     }
   }
 
+  /** Instantiate `param` to `tp` if the constraint stays satisfiable */
+  protected def tryInstantiate(param: PolyParam, tp: Type): Boolean = {
+    val saved = constraint
+    constraint =
+      if (addConstraint(param, tp, fromBelow = true) &&
+          addConstraint(param, tp, fromBelow = false)) constraint.replace(param, tp)
+      else saved
+    constraint ne saved
+  }
+
   /** Check that constraint is fully propagated. See comment in Config.checkConstraintsPropagated */
   def checkPropagated(msg: => String)(result: Boolean): Boolean = {
     if (Config.checkConstraintsPropagated && result && addConstraintInvocations == 0) {
diff --git a/src/dotty/tools/dotc/core/Definitions.scala b/src/dotty/tools/dotc/core/Definitions.scala
index 4556dd9d5..5f794f2d5 100644
--- a/src/dotty/tools/dotc/core/Definitions.scala
+++ b/src/dotty/tools/dotc/core/Definitions.scala
@@ -588,6 +588,12 @@ class Definitions {
     name.startsWith(prefix) && name.drop(prefix.length).forall(_.isDigit)
   }
 
+  def isBottomClass(cls: Symbol) = cls == NothingClass || cls == NullClass
+  def isBottomType(tp: Type) = tp match {
+    case tp: TypeRef => isBottomClass(tp.symbol)
+    case _ => false
+  }
+
   def isFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.Function)
   def isAbstractFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.AbstractFunction)
   def isTupleClass(cls: Symbol) = isVarArityClass(cls, tpnme.Tuple)
@@ -642,9 +648,9 @@ class Definitions {
    *  to be the type parameters of a higher-kided type). This is a class symbol that
    *  would be generated by the following schema.
    *
-   *      class LambdaXYZ extends Object with P1 with ... with Pn {
-   *        type v_1 $hk$Arg0; ...; type v_N $hk$ArgN;
-   *        type Apply
+   *      trait LambdaXYZ extends Object with P1 with ... with Pn {
+   *        type v_1 hk$0; ...; type v_N hk$N;
+   *        type +$Apply
    *      }
    *
    *  Here:
@@ -669,7 +675,7 @@ class Definitions {
         val cls = denot.asClass.classSymbol
         val paramDecls = newScope
         for (i <- 0 until vcs.length)
-          newTypeParam(cls, tpnme.LambdaArgName(i), varianceFlags(vcs(i)), paramDecls)
+          newTypeParam(cls, tpnme.hkArg(i), varianceFlags(vcs(i)), paramDecls)
         newTypeField(cls, tpnme.hkApply, Covariant, paramDecls)
         val parentTraitRefs =
           for (i <- 0 until vcs.length if vcs(i) != 0)
@@ -679,7 +685,7 @@ class Definitions {
       }
     }
 
-    val traitName = tpnme.LambdaTraitName(vcs)
+    val traitName = tpnme.hkLambda(vcs)
 
     def createTrait = {
       val cls = newClassSymbol(
diff --git a/src/dotty/tools/dotc/core/Denotations.scala b/src/dotty/tools/dotc/core/Denotations.scala
index b6519e1cd..fcd60ef16 100644
--- a/src/dotty/tools/dotc/core/Denotations.scala
+++ b/src/dotty/tools/dotc/core/Denotations.scala
@@ -324,7 +324,7 @@ object Denotations {
                         info1 // follow Scala2 linearization -
                               // compare with way merge is performed in SymDenotation#computeMembersNamed
                       else
-                        throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}")
+                        throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}", ex.tp1, ex.tp2)
                   }
                 new JointRefDenotation(sym, jointInfo, denot1.validFor & denot2.validFor)
               }
diff --git a/src/dotty/tools/dotc/core/Flags.scala b/src/dotty/tools/dotc/core/Flags.scala
index c8a9dc13b..42d06c2ab 100644
--- a/src/dotty/tools/dotc/core/Flags.scala
+++ b/src/dotty/tools/dotc/core/Flags.scala
@@ -306,10 +306,12 @@ object Flags {
   /** A case parameter accessor */
   final val CaseAccessor = termFlag(25, "<caseaccessor>")
 
-  /** An type parameter which is an alias for some other (non-visible) type parameter */
-  final val TypeArgument = typeFlag(25, "<type-param-inst>")
+  /** A binding for a type parameter of a base class or trait.
+   *  TODO: Replace with combination of isType, ExpandedName, and Override?
+   */
+  final val BaseTypeArg = typeFlag(25, "<basetypearg>")
 
-  final val CaseAccessorOrTypeArgument = CaseAccessor.toCommonFlags
+  final val CaseAccessorOrBaseTypeArg = CaseAccessor.toCommonFlags
 
   /** A super accessor */
   final val SuperAccessor = termFlag(26, "<superaccessor>")
@@ -446,7 +448,7 @@ object Flags {
   final val FromStartFlags =
     AccessFlags | Module | Package | Deferred | Final | MethodOrHKCommon | Param | ParamAccessor | Scala2ExistentialCommon |
     InSuperCall | Touched | JavaStatic | CovariantOrOuter | ContravariantOrLabel | ExpandedName | AccessorOrSealed |
-    CaseAccessorOrTypeArgument | Fresh | Frozen | Erroneous | ImplicitCommon | Permanent |
+    CaseAccessorOrBaseTypeArg | Fresh | Frozen | Erroneous | ImplicitCommon | Permanent |
     LazyOrTrait | SuperAccessorOrScala2x | SelfNameOrImplClass
 
   assert(FromStartFlags.isTermFlags && FromStartFlags.isTypeFlags)
@@ -545,7 +547,7 @@ object Flags {
   final val TypeParamOrAccessor = TypeParam | TypeParamAccessor
 
   /** If symbol of a type alias has these flags, prefer the alias */
-  final val AliasPreferred = TypeParam | TypeArgument | ExpandedName
+  final val AliasPreferred = TypeParam | BaseTypeArg | ExpandedName
 
   /** A covariant type parameter instance */
   final val LocalCovariant = allOf(Local, Covariant)
@@ -583,6 +585,9 @@ object Flags {
   /** A private[this] parameter accessor */
   final val PrivateLocalParamAccessor = allOf(Private, Local, ParamAccessor)
 
+  /** A parameter forwarder */
+  final val ParamForwarder = allOf(Method, Stable, ParamAccessor)
+
   /** A private[this] parameter */
   final val PrivateLocalParam = allOf(Private, Local, Param)
 
diff --git a/src/dotty/tools/dotc/core/NameOps.scala b/src/dotty/tools/dotc/core/NameOps.scala
index 1a2646347..6c1930c9f 100644
--- a/src/dotty/tools/dotc/core/NameOps.scala
+++ b/src/dotty/tools/dotc/core/NameOps.scala
@@ -81,6 +81,7 @@ object NameOps {
     def isScala2LocalSuffix = name.endsWith(" ")
     def isModuleVarName(name: Name): Boolean =
       name.stripAnonNumberSuffix endsWith MODULE_VAR_SUFFIX
+    def isSelectorName = name.startsWith(" ") && name.tail.forall(_.isDigit)
 
     /** Is name a variable name? */
     def isVariableName: Boolean = name.length > 0 && {
@@ -99,19 +100,22 @@ object NameOps {
     }
 
     /** Is this the name of a higher-kinded type parameter of a Lambda? */
-    def isLambdaArgName =
+    def isHkArgName =
       name.length > 0 &&
-      name.head == tpnme.LAMBDA_ARG_PREFIXhead &&
-      name.startsWith(tpnme.LAMBDA_ARG_PREFIX) && {
-        val digits = name.drop(tpnme.LAMBDA_ARG_PREFIX.length)
+      name.head == tpnme.hkArgPrefixHead &&
+      name.startsWith(tpnme.hkArgPrefix) && {
+        val digits = name.drop(tpnme.hkArgPrefixLength)
         digits.length <= 4 && digits.forall(_.isDigit)
       }
 
     /** The index of the higher-kinded type parameter with this name.
      *  Pre: isLambdaArgName.
      */
-    def LambdaArgIndex: Int =
-      name.drop(tpnme.LAMBDA_ARG_PREFIX.length).toString.toInt
+    def hkArgIndex: Int =
+      name.drop(tpnme.hkArgPrefixLength).toString.toInt
+
+    def isLambdaTraitName(implicit ctx: Context): Boolean =
+      name.startsWith(tpnme.hkLambdaPrefix)
 
     /** If the name ends with $nn where nn are
       * all digits, strip the $ and the digits.
diff --git a/src/dotty/tools/dotc/core/StdNames.scala b/src/dotty/tools/dotc/core/StdNames.scala
index e8cddd3d4..e2add1a52 100644
--- a/src/dotty/tools/dotc/core/StdNames.scala
+++ b/src/dotty/tools/dotc/core/StdNames.scala
@@ -173,8 +173,6 @@ object StdNames {
     final val WILDCARD_STAR: N                  = "_*"
     final val REIFY_TREECREATOR_PREFIX: N       = "$treecreator"
     final val REIFY_TYPECREATOR_PREFIX: N       = "$typecreator"
-    final val LAMBDA_ARG_PREFIX: N              = "hk$"
-    final val LAMBDA_ARG_PREFIXhead: Char       = LAMBDA_ARG_PREFIX.head
 
     final val AbstractFunction: N    = "AbstractFunction"
     final val Any: N                 = "Any"
@@ -314,7 +312,6 @@ object StdNames {
 
     val AnnotatedType: N        = "AnnotatedType"
     val AppliedTypeTree: N      = "AppliedTypeTree"
-    val hkApply: N              = "$apply"
     val ArrayAnnotArg: N        = "ArrayAnnotArg"
     val Constant: N             = "Constant"
     val ConstantType: N         = "ConstantType"
@@ -322,7 +319,6 @@ object StdNames {
     val Flag : N                = "Flag"
     val Ident: N                = "Ident"
     val Import: N               = "Import"
-    val LambdaPrefix: N         = "Lambda$"
     val Literal: N              = "Literal"
     val LiteralAnnotArg: N      = "LiteralAnnotArg"
     val Modifiers: N            = "Modifiers"
@@ -530,9 +526,14 @@ object StdNames {
     val nothingRuntimeClass: N  = "scala.runtime.Nothing$"
     val nullRuntimeClass: N     = "scala.runtime.Null$"
 
-
     val synthSwitch: N          = "$synthSwitch"
 
+    val hkApply: N              = "$Apply"
+    val hkArgPrefix: N          = "$hk"
+    val hkLambdaPrefix: N       = "Lambda$"
+    val hkArgPrefixHead: Char   = hkArgPrefix.head
+    val hkArgPrefixLength: Int  = hkArgPrefix.length
+
     // unencoded operators
     object raw {
       final val AMP  : N  = "&"
@@ -742,14 +743,13 @@ object StdNames {
     def syntheticTypeParamNames(num: Int): List[TypeName] =
       (0 until num).map(syntheticTypeParamName)(breakOut)
 
-    def LambdaTraitName(vcs: List[Int]): TypeName = LambdaPrefix ++ vcs.map(varianceSuffix).mkString
-    def LambdaArgName(n: Int) = LAMBDA_ARG_PREFIX ++ n.toString
-
-    final val Conforms = encode("<:<")
+    def hkLambda(vcs: List[Int]): TypeName = hkLambdaPrefix ++ vcs.map(varianceSuffix).mkString
+    def hkArg(n: Int): TypeName = hkArgPrefix ++ n.toString
 
     def varianceSuffix(v: Int): Char = varianceSuffixes.charAt(v + 1)
-
     val varianceSuffixes = "NIP"
+
+    final val Conforms = encode("<:<")
   }
 
   abstract class JavaNames[N <: Name] extends DefinedNames[N] {
diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index d7fa183c9..e351557a3 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -167,7 +167,6 @@ object SymDenotations {
         completions.println(i"${"  " * indent}completing ${if (isType) "type" else "val"} $name")
         indent += 1
       }
-      indent += 1
       if (myFlags is Touched) throw CyclicReference(this)
       myFlags |= Touched
 
@@ -446,7 +445,7 @@ object SymDenotations {
 
     /** is this symbol a trait representing a type lambda? */
     final def isLambdaTrait(implicit ctx: Context): Boolean =
-      isClass && name.startsWith(tpnme.LambdaPrefix) && owner == defn.ScalaPackageClass
+      isClass && name.startsWith(tpnme.hkLambdaPrefix) && owner == defn.ScalaPackageClass
 
     /** Is this symbol a package object or its module class? */
     def isPackageObject(implicit ctx: Context): Boolean = {
diff --git a/src/dotty/tools/dotc/core/Symbols.scala b/src/dotty/tools/dotc/core/Symbols.scala
index e33f9651e..2a76f18d8 100644
--- a/src/dotty/tools/dotc/core/Symbols.scala
+++ b/src/dotty/tools/dotc/core/Symbols.scala
@@ -368,7 +368,7 @@ object Symbols {
 
     type ThisName <: Name
 
-    //assert(_id != 30214)
+    //assert(id != 4285)
 
     /** The last denotation of this symbol */
     private[this] var lastDenot: SymDenotation = _
diff --git a/src/dotty/tools/dotc/core/TypeApplications.scala b/src/dotty/tools/dotc/core/TypeApplications.scala
index 661975dab..fbab4ee39 100644
--- a/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -36,6 +36,145 @@ object TypeApplications {
     case tp: TypeBounds => tp.hi
     case _ => tp
   }
+
+  /** Does the variance of `sym1` conform to the variance of `sym2`?
+   *  This is the case if the variances are the same or `sym` is nonvariant.
+   */
+  def varianceConforms(sym1: TypeSymbol, sym2: TypeSymbol)(implicit ctx: Context) =
+    sym1.variance == sym2.variance || sym2.variance == 0
+
+  def variancesConform(syms1: List[TypeSymbol], syms2: List[TypeSymbol])(implicit ctx: Context) =
+    syms1.corresponds(syms2)(varianceConforms)
+
+  /** Extractor for
+   *
+   *    [v1 X1: B1, ..., vn Xn: Bn] -> T
+   *    ==>
+   *    Lambda$_v1...vn { type $hk_i: B_i, type $Apply = [X_i := this.$Arg_i] T }
+   */
+  object TypeLambda {
+    def apply(variances: List[Int],
+              argBoundsFns: List[RefinedType => TypeBounds],
+              bodyFn: RefinedType => Type)(implicit ctx: Context): Type = {
+      def argRefinements(parent: Type, i: Int, bs: List[RefinedType => TypeBounds]): Type = bs match {
+        case b :: bs1 =>
+          argRefinements(RefinedType(parent, tpnme.hkArg(i), b), i + 1, bs1)
+        case nil =>
+          parent
+      }
+      assert(variances.nonEmpty)
+      assert(argBoundsFns.length == variances.length)
+      RefinedType(
+        argRefinements(defn.LambdaTrait(variances).typeRef, 0, argBoundsFns),
+        tpnme.hkApply, bodyFn(_).bounds.withVariance(1))
+    }
+
+    def unapply(tp: Type)(implicit ctx: Context): Option[(List[Int], List[TypeBounds], Type)] = tp match {
+      case app @ RefinedType(parent, tpnme.hkApply) =>
+        val cls = parent.typeSymbol
+        val variances = cls.typeParams.map(_.variance)
+        def collectBounds(t: Type, acc: List[TypeBounds]): List[TypeBounds] = t match {
+          case t @ RefinedType(p, rname) =>
+            assert(rname.isHkArgName)
+            collectBounds(p, t.refinedInfo.bounds :: acc)
+          case TypeRef(_, lname) =>
+            assert(lname.isLambdaTraitName)
+            acc
+        }
+        val argBounds = collectBounds(parent, Nil)
+        Some((variances, argBounds, app.refinedInfo.argInfo))
+      case _ =>
+        None
+    }
+  }
+
+  /** Extractor for
+   *
+   *    [v1 X1: B1, ..., vn Xn: Bn] -> C[X1, ..., Xn]
+   *
+   *  where v1, ..., vn and B1, ..., Bn are the variances and bounds of the type parameters
+   *  of the class C.
+   *
+   *  @param tycon     C
+   */
+  object EtaExpansion {
+    def apply(tycon: TypeRef)(implicit ctx: Context) = {
+      assert(tycon.isEtaExpandable)
+      tycon.EtaExpand(tycon.typeParams)
+    }
+
+    def unapply(tp: Type)(implicit ctx: Context): Option[TypeRef] = {
+      def argsAreForwarders(args: List[Type], n: Int): Boolean = args match {
+        case Nil =>
+          n == 0
+        case TypeRef(RefinedThis(rt), sel) :: args1 =>
+          rt.eq(tp) && sel == tpnme.hkArg(n - 1) && argsAreForwarders(args1, n - 1)
+        case _ =>
+          false
+      }
+      tp match {
+        case TypeLambda(_, argBounds, AppliedType(fn: TypeRef, args))
+        if argsAreForwarders(args, tp.typeParams.length) => Some(fn)
+        case _ => None
+      }
+    }
+  }
+
+  /** Extractor for type application T[U_1, ..., U_n]. This is the refined type
+   *
+   *     T { type p_1 v_1= U_1; ...; type p_n v_n= U_n }
+   *
+   *  where v_i, p_i are the variances and names of the type parameters of T,
+   *  If `T`'s class symbol is a lambda trait, follow the refined type with a
+   *  projection
+   *
+   *      T { ... } # $Apply
+   */
+  object AppliedType {
+    def apply(tp: Type, args: List[Type])(implicit ctx: Context): Type = tp.appliedTo(args)
+
+    def unapply(tp: Type)(implicit ctx: Context): Option[(Type, List[Type])] = tp match {
+      case TypeRef(prefix, tpnme.hkApply) => unapp(prefix)
+      case _ =>
+        unapp(tp) match {
+          case Some((tycon: TypeRef, _)) if tycon.symbol.isLambdaTrait =>
+            // We are seeing part of a lambda abstraction, not an applied type
+            None
+          case x => x
+        }
+    }
+
+    private def unapp(tp: Type)(implicit ctx: Context): Option[(Type, List[Type])] = tp match {
+      case _: RefinedType =>
+        val tparams = tp.classSymbol.typeParams
+        if (tparams.isEmpty) None
+        else {
+          val argBuf = new mutable.ListBuffer[Type]
+          def stripArgs(tp: Type, n: Int): Type =
+            if (n == 0) tp
+            else tp match {
+              case tp @ RefinedType(parent, pname) if pname == tparams(n - 1).name =>
+                val res = stripArgs(parent, n - 1)
+                if (res.exists) argBuf += tp.refinedInfo.argInfo
+                res
+              case _ =>
+                NoType
+            }
+          val res = stripArgs(tp, tparams.length)
+          if (res.exists) Some((res, argBuf.toList)) else None
+        }
+      case _ => None
+    }
+  }
+
+   /** Adapt all arguments to possible higher-kinded type parameters using etaExpandIfHK
+   */
+  def etaExpandIfHK(tparams: List[Symbol], args: List[Type])(implicit ctx: Context): List[Type] =
+    if (tparams.isEmpty) args
+    else args.zipWithConserve(tparams)((arg, tparam) => arg.etaExpandIfHK(tparam.infoOrCompleter))
+
+  def argRefs(rt: RefinedType, n: Int)(implicit ctx: Context) =
+    List.range(0, n).map(i => RefinedThis(rt).select(tpnme.hkArg(i)))
 }
 
 import TypeApplications._
@@ -61,27 +200,27 @@ class TypeApplications(val self: Type) extends AnyVal {
       case self: ClassInfo =>
         self.cls.typeParams
       case self: TypeRef =>
-        val tsym = self.typeSymbol
+        val tsym = self.symbol
         if (tsym.isClass) tsym.typeParams
         else if (tsym.isAliasType) self.underlying.typeParams
-        else {
-          val lam = LambdaClass(forcing = false)
-          if (lam.exists) lam.typeParams else Nil
-        }
+        else if (tsym.isCompleting)
+          // We are facing a problem when computing the type parameters of an uncompleted
+          // abstract type. We can't access the bounds of the symbol yet because that
+          // would cause a cause a cyclic reference. So we return `Nil` instead
+          // and try to make up for it later. The acrobatics in Scala2Unpicker#readType
+          // for reading a TypeRef show what's neeed.
+          Nil
+        else tsym.info.typeParams
       case self: RefinedType =>
-        def isBoundedLambda(tp: Type): Boolean = tp match {
-          case tp: TypeRef => tp.typeSymbol.isLambdaTrait
-          case tp: RefinedType => tp.refinedName.isLambdaArgName && isBoundedLambda(tp.parent)
-          case _ => false
-        }
-        val tparams = self.parent.typeParams
-        self.refinedInfo match {
-          case rinfo: TypeBounds if rinfo.isAlias || isBoundedLambda(self) =>
-            tparams.filterNot(_.name == self.refinedName)
-          case _ =>
-            tparams
+        // inlined and optimized version of
+        //   val sym = self.LambdaTrait
+        //   if (sym.exists) return sym.typeParams
+        if (self.refinedName == tpnme.hkApply) {
+          val sym = self.parent.classSymbol
+          if (sym.isLambdaTrait) return sym.typeParams
         }
-     case self: SingletonType =>
+        self.parent.typeParams.filterNot(_.name == self.refinedName)
+      case self: SingletonType =>
         Nil
       case self: TypeProxy =>
         self.underlying.typeParams
@@ -90,76 +229,24 @@ class TypeApplications(val self: Type) extends AnyVal {
     }
   }
 
-  /** The type parameters of the underlying class.
-   *  This is like `typeParams`, except for 3 differences.
-   *  First, it does not adjust type parameters in refined types. I.e. type arguments
-   *  do not remove corresponding type parameters.
-   *  Second, it will return Nil for BoundTypes because we might get a NullPointer exception
-   *  on PolyParam#underlying otherwise (demonstrated by showClass test).
-   *  Third, it won't return abstract higher-kinded type parameters, i.e. the type parameters of
-   *  an abstract type are always empty.
-   */
-  final def rawTypeParams(implicit ctx: Context): List[TypeSymbol] = {
-    self match {
-      case self: ClassInfo =>
-        self.cls.typeParams
-      case self: TypeRef =>
-        val tsym = self.typeSymbol
-        if (tsym.isClass) tsym.typeParams
-        else if (tsym.isAliasType) self.underlying.rawTypeParams
-        else Nil
-      case _: BoundType | _: SingletonType =>
-        Nil
-      case self: TypeProxy =>
-        self.underlying.rawTypeParams
-      case _ =>
-        Nil
-    }
+  /** The Lambda trait underlying a type lambda */
+  def LambdaTrait(implicit ctx: Context): Symbol = self.stripTypeVar match {
+    case RefinedType(parent, tpnme.hkApply) =>
+      val sym = self.classSymbol
+      if (sym.isLambdaTrait) sym else NoSymbol
+    case TypeBounds(lo, hi) => hi.LambdaTrait
+    case _ => NoSymbol
   }
 
-  /** If type `self` is equal, aliased-to, or upperbounded-by a type of the form
-   *  `LambdaXYZ { ... }`, the class symbol of that type, otherwise NoSymbol.
-   *  symbol of that type, otherwise NoSymbol.
-   *  @param forcing  if set, might force completion. If not, never forces
-   *                  but returns NoSymbol when it would have to otherwise.
-   */
-  def LambdaClass(forcing: Boolean)(implicit ctx: Context): Symbol = track("LambdaClass") { self.stripTypeVar match {
-    case self: TypeRef =>
-      val sym = self.symbol
-      if (sym.isLambdaTrait) sym
-      else if (sym.isClass || sym.isCompleting && !forcing) NoSymbol
-      else self.info.LambdaClass(forcing)
-    case self: TypeProxy =>
-      self.underlying.LambdaClass(forcing)
-    case _ =>
-      NoSymbol
-  }}
-
-  /** Is type `self` equal, aliased-to, or upperbounded-by a type of the form
-   *  `LambdaXYZ { ... }`?
-   */
-  def isLambda(implicit ctx: Context): Boolean =
-    LambdaClass(forcing = true).exists
-
-  /** Same is `isLambda`, except that symbol denotations are not forced
-   *  Symbols in completion count as not lambdas.
-   */
-  def isSafeLambda(implicit ctx: Context): Boolean =
-    LambdaClass(forcing = false).exists
-
-  /** Is type `self` a Lambda with all hk$i fields fully instantiated? */
-  def isInstantiatedLambda(implicit ctx: Context): Boolean =
-    isSafeLambda && typeParams.isEmpty
-
   /** Is receiver type higher-kinded (i.e. of kind != "*")? */
   def isHK(implicit ctx: Context): Boolean = self.dealias match {
     case self: TypeRef => self.info.isHK
-    case RefinedType(_, name) => name == tpnme.hkApply || name.isLambdaArgName
+    case RefinedType(_, name) => name == tpnme.hkApply
     case TypeBounds(_, hi) => hi.isHK
     case _ => false
   }
 
-  /** is receiver of the form T#$apply? */
+  /** is receiver of the form T#$Apply? */
   def isHKApply: Boolean = self match {
     case TypeRef(_, name) => name == tpnme.hkApply
     case _ => false
@@ -173,101 +260,237 @@ class TypeApplications(val self: Type) extends AnyVal {
    *
    *  but without forcing anything.
    */
-  def noHK(implicit ctx: Context): Boolean = self.stripTypeVar match {
+  def classNotLambda(implicit ctx: Context): Boolean = self.stripTypeVar match {
     case self: RefinedType =>
-      self.parent.noHK
+      self.parent.classNotLambda
     case self: TypeRef =>
-      (self.denot.exists) && {
+      self.denot.exists && {
         val sym = self.symbol
         if (sym.isClass) !sym.isLambdaTrait
-        else sym.isCompleted && self.info.isAlias && self.info.bounds.hi.noHK
+        else sym.isCompleted && self.info.isAlias && self.info.bounds.hi.classNotLambda
       }
     case _ =>
       false
   }
 
-  /** Encode the type resulting from applying this type to given arguments */
-  final def appliedTo(args: List[Type])(implicit ctx: Context): Type = /*>|>*/ track("appliedTo") /*<|<*/ {
-    def matchParams(tp: Type, tparams: List[TypeSymbol], args: List[Type]): Type = args match {
-      case arg :: args1 =>
-        if (tparams.isEmpty) {
-          println(s"applied type mismatch: $self $args, typeParams = $typeParams, tsym = ${self.typeSymbol.debugString}") // !!! DEBUG
-          println(s"precomplete decls = ${self.typeSymbol.unforcedDecls.toList.map(_.denot).mkString("\n  ")}")
-        }
-        val tparam = tparams.head
-        val tp1 = RefinedType(tp, tparam.name, arg.toBounds(tparam))
-        matchParams(tp1, tparams.tail, args1)
-      case nil => tp
-    }
+  /** Lambda abstract `self` with given type parameters. Examples:
+   *
+   *      type T[X] = U        becomes    type T = [X] -> U
+   *      type T[X] >: L <: U  becomes    type T >: L <: ([X] -> _ <: U)
+   */
+  def LambdaAbstract(tparams: List[Symbol])(implicit ctx: Context): Type = {
 
-    /** Instantiate type `tp` with `args`.
-     *  @param original  The original type for which we compute the type parameters
-     *                   This makes a difference for refinement types, because
-     *                   refinements bind type parameters and thereby remove them
-     *                   from `typeParams`.
+    /** Replace references to type parameters with references to hk arguments `this.$hk_i`
+     * Care is needed not to cause cycles, hence `SafeSubstMap`.
      */
-    def instantiate(tp: Type, original: Type): Type = tp match {
-      case tp: TypeRef =>
-        val tsym = tp.symbol
-        if (tsym.isAliasType) tp.underlying.appliedTo(args)
-        else {
-          val safeTypeParams =
-            if (tsym.isClass || !tp.typeSymbol.isCompleting) original.typeParams
-            else {
-              ctx.warning(i"encountered F-bounded higher-kinded type parameters for $tsym; assuming they are invariant")
-              defn.LambdaTrait(args map alwaysZero).typeParams // @@@ can we force?
-            }
-          matchParams(tp, safeTypeParams, args)
-        }
-      case tp: RefinedType =>
-        val redux = tp.EtaReduce
-        if (redux.exists) redux.appliedTo(args) // Rewrite ([hk$0] => C[hk$0])(T)   to   C[T]
-        else tp.derivedRefinedType(
-          instantiate(tp.parent, original),
-          tp.refinedName,
-          tp.refinedInfo)
-      case tp: TypeProxy =>
-        instantiate(tp.underlying, original)
-      case tp: PolyType =>
-        tp.instantiate(args)
-      case ErrorType =>
-        tp
+    def internalize[T <: Type](tp: T) =
+      (rt: RefinedType) =>
+        new ctx.SafeSubstMap(tparams, argRefs(rt, tparams.length))
+          .apply(tp).asInstanceOf[T]
+
+    def expand(tp: Type) = {
+      TypeLambda(
+        tparams.map(_.variance),
+        tparams.map(tparam => internalize(self.memberInfo(tparam).bounds)),
+        internalize(tp))
     }
+    self match {
+      case self: TypeAlias =>
+        self.derivedTypeAlias(expand(self.alias))
+      case self @ TypeBounds(lo, hi) =>
+        self.derivedTypeBounds(lo, expand(TypeBounds.upper(hi)))
+      case _ => expand(self)
+    }
+  }
 
-    /** Same as isHK, except we classify all abstract types as HK,
-     *  (they must be, because they are applied). This avoids some forcing and
-     *  CyclicReference errors of the standard isHK.
-     */
-    def isKnownHK(tp: Type): Boolean = tp match {
-      case tp: TypeRef =>
-        val sym = tp.symbol
-        if (sym.isClass) sym.isLambdaTrait
-        else !sym.isAliasType || isKnownHK(tp.info)
-      case tp: TypeProxy => isKnownHK(tp.underlying)
-      case _ => false
+  /** A type ref is eta expandable if it refers to a non-lambda class.
+   *  In that case we can look for parameterized base types of the type
+   *  to eta expand them.
+   */
+  def isEtaExpandable(implicit ctx: Context) = self match {
+    case self: TypeRef => self.symbol.isClass && !self.name.isLambdaTraitName
+    case _ => false
+  }
+
+  /** Convert a type constructor `TC` which has type parameters `T1, ..., Tn`
+   *  in a context where type parameters `U1,...,Un` are expected to
+   *
+   *     LambdaXYZ { Apply = TC[hk$0, ..., hk$n] }
+   *
+   *  Here, XYZ corresponds to the variances of
+   *   - `U1,...,Un` if the variances of `T1,...,Tn` are pairwise compatible with `U1,...,Un`,
+   *   - `T1,...,Tn` otherwise.
+   *  v1 is compatible with v2, if v1 = v2 or v2 is non-variant.
+   */
+  def EtaExpand(tparams: List[TypeSymbol])(implicit ctx: Context): Type = {
+    val tparamsToUse = if (variancesConform(typeParams, tparams)) tparams else typeParams
+    self.appliedTo(tparams map (_.typeRef)).LambdaAbstract(tparamsToUse)
+      //.ensuring(res => res.EtaReduce =:= self, s"res = $res, core = ${res.EtaReduce}, self = $self, hc = ${res.hashCode}")
+  }
+
+  /** Eta expand the prefix in front of any refinements.
+   *  @param  tparamsForBottom   Type parameters to use if core is a bottom type
+   */
+  def EtaExpandCore(tparamsForBottom: List[TypeSymbol])(implicit ctx: Context): Type = self.stripTypeVar match {
+    case self: RefinedType =>
+      self.derivedRefinedType(self.parent.EtaExpandCore(tparamsForBottom), self.refinedName, self.refinedInfo)
+    case tp: TypeRef if defn.isBottomClass(tp.symbol) =>
+      self.LambdaAbstract(tparamsForBottom)
+    case _ =>
+      self.EtaExpand(self.typeParams)
+  }
+
+  /** Eta expand if `self` is a (non-lambda) class reference and `bound` is a higher-kinded type */
+  def etaExpandIfHK(bound: Type)(implicit ctx: Context): Type = {
+    val boundLambda = bound.LambdaTrait
+    val hkParams = boundLambda.typeParams
+    if (hkParams.isEmpty) self
+    else self match {
+      case self: TypeRef if self.symbol.isClass && self.typeParams.length == hkParams.length =>
+        EtaExpansion(self)
+      case _ => self
     }
+  }
 
-    if (args.isEmpty || ctx.erasedTypes) self
+  /** If argument A and type parameter P are higher-kinded, adapt the variances
+   *  of A to those of P, ensuring that the variances of the type lambda A
+   *  agree with the variances of corresponding higher-kinded type parameters of P. Example:
+   *
+   *     class GenericCompanion[+CC[X]]
+   *     GenericCompanion[List]
+   *
+   *  with adaptHkVariances, the argument `List` will expand to
+   *
+   *     [X] => List[X]
+   *
+   *  instead of
+   *
+   *     [+X] => List[X]
+   *
+   *  even though `List` is covariant. This adaptation is necessary to ignore conflicting
+   *  variances in overriding members that have types of hk-type parameters such as
+   *  `GenericCompanion[GenTraversable]` or `GenericCompanion[ListBuffer]`.
+   *  When checking overriding, we need to validate the subtype relationship
+   *
+   *      GenericCompanion[[X] -> ListBuffer[X]] <: GenericCompanion[[+X] -> GenTraversable[X]]
+   *
+   *   Without adaptation, this would be false, and hence an overriding error would
+   *   result. But with adaptation, the rhs argument will be adapted to
+   *
+   *     [X] -> GenTraversable[X]
+   *
+   *   which makes the subtype test succeed. The crucial point here is that, since
+   *   GenericCompanion only expects a non-variant CC, the fact that GenTraversable
+   *   is covariant is irrelevant, so can be ignored.
+   */
+  def adaptHkVariances(bound: Type)(implicit ctx: Context): Type = {
+    val boundLambda = bound.LambdaTrait
+    val hkParams = boundLambda.typeParams
+    if (hkParams.isEmpty) self
     else {
-      val res = instantiate(self, self)
-      if (isKnownHK(res)) TypeRef(res, tpnme.hkApply) else res
+      def adaptArg(arg: Type): Type = arg match {
+        case arg: TypeRef if arg.symbol.isLambdaTrait &&
+          !arg.symbol.typeParams.corresponds(hkParams)(_.variance == _.variance) &&
+          arg.symbol.typeParams.corresponds(hkParams)(varianceConforms) =>
+          arg.prefix.select(boundLambda)
+        case arg: RefinedType =>
+          arg.derivedRefinedType(adaptArg(arg.parent), arg.refinedName, arg.refinedInfo)
+        case arg @ TypeAlias(alias) =>
+          arg.derivedTypeAlias(adaptArg(alias))
+        case arg @ TypeBounds(lo, hi) =>
+          arg.derivedTypeBounds(lo, adaptArg(hi))
+        case _ =>
+          arg
+      }
+      adaptArg(self)
     }
   }
 
-  /** Simplify a fully instantiated type of the form `LambdaX{... type Apply = T } # Apply` to `T`.
+  /** Encode
+   *
+   *     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`.
    */
-  def simplifyApply(implicit ctx: Context): Type = self match {
-    case self @ TypeRef(prefix, tpnme.hkApply) if prefix.isInstantiatedLambda =>
-      prefix.member(tpnme.hkApply).info match {
-        case TypeAlias(alias) => alias
-        case _ => self
+  final def appliedTo(args: List[Type])(implicit ctx: Context): Type = /*>|>*/ track("appliedTo") /*<|<*/ {
+    def substHkArgs = new TypeMap {
+      def apply(tp: Type): Type = tp match {
+        case TypeRef(RefinedThis(rt), name) if rt.eq(self) && name.isHkArgName =>
+          args(name.hkArgIndex)
+        case _ =>
+          mapOver(tp)
       }
-    case _ => self
+    }
+    if (args.isEmpty || ctx.erasedTypes) self
+    else self.stripTypeVar match {
+      case EtaExpansion(self1) =>
+        self1.appliedTo(args)
+      case TypeLambda(_, _, body) if !args.exists(_.isInstanceOf[TypeBounds]) =>
+        substHkArgs(body)
+      case self: PolyType =>
+        self.instantiate(args)
+      case _ =>
+        appliedTo(args, typeParams)
+    }
+  }
+
+  /** Encode application `T[U1, ..., Un]` without simplifications, where
+   *  @param self     = `T`
+   *  @param args     = `U1, ..., Un`
+   *  @param tparams  are assumed to be the type parameters of `T`.
+   */
+  final def appliedTo(args: List[Type], typParams: List[TypeSymbol])(implicit ctx: Context): Type = {
+    def matchParams(t: Type, tparams: List[TypeSymbol], args: List[Type])(implicit ctx: Context): Type = args match {
+      case arg :: args1 =>
+        try {
+          val tparam :: tparams1 = tparams
+          matchParams(RefinedType(t, tparam.name, arg.toBounds(tparam)), tparams1, args1)
+        } catch {
+          case ex: MatchError =>
+            println(s"applied type mismatch: $self $args, typeParams = $typParams") // !!! DEBUG
+            //println(s"precomplete decls = ${self.typeSymbol.unforcedDecls.toList.map(_.denot).mkString("\n  ")}")
+            throw ex
+        }
+      case nil => t
+    }
+    assert(args.nonEmpty)
+    matchParams(self, typParams, args) match {
+      case refined @ RefinedType(_, pname) if pname.isHkArgName =>
+        TypeRef(refined, tpnme.hkApply)
+      case refined =>
+        refined
+    }
   }
 
   final def appliedTo(arg: Type)(implicit ctx: Context): Type = appliedTo(arg :: Nil)
   final def appliedTo(arg1: Type, arg2: Type)(implicit ctx: Context): Type = appliedTo(arg1 :: arg2 :: Nil)
 
+  /** A cycle-safe version of `appliedTo` where computing type parameters do not force
+   *  the typeconstructor. Instead, if the type constructor is completing, we make
+   *  up hk type parameters matching the arguments. This is needed when unpickling
+   *  Scala2 files such as `scala.collection.generic.Mapfactory`.
+   */
+  final def safeAppliedTo(args: List[Type])(implicit ctx: Context) = {
+    val safeTypeParams = self match {
+      case self: TypeRef if !self.symbol.isClass && self.symbol.isCompleting =>
+        // This happens when unpickling e.g. scala$collection$generic$GenMapFactory$$CC
+        ctx.warning(i"encountered F-bounded higher-kinded type parameters for ${self.symbol}; assuming they are invariant")
+        defn.LambdaTrait(args map alwaysZero).typeParams
+      case _ =>
+        typeParams
+    }
+    appliedTo(args, safeTypeParams)
+  }
+
   /** Turn this type, which is used as an argument for
    *  type parameter `tparam`, into a TypeBounds RHS
    */
@@ -281,7 +504,7 @@ class TypeApplications(val self: Type) extends AnyVal {
       else TypeAlias(self, v)
   }
 
-  /** The type arguments of this type's base type instance wrt.`base`.
+  /** The type arguments of this type's base type instance wrt. `base`.
    *  Existential types in arguments are returned as TypeBounds instances.
    */
   final def baseArgInfos(base: Symbol)(implicit ctx: Context): List[Type] =
@@ -370,26 +593,9 @@ class TypeApplications(val self: Type) extends AnyVal {
    *  otherwise return Nil.
    *  Existential types in arguments are returned as TypeBounds instances.
    */
-  final def argInfos(implicit ctx: Context): List[Type] = {
-    var tparams: List[TypeSymbol] = null
-    def recur(tp: Type, refineCount: Int): mutable.ListBuffer[Type] = tp.stripTypeVar match {
-      case tp @ RefinedType(tycon, name) =>
-        val buf = recur(tycon, refineCount + 1)
-        if (buf == null) null
-        else {
-          if (tparams == null) tparams = tycon.typeParams
-          if (buf.size < tparams.length) {
-            val tparam = tparams(buf.size)
-            if (name == tparam.name) buf += tp.refinedInfo.argInfo
-            else null
-          } else null
-        }
-      case _ =>
-        if (refineCount == 0) null
-        else new mutable.ListBuffer[Type]
-    }
-    val buf = recur(self, 0)
-    if (buf == null) Nil else buf.toList
+  final def argInfos(implicit ctx: Context): List[Type] = self match {
+    case AppliedType(tycon, args) => args
+    case _ => Nil
   }
 
   /** Argument types where existential types in arguments are disallowed */
@@ -412,6 +618,11 @@ class TypeApplications(val self: Type) extends AnyVal {
       self
   }
 
+  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.
    */
@@ -421,6 +632,12 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => NoType
   }
 
+  /** If this is a type alias, its underlying type, otherwise the type itself */
+  def dropAlias(implicit ctx: Context): Type = self match {
+    case TypeAlias(alias) => alias
+    case _ => self
+  }
+
   /** The element type of a sequence or array */
   def elemType(implicit ctx: Context): Type = self match {
     case defn.ArrayOf(elemtp) => elemtp
@@ -428,6 +645,9 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => firstBaseArgInfo(defn.SeqClass)
   }
 
+  /** Does this type contain RefinedThis type with `target` as its underling
+   *  refinement type?
+   */
   def containsRefinedThis(target: Type)(implicit ctx: Context): Boolean = {
     def recur(tp: Type): Boolean = tp.stripTypeVar match {
       case RefinedThis(tp) =>
@@ -451,201 +671,4 @@ class TypeApplications(val self: Type) extends AnyVal {
     }
     recur(self)
   }
-
-  /** The typed lambda abstraction of this type `T` relative to `boundSyms`.
-   *  This is:
-   *
-   *      LambdaXYZ{ bounds }{ type Apply = toHK(T) }
-   *
-   *  where
-   *   - XYZ reflects the variances of the bound symbols,
-   *   - `bounds` consists of type declarations `type hk$i >: toHK(L) <: toHK(U),
-   *     one for each type parameter in `T` with non-trivial bounds L,U.
-   *   - `toHK` is a substitution that replaces every bound symbol sym_i by
-   *     `this.hk$i`.
-   *
-   *  TypeBounds are lambda abstracting by lambda abstracting their upper bound.
-   *
-   *  @param cycleParanoid   If `true` don't force denotation of a TypeRef unless
-   *                         its name matches one of `boundSyms`. Needed to avoid cycles
-   *                         involving F-boundes hk-types when reading Scala2 collection classes
-   *                         with new hk-scheme.
-   */
-  def LambdaAbstract(boundSyms: List[Symbol], cycleParanoid: Boolean = false)(implicit ctx: Context): Type = {
-    def expand(tp: Type): Type = {
-      val lambda = defn.LambdaTrait(boundSyms.map(_.variance))
-      def toHK(tp: Type) = (rt: RefinedType) => {
-        val argRefs = boundSyms.indices.toList.map(i =>
-          RefinedThis(rt).select(tpnme.LambdaArgName(i)))
-        val substituted =
-          if (cycleParanoid) new ctx.SafeSubstMap(boundSyms, argRefs).apply(tp)
-          else tp.subst(boundSyms, argRefs)
-        substituted.bounds.withVariance(1)
-      }
-      val boundNames = new mutable.ListBuffer[Name]
-      val boundss = new mutable.ListBuffer[TypeBounds]
-      for (sym <- boundSyms) {
-        val bounds = sym.info.bounds
-        if (!(TypeBounds.empty frozen_<:< bounds)) {
-          boundNames += sym.name
-          boundss += bounds
-        }
-      }
-      val lambdaWithBounds =
-        RefinedType.make(lambda.typeRef, boundNames.toList, boundss.toList.map(toHK))
-      RefinedType(lambdaWithBounds, tpnme.hkApply, toHK(tp))
-    }
-    self match {
-      case self @ TypeBounds(lo, hi) =>
-        self.derivedTypeBounds(lo, expand(TypeBounds.upper(hi)))
-      case _ =>
-        expand(self)
-    }
-  }
-
-  /** Convert a type constructor `TC` which has type parameters `T1, ..., Tn`
-   *  in a context where type parameters `U1,...,Un` are expected to
-   *
-   *     LambdaXYZ { Apply = TC[hk$0, ..., hk$n] }
-   *
-   *  Here, XYZ corresponds to the variances of
-   *   - `U1,...,Un` if the variances of `T1,...,Tn` are pairwise compatible with `U1,...,Un`,
-   *   - `T1,...,Tn` otherwise.
-   *  v1 is compatible with v2, if v1 = v2 or v2 is non-variant.
-   */
-  def EtaExpand(tparams: List[Symbol])(implicit ctx: Context): Type = {
-    def varianceCompatible(actual: Symbol, formal: Symbol) =
-      formal.variance == 0 || actual.variance == formal.variance
-    val tparamsToUse =
-      if (typeParams.corresponds(tparams)(varianceCompatible)) tparams else typeParams
-    self.appliedTo(tparams map (_.typeRef)).LambdaAbstract(tparamsToUse)
-      //.ensuring(res => res.EtaReduce =:= self, s"res = $res, core = ${res.EtaReduce}, self = $self, hc = ${res.hashCode}")
-  }
-
-  /** Eta expand if `bound` is a higher-kinded type */
-  def EtaExpandIfHK(bound: Type)(implicit ctx: Context): Type =
-    if (bound.isHK && !isHK && self.typeSymbol.isClass && typeParams.nonEmpty) EtaExpand(bound.typeParams)
-    else self
-
-  /** Eta expand the prefix in front of any refinements. */
-  def EtaExpandCore(implicit ctx: Context): Type = self.stripTypeVar match {
-    case self: RefinedType =>
-      self.derivedRefinedType(self.parent.EtaExpandCore, self.refinedName, self.refinedInfo)
-    case _ =>
-      self.EtaExpand(self.typeParams)
-  }
-
-  /** If `self` is a (potentially partially instantiated) eta expansion of type T, return T,
-   *  otherwise NoType. More precisely if `self` is of the form
-   *
-   *    T { type $apply = U[T1, ..., Tn] }
-   *
-   *  where
-   *
-   *   - hk$0, ..., hk${m-1} are the type parameters of T
-   *   - a sublist of the arguments Ti_k (k = 0,...,m_1) are of the form T{...}.this.hk$i_k
-   *
-   *  rewrite `self` to
-   *
-   *    U[T'1,...T'j]
-   *
-   *   where
-   *
-   *      T'j = _ >: Lj <: Uj   if j is in the i_k list defined above
-   *                            where Lj and Uj are the bounds of hk$j mapped using `fromHK`.
-   *          = fromHK(Tj)   otherwise.
-   *
-   *   `fromHK` is the function that replaces every occurrence of `<self>.this.hk$i` by the
-   *   corresponding parameter reference in `U[T'1,...T'j]`
-   */
-  def EtaReduce(implicit ctx: Context): Type = {
-    def etaCore(tp: Type, tparams: List[Symbol]): Type = tparams match {
-      case Nil => tp
-      case tparam :: otherParams =>
-        tp match {
-          case tp: RefinedType =>
-            tp.refinedInfo match {
-              case TypeAlias(TypeRef(RefinedThis(rt), rname))
-              if (rname == tparam.name) && (rt eq self) =>
-                // we have a binding T = Lambda$XYZ{...}.this.hk$i where hk$i names the current `tparam`.
-                val pcore = etaCore(tp.parent, otherParams)
-                val hkBounds = self.member(rname).info.bounds
-                if (TypeBounds.empty frozen_<:< hkBounds) pcore
-                else tp.derivedRefinedType(pcore, tp.refinedName, hkBounds)
-              case _ =>
-                val pcore = etaCore(tp.parent, tparams)
-                if (pcore.exists) tp.derivedRefinedType(pcore, tp.refinedName, tp.refinedInfo)
-                else NoType
-            }
-          case _ =>
-            NoType
-        }
-    }
-    // Map references `Lambda$XYZ{...}.this.hk$i to corresponding parameter references of the reduced core.
-    def fromHK(reduced: Type) = reduced match {
-      case reduced: RefinedType =>
-        new TypeMap {
-          def apply(tp: Type): Type = tp match {
-            case TypeRef(RefinedThis(binder), name) if binder eq self =>
-              assert(name.isLambdaArgName)
-              RefinedThis(reduced).select(reduced.typeParams.apply(name.LambdaArgIndex))
-            case _ =>
-              mapOver(tp)
-          }
-        }.apply(reduced)
-      case _ =>
-        reduced
-    }
-
-    self match {
-      case self @ RefinedType(parent, tpnme.hkApply) =>
-        val lc = parent.LambdaClass(forcing = false)
-        self.refinedInfo match {
-          case TypeAlias(alias) if lc.exists =>
-            fromHK(etaCore(alias, lc.typeParams.reverse))
-          case _ => NoType
-        }
-      case _ => NoType
-    }
-  }
-
-  /** Test whether this type has a base type of the form `B[T1, ..., Tn]` where
-   *  the type parameters of `B` match one-by-one the variances of `tparams`,
-   *  and where the lambda abstracted type
-   *
-   *     LambdaXYZ { type Apply = B[hk$0, ..., hk${n-1}] }
-   *               { type hk$0 = T1; ...; type hk${n-1} = Tn }
-   *
-   *  satisfies predicate `p`. Try base types in the order of their occurrence in `baseClasses`.
-   *  A type parameter matches a variance V if it has V as its variance or if V == 0.
-   *  @param classBounds  A hint to bound the search. Only types that derive from one of the
-   *                      classes in classBounds are considered.
-   */
-  def testLifted(tparams: List[Symbol], p: Type => Boolean, classBounds: List[ClassSymbol])(implicit ctx: Context): Boolean = {
-    def tryLift(bcs: List[ClassSymbol]): Boolean = bcs match {
-      case bc :: bcs1 =>
-        val tp = self.baseTypeWithArgs(bc)
-        val targs = tp.argInfos
-        val tycon = tp.withoutArgs(targs)
-        def variancesMatch(param1: Symbol, param2: Symbol) =
-          param2.variance == param2.variance || param2.variance == 0
-        if (classBounds.exists(tycon.derivesFrom(_)) &&
-            tycon.typeParams.corresponds(tparams)(variancesMatch)) {
-          val expanded = tycon.EtaExpand(tparams)
-          val lifted = (expanded /: targs) { (partialInst, targ) =>
-            val tparam = partialInst.typeParams.head
-            RefinedType(partialInst, tparam.name, targ.bounds.withVariance(tparam.variance))
-          }
-          ctx.traceIndented(i"eta lifting $self --> $lifted", hk) {
-            p(lifted) || tryLift(bcs1)
-          }
-        }
-        else tryLift(bcs1)
-      case nil =>
-        false
-    }
-    tparams.nonEmpty &&
-      (typeParams.hasSameLengthAs(tparams) && p(EtaExpand(tparams)) ||
-       classBounds.nonEmpty && tryLift(self.baseClasses))
-  }
 }
diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index 1d9928e2d..163fa4919 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -12,6 +12,7 @@ import util.{Stats, DotClass, SimpleMap}
 import config.Config
 import config.Printers._
 import TypeErasure.{erasedLub, erasedGlb}
+import TypeApplications._
 import scala.util.control.NonFatal
 
 /** Provides methods to compare types.
@@ -143,27 +144,46 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       def compareNamed(tp1: Type, tp2: NamedType): Boolean = {
         implicit val ctx: Context = this.ctx
         tp2.info match {
-          case info2: TypeAlias => firstTry(tp1, info2.alias)
+          case info2: TypeAlias if tp2.prefix.isStable =>
+            // If prefix is not stable (i.e. is not a path), then we have a true
+            // projection `T # A` which is treated as the existential type
+            // `ex(x: T)x.A`. We need to deal with the existential first before
+            // following the alias. If we did follow the alias we could be
+            // unsound as well as incomplete. An example of this was discovered in Iter2.scala.
+            // It failed to validate the subtype test
+            //
+            //   (([+X] -> Seq[X]) & C)[SA] <: C[SA]
+            //
+            // Both sides are projections of $Apply. The left $Apply does have an
+            // aliased info, namely, Seq[SA]. But that is not a subtype of C[SA].
+            // The problem is that, with the prefix not being a path, an aliased info
+            // does not necessarily give all of the information of the original projection.
+            // So we can't follow the alias without a backup strategy. If the alias
+            // would appear on the right then I believe this can be turned into a case
+            // of unsoundness.
+            isSubType(tp1, info2.alias)
           case _ => tp1 match {
             case tp1: NamedType =>
               tp1.info match {
-                case info1: TypeAlias => compareNamed(info1.alias, tp2)
+                case info1: TypeAlias if tp1.prefix.isStable =>
+                  isSubType(info1.alias, tp2)
                 case _ =>
                   val sym1 = tp1.symbol
-                  (if ((sym1 ne NoSymbol) && (sym1 eq tp2.symbol))
-                     ctx.erasedTypes ||
-                     sym1.isStaticOwner ||
-                     isSubType(tp1.prefix, tp2.prefix) ||
-                     thirdTryNamed(tp1, tp2)
-                   else
-                     (tp1.name eq tp2.name) &&
-                     isSubType(tp1.prefix, tp2.prefix) &&
-                     (tp1.signature == tp2.signature) &&
-                     !tp1.isInstanceOf[WithFixedSym] &&
-                     !tp2.isInstanceOf[WithFixedSym] ||
-                     compareHK(tp1, tp2, inOrder = true) ||
-                     compareHK(tp2, tp1, inOrder = false) ||
-                     thirdTryNamed(tp1, tp2))
+                  if ((sym1 ne NoSymbol) && (sym1 eq tp2.symbol))
+                    ctx.erasedTypes ||
+                    sym1.isStaticOwner ||
+                    isSubType(tp1.prefix, tp2.prefix) ||
+                    thirdTryNamed(tp1, tp2)
+                  else
+                    (  (tp1.name eq tp2.name)
+                    && isSubType(tp1.prefix, tp2.prefix)
+                    && tp1.signature == tp2.signature
+                    && !tp1.isInstanceOf[WithFixedSym]
+                    && !tp2.isInstanceOf[WithFixedSym]
+                    ) ||
+                    compareHK(tp1, tp2, inOrder = true) ||
+                    compareHK(tp2, tp1, inOrder = false) ||
+                    thirdTryNamed(tp1, tp2)
               }
             case _ =>
               compareHK(tp2, tp1, inOrder = false) ||
@@ -239,9 +259,9 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case tp1: NamedType =>
       tp1.info match {
         case info1: TypeAlias => isSubType(info1.alias, tp2)
-        case _ => compareHK(tp1, tp2, inOrder = true) || thirdTry(tp1, tp2)
-          // Note: If we change the order here, doing compareHK first and following aliases second,
-          // we get a -Ycheck error when compiling dotc/transform. Need to investigate.
+        case _ =>
+          compareHK(tp1, tp2, inOrder = true) ||
+          thirdTry(tp1, tp2)
       }
     case tp1: PolyParam =>
       def flagNothingBound = {
@@ -336,21 +356,23 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         isSubType(tp1, tp2.parent) &&
           (name2 == nme.WILDCARD || hasMatchingMember(name2, tp1, tp2))
       }
+      def etaExpandedSubType(tp1: Type) =
+        isSubType(tp1.typeConstructor.EtaExpand(tp2.typeParams), tp2)
       def compareRefined: Boolean = {
         val tp1w = tp1.widen
         val skipped2 = skipMatching(tp1w, tp2)
-        if ((skipped2 eq tp2) || !Config.fastPathForRefinedSubtype) {
-          val normalPath = tp1 match {
+        if ((skipped2 eq tp2) || !Config.fastPathForRefinedSubtype)
+          tp1 match {
             case tp1: AndType =>
               // Delay calling `compareRefinedSlow` because looking up a member
               // of an `AndType` can lead to a cascade of subtyping checks
+              // This twist is needed to make collection/generic/ParFactory.scala compile
               fourthTry(tp1, tp2) || compareRefinedSlow
             case _ =>
-              compareRefinedSlow || fourthTry(tp1, tp2)
+              compareRefinedSlow ||
+              fourthTry(tp1, tp2) ||
+              needsEtaLift(tp1, tp2) && testLifted(tp1, tp2, tp2.typeParams, etaExpandedSubType)
           }
-          normalPath ||
-            needsEtaLift(tp1, tp2) && tp1.testLifted(tp2.typeParams, isSubType(_, tp2), classBounds(tp2))
-        }
         else // fast path, in particular for refinements resulting from parameterization.
           isSubType(tp1, skipped2) &&
           isSubRefinements(tp1w.asInstanceOf[RefinedType], tp2, skipped2)
@@ -412,8 +434,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           (tp2.variance > 0 && tp1.variance >= 0 || (lo2 eq NothingType) || isSubType(lo2, lo1)) &&
           (tp2.variance < 0 && tp1.variance <= 0 || (hi2 eq AnyType) || isSubType(hi1, hi2))
         case tp1: ClassInfo =>
-          val tt = tp1.typeRef
-          isSubType(lo2, tt) && isSubType(tt, hi2)
+          tp2 contains tp1
         case _ =>
           false
       }
@@ -471,7 +492,9 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       isNewSubType(tp1.underlying.widenExpr, tp2) || comparePaths
     case tp1: RefinedType =>
        isNewSubType(tp1.parent, tp2) ||
-         needsEtaLift(tp2, tp1) && tp2.testLifted(tp1.typeParams, isSubType(tp1, _), Nil)
+         needsEtaLift(tp2, tp1) &&
+         tp2.typeParams.length == tp1.typeParams.length &&
+         isSubType(tp1, tp2.EtaExpand(tp1.typeParams))
     case AndType(tp11, tp12) =>
       // Rewrite (T111 | T112) & T12 <: T2 to (T111 & T12) <: T2 and (T112 | T12) <: T2
       // and analogously for T11 & (T121 | T122) & T12 <: T2
@@ -502,19 +525,89 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       false
   }
 
-  /** If `projection` is a hk projection T#$apply
-   *  and `other` is not a hk projection, then convert `other` to a hk projection `U`, and
-   *  continue with `T <:< U` if `inOrder` is true and `U <:< T` otherwise.
+  /** Does `tp` need to be eta lifted to be comparable to `target`?
+   *  This is the case if:
+   *   - target is a type lambda, and
+   *   - `tp` is eta-expandable (i.e. is a non-lambda class ref)
    */
-  def compareHK(projection: NamedType, other: Type, inOrder: Boolean) =
-    projection.name == tpnme.hkApply &&
-    !other.isHKApply &&
-    other.testLifted(projection.prefix.LambdaClass(forcing = true).typeParams,
-      if (inOrder) isSubType(projection.prefix, _) else isSubType(_, projection.prefix),
-      if (inOrder) Nil else classBounds(projection.prefix))
+  private def needsEtaLift(tp: Type, target: RefinedType): Boolean =
+    target.refinedName == tpnme.hkApply && tp.isEtaExpandable
 
-  /** The class symbols bounding the type of the `Apply` member of `tp` */
-  private def classBounds(tp: Type) = tp.member(tpnme.hkApply).info.classSymbols
+  /** Test whether `tp1` has a base type of the form `B[T1, ..., Tn]` where
+   *   - `B` derives from one of the class symbols of `tp2`,
+   *   - the type parameters of `B` match one-by-one the variances of `tparams`,
+   *   - `B` satisfies predicate `p`.
+   */
+  private def testLifted(tp1: Type, tp2: Type, tparams: List[TypeSymbol], p: Type => Boolean): Boolean = {
+    val classBounds = tp2.member(tpnme.hkApply).info.classSymbols
+    def recur(bcs: List[ClassSymbol]): Boolean = bcs match {
+      case bc :: bcs1 =>
+        val baseRef = tp1.baseTypeRef(bc)
+        (classBounds.exists(bc.derivesFrom) &&
+         variancesConform(baseRef.typeParams, tparams) &&
+         p(baseRef.appliedTo(tp1.baseArgInfos(bc)))
+         ||
+         recur(bcs1))
+      case nil =>
+        false
+    }
+    recur(tp1.baseClasses)
+  }
+
+  /** If `projection` is a hk projection T#$apply with a constrainable poly param
+   *  as type constructor and `other` is not a hk projection, then perform the following
+   *  steps:
+   *
+   *   (1) If not `inOrder` then perform the next steps until they all succeed
+   *       for each base type of other which
+   *        - derives from a class bound of `projection`,
+   *        - has the same number of type parameters than `projection`
+   *        - has type parameter variances which conform to those of `projection`.
+   *       If `inOrder` then perform the same steps on the original `other` type.
+   *
+   *   (2) Try to eta expand the constructor of `other`.
+   *
+   *   (3a) In mode `TypevarsMissConetxt` replace the projection's hk constructor parameter
+   *        by the eta expansion of step (2) reapplied to the projection's arguments.
+   *   (3b) In normal mode, try to unify the projection's hk constructor parameter with
+   *        the eta expansion of step(2)
+   *
+   *   (4) If `inOrder`, test `projection <: other` else test `other <: projection`.
+   */
+  def compareHK(projection: NamedType, other: Type, inOrder: Boolean): Boolean = {
+    def tryInfer(tp: Type): Boolean = ctx.traceIndented(i"compareHK($projection, $other, inOrder = $inOrder, constr = $tp)", subtyping) {
+      tp match {
+        case tp: TypeVar => tryInfer(tp.underlying)
+        case param: PolyParam if canConstrain(param) =>
+
+          def unifyWith(liftedOther: Type): Boolean = {
+            subtyping.println(i"unify with $liftedOther")
+            liftedOther.typeConstructor.widen match {
+              case tycon: TypeRef if tycon.isEtaExpandable && tycon.typeParams.nonEmpty =>
+                val (ok, projection1) =
+                  if (ctx.mode.is(Mode.TypevarsMissContext))
+                    (true, EtaExpansion(tycon).appliedTo(projection.argInfos))
+                  else
+                    (tryInstantiate(param, EtaExpansion(tycon)), projection)
+                ok &&
+                (if (inOrder) isSubType(projection1, other) else isSubType(other, projection1))
+              case _ =>
+                false
+            }
+          }
+          val hkTypeParams = param.typeParams
+          subtyping.println(i"classBounds = ${projection.prefix.member(tpnme.hkApply).info.classSymbols}")
+          subtyping.println(i"base classes = ${other.baseClasses}")
+          subtyping.println(i"type params = $hkTypeParams")
+          if (inOrder) unifyWith(other)
+          else testLifted(other, projection.prefix, hkTypeParams, unifyWith)
+        case _ =>
+          false
+      }
+    }
+    projection.name == tpnme.hkApply && !other.isHKApply &&
+    tryInfer(projection.prefix.typeConstructor.dealias)
+  }
 
   /** Returns true iff either `tp11 <:< tp21` or `tp12 <:< tp22`, trying at the same time
    *  to keep the constraint as wide as possible. Specifically, if
@@ -627,11 +720,19 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
    *       Further, no refinement refers back to the refined type via a refined this.
    *  The precondition is established by `skipMatching`.
    */
-  private def isSubRefinements(tp1: RefinedType, tp2: RefinedType, limit: Type): Boolean =
-    isSubType(tp1.refinedInfo, tp2.refinedInfo) && (
+  private def isSubRefinements(tp1: RefinedType, tp2: RefinedType, limit: Type): Boolean = {
+    def hasSubRefinement(tp1: RefinedType, refine2: Type): Boolean = {
+      isSubType(tp1.refinedInfo, refine2) || {
+        // last effort: try to adapt variances of higher-kinded types if this is sound.
+        val adapted2 = refine2.adaptHkVariances(tp1.parent.member(tp1.refinedName).symbol.info)
+        adapted2.ne(refine2) && hasSubRefinement(tp1, adapted2)
+      }
+    }
+    hasSubRefinement(tp1, tp2.refinedInfo) && (
       (tp2.parent eq limit) ||
       isSubRefinements(
         tp1.parent.asInstanceOf[RefinedType], tp2.parent.asInstanceOf[RefinedType], limit))
+  }
 
   /** A type has been covered previously in subtype checking if it
    *  is some combination of TypeRefs that point to classes, where the
@@ -665,14 +766,6 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case _ => false
   }
 
-  /** Does `tp` need to be eta lifted to be comparable to `target`? */
-  private def needsEtaLift(tp: Type, target: RefinedType): Boolean = {
-    // if (tp.isLambda != tp.isHK) println(i"discrepancy for $tp, isLambda = ${tp.isLambda}, isHK = ${tp.isHK}")
-    val name = target.refinedName
-    (name.isLambdaArgName || (name eq tpnme.hkApply)) &&
-    tp.exists && !tp.isLambda // we do encounter Lambda classes without any arguments here
-  }
-
   /** Narrow gadt.bounds for the type parameter referenced by `tr` to include
    *  `bound` as an upper or lower bound (which depends on `isUpper`).
    *  Test that the resulting bounds are still satisfiable.
@@ -969,13 +1062,13 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case tp1: TypeBounds =>
       tp2 match {
         case tp2: TypeBounds => tp1 & tp2
-        case tp2: ClassInfo if tp1 contains tp2.typeRef => tp2
+        case tp2: ClassInfo if tp1 contains tp2 => tp2
         case _ => mergeConflict(tp1, tp2)
       }
     case tp1: ClassInfo =>
       tp2 match {
         case tp2: ClassInfo if tp1.cls eq tp2.cls => tp1.derivedClassInfo(tp1.prefix & tp2.prefix)
-        case tp2: TypeBounds if tp2 contains tp1.typeRef => tp1
+        case tp2: TypeBounds if tp2 contains tp1 => tp1
         case _ => mergeConflict(tp1, tp2)
       }
     case tp1 @ MethodType(names1, formals1) =>
@@ -1028,13 +1121,13 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case tp1: TypeBounds =>
       tp2 match {
         case tp2: TypeBounds => tp1 | tp2
-        case tp2: ClassInfo if tp1 contains tp2.typeRef => tp1
+        case tp2: ClassInfo if tp1 contains tp2 => tp1
         case _ => mergeConflict(tp1, tp2)
       }
     case tp1: ClassInfo =>
       tp2 match {
         case tp2: ClassInfo if tp1.cls eq tp2.cls => tp1.derivedClassInfo(tp1.prefix | tp2.prefix)
-        case tp2: TypeBounds if tp2 contains tp1.typeRef => tp2
+        case tp2: TypeBounds if tp2 contains tp1 => tp2
         case _ => mergeConflict(tp1, tp2)
       }
     case tp1 @ MethodType(names1, formals1) =>
@@ -1074,7 +1167,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       case bounds: TypeBounds => i"type bounds $bounds"
       case _ => tp.show
     }
-    throw new MergeError(s"cannot merge ${showType(tp1)} with ${showType(tp2)}")
+    throw new MergeError(s"cannot merge ${showType(tp1)} with ${showType(tp2)}", tp1, tp2)
   }
 
   /** Merge two lists of names. If names in corresponding positions match, keep them,
diff --git a/src/dotty/tools/dotc/core/TypeOps.scala b/src/dotty/tools/dotc/core/TypeOps.scala
index f3884e11a..3dfe698f0 100644
--- a/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/src/dotty/tools/dotc/core/TypeOps.scala
@@ -61,18 +61,19 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
     def toPrefix(pre: Type, cls: Symbol, thiscls: ClassSymbol): Type = /*>|>*/ ctx.conditionalTraceIndented(TypeOps.track, s"toPrefix($pre, $cls, $thiscls)") /*<|<*/ {
       if ((pre eq NoType) || (pre eq NoPrefix) || (cls is PackageClass))
         tp
-      else if (thiscls.derivesFrom(cls) && pre.baseTypeRef(thiscls).exists) {
-        if (theMap != null && theMap.currentVariance <= 0 && !isLegalPrefix(pre))
-          theMap.unstable = true
-        pre match {
-          case SuperType(thispre, _) => thispre
-          case _ => pre
-        }
+      else pre match {
+        case pre: SuperType => toPrefix(pre.thistpe, cls, thiscls)
+        case _ =>
+          if (thiscls.derivesFrom(cls) && pre.baseTypeRef(thiscls).exists) {
+            if (theMap != null && theMap.currentVariance <= 0 && !isLegalPrefix(pre))
+              theMap.unstable = true
+            pre
+          }
+          else if ((pre.termSymbol is Package) && !(thiscls is Package))
+            toPrefix(pre.select(nme.PACKAGE), cls, thiscls)
+          else
+            toPrefix(pre.baseTypeRef(cls).normalizedPrefix, cls.owner, thiscls)
       }
-      else if ((pre.termSymbol is Package) && !(thiscls is Package))
-        toPrefix(pre.select(nme.PACKAGE), cls, thiscls)
-      else
-        toPrefix(pre.baseTypeRef(cls).normalizedPrefix, cls.owner, thiscls)
     }
 
     /*>|>*/ ctx.conditionalTraceIndented(TypeOps.track, s"asSeen ${tp.show} from (${pre.show}, ${cls.show})", show = true) /*<|<*/ { // !!! DEBUG
@@ -384,10 +385,9 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
         denot.info = info
       }
     }
-    val typeArgFlag = if (formal is Local) TypeArgument else EmptyFlags
     val sym = ctx.newSymbol(
       cls, formal.name,
-      formal.flagsUNSAFE & RetainedTypeArgFlags | typeArgFlag | Override,
+      formal.flagsUNSAFE & RetainedTypeArgFlags | BaseTypeArg | Override,
       lazyInfo,
       coord = cls.coord)
     cls.enter(sym, decls)
@@ -463,14 +463,20 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
       case to @ TypeBounds(lo1, hi1) if lo1 eq hi1 =>
         for (pref <- prefs)
           for (argSym <- pref.decls)
-            if (argSym is TypeArgument)
+            if (argSym is BaseTypeArg)
               forwardRef(argSym, from, to, cls, decls)
       case _ =>
     }
 
     // println(s"normalizing $parents of $cls in ${cls.owner}") // !!! DEBUG
+
+    // A map consolidating all refinements arising from parent type parameters
     var refinements: SimpleMap[TypeName, Type] = SimpleMap.Empty
-    var formals: SimpleMap[TypeName, Symbol] = SimpleMap.Empty
+
+    // A map of all formal type parameters of base classes that get refined
+    var formals: SimpleMap[TypeName, Symbol] = SimpleMap.Empty // A map of all formal parent parameter
+
+    // Strip all refinements from parent type, populating `refinements` and `formals` maps.
     def normalizeToRef(tp: Type): TypeRef = tp.dealias match {
       case tp: TypeRef =>
         tp
@@ -488,13 +494,17 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
         throw new TypeError(s"unexpected parent type: $tp")
     }
     val parentRefs = parents map normalizeToRef
+
+    // Enter all refinements into current scope.
     refinements foreachBinding { (name, refinedInfo) =>
       assert(decls.lookup(name) == NoSymbol, // DEBUG
         s"redefinition of ${decls.lookup(name).debugString} in ${cls.showLocated}")
       enterArgBinding(formals(name), refinedInfo, cls, decls)
     }
-    // These two loops cannot be fused because second loop assumes that
-    // all arguments have been entered in `decls`.
+    // Forward definitions in super classes that have one of the refined paramters
+    // as aliases directly to the refined info.
+    // Note that this cannot be fused bwith the previous loop because we now
+    // assume that all arguments have been entered in `decls`.
     refinements foreachBinding { (name, refinedInfo) =>
       forwardRefs(formals(name), refinedInfo, parentRefs)
     }
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 2558dcbb7..21b74e07b 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -100,7 +100,7 @@ object Types {
       case _ => false
     }
 
-    /** Is this type a (possibly aliased and/or partially applied) type reference
+    /** Is this type a (possibly refined or applied or aliased) type reference
      *  to the given type symbol?
      *  @sym  The symbol to compare to. It must be a class symbol or abstract type.
      *        It makes no sense for it to be an alias type because isRef would always
@@ -113,8 +113,7 @@ object Types {
           case _ =>  this1.symbol eq sym
         }
       case this1: RefinedType =>
-        // make sure all refinements are type arguments
-        this1.parent.isRef(sym) && this.argInfos.nonEmpty
+        this1.parent.isRef(sym)
       case _ =>
         false
     }
@@ -142,20 +141,23 @@ object Types {
     }
 
     /** Is this type an instance of a non-bottom subclass of the given class `cls`? */
-    final def derivesFrom(cls: Symbol)(implicit ctx: Context): Boolean = this match {
-      case tp: TypeRef =>
-        val sym = tp.symbol
-        if (sym.isClass) sym.derivesFrom(cls) else tp.underlying.derivesFrom(cls)
-      case tp: TypeProxy =>
-        tp.underlying.derivesFrom(cls)
-      case tp: AndType =>
-        tp.tp1.derivesFrom(cls) || tp.tp2.derivesFrom(cls)
-      case tp: OrType =>
-        tp.tp1.derivesFrom(cls) && tp.tp2.derivesFrom(cls)
-      case tp: JavaArrayType =>
-        cls == defn.ObjectClass
-      case _ =>
-        false
+    final def derivesFrom(cls: Symbol)(implicit ctx: Context): Boolean = {
+      def loop(tp: Type) = tp match {
+        case tp: TypeRef =>
+          val sym = tp.symbol
+          if (sym.isClass) sym.derivesFrom(cls) else tp.underlying.derivesFrom(cls)
+        case tp: TypeProxy =>
+          tp.underlying.derivesFrom(cls)
+        case tp: AndType =>
+          tp.tp1.derivesFrom(cls) || tp.tp2.derivesFrom(cls)
+        case tp: OrType =>
+          tp.tp1.derivesFrom(cls) && tp.tp2.derivesFrom(cls)
+        case tp: JavaArrayType =>
+          cls == defn.ObjectClass
+        case _ =>
+          false
+      }
+      cls == defn.AnyClass || loop(this)
     }
 
     /** Is this type guaranteed not to have `null` as a value?
@@ -448,7 +450,18 @@ object Types {
             if (rinfo.isAlias) rinfo
             else if (pdenot.info.isAlias) pdenot.info
             else if (ctx.pendingMemberSearches.contains(name)) safeAnd(pdenot.info, rinfo)
-            else pdenot.info & rinfo
+            else
+              try pdenot.info & rinfo
+              catch {
+                case ex: CyclicReference =>
+                  // happens for tests/pos/sets.scala. findMember is called from baseTypeRef.
+                  // The & causes a subtype check which calls baseTypeRef again with the same
+                  // superclass. In the observed case, the superclass was Any, and
+                  // the special shortcut for Any in derivesFrom was as yet absent. To reproduce,
+                  // remove the special treatment of Any in derivesFrom and compile
+                  // sets.scala.
+                  safeAnd(pdenot.info, rinfo)
+              }
           pdenot.asSingleDenotation.derivedSingleDenotation(pdenot.symbol, jointInfo)
         } else
           pdenot & (new JointRefDenotation(NoSymbol, rinfo, Period.allInRun(ctx.runId)), pre)
@@ -856,7 +869,7 @@ object Types {
           object instantiate extends TypeMap {
             var isSafe = true
             def apply(tp: Type): Type = tp match {
-              case TypeRef(RefinedThis(`pre`), name) if name.isLambdaArgName =>
+              case TypeRef(RefinedThis(`pre`), name) if name.isHkArgName =>
                 member(name).info match {
                   case TypeAlias(alias) => alias
                   case _ => isSafe = false; tp
@@ -869,7 +882,7 @@ object Types {
             }
           }
           def instArg(tp: Type): Type = tp match {
-            case tp @ TypeAlias(TypeRef(RefinedThis(`pre`), name)) if name.isLambdaArgName =>
+            case tp @ TypeAlias(TypeRef(RefinedThis(`pre`), name)) if name.isHkArgName =>
               member(name).info match {
                 case TypeAlias(alias) => tp.derivedTypeAlias(alias) // needed to keep variance
                 case bounds => bounds
@@ -884,19 +897,17 @@ object Types {
               instantiate(tp)
           }
           /** Reduce rhs of $hkApply to make it stand alone */
-          def betaReduce(tp: Type) =
-            if (pre.parent.isSafeLambda) {
-              val reduced = instTop(tp)
-              if (instantiate.isSafe) reduced else NoType
-            }
-            else NoType
+          def betaReduce(tp: Type) = {
+            val reduced = instTop(tp)
+            if (instantiate.isSafe) reduced else NoType
+          }
           pre.refinedInfo match {
             case TypeAlias(alias) =>
               if (pre.refinedName ne name) loop(pre.parent)
               else if (!pre.refinementRefersToThis) alias
               else alias match {
                 case TypeRef(RefinedThis(`pre`), aliasName) => lookupRefined(aliasName) // (1)
-                case _ => if (name == tpnme.hkApply) betaReduce(alias) else NoType // (2)
+                case _ => if (name == tpnme.hkApply) betaReduce(alias) else NoType // (2) // ### use TypeApplication's betaReduce
               }
             case _ => loop(pre.parent)
           }
@@ -1375,7 +1386,11 @@ object Types {
       if (owner.isTerm) d else d.asSeenFrom(prefix)
     }
 
-    private def checkSymAssign(sym: Symbol)(implicit ctx: Context) =
+    private def checkSymAssign(sym: Symbol)(implicit ctx: Context) = {
+      def selfTypeOf(sym: Symbol) = sym.owner.info match {
+        case info: ClassInfo => info.givenSelfType
+        case _ => NoType
+      }
       assert(
         (lastSymbol eq sym) ||
         (lastSymbol eq null) || {
@@ -1387,9 +1402,16 @@ object Types {
           (lastDefRunId == NoRunId)
         } ||
         (lastSymbol.infoOrCompleter == ErrorType ||
-        sym.owner.derivesFrom(lastSymbol.owner) && sym.owner != lastSymbol.owner
-      ),
-        s"data race? overwriting symbol of ${this.show} / $this / ${this.getClass} / ${lastSymbol.id} / ${sym.id} / ${sym.owner} / ${lastSymbol.owner} / ${ctx.phase} at run ${ctx.runId}")
+        sym.owner != lastSymbol.owner &&
+        (sym.owner.derivesFrom(lastSymbol.owner) ||
+         selfTypeOf(sym).derivesFrom(lastSymbol.owner) ||
+         selfTypeOf(lastSymbol).derivesFrom(sym.owner))),
+        s"""data race? overwriting symbol of type ${this.show},
+           |long form = $this of class ${this.getClass},
+           |last sym id = ${lastSymbol.id}, new sym id = ${sym.id},
+           |last owner = ${lastSymbol.owner}, new owner = ${sym.owner},
+           |period = ${ctx.phase} at run ${ctx.runId}""".stripMargin)
+    }
 
     protected def sig: Signature = Signature.NotAMethod
 
@@ -1516,7 +1538,12 @@ object Types {
       else {
         val res = prefix.lookupRefined(name)
         if (res.exists) res
-        else if (name == tpnme.hkApply && prefix.noHK) derivedSelect(prefix.EtaExpandCore)
+        else if (name == tpnme.hkApply && prefix.classNotLambda) {
+          // After substitution we might end up with a type like
+          // `C { type hk$0 = T0; ...; type hk$n = Tn } # $Apply`
+          // where C is a class. In that case we eta expand `C`.
+          derivedSelect(prefix.EtaExpandCore(this.prefix.typeConstructor.typeParams))
+        }
         else newLikeThis(prefix)
       }
 
@@ -1560,6 +1587,15 @@ object Types {
       case _ =>
         false
     }
+
+    /* A version of toString which also prints aliases. Can be used for debugging
+    override def toString =
+      if (isTerm) s"TermRef($prefix, $name)"
+      else s"TypeRef($prefix, $name)${
+        if (lastDenotation != null && lastDenotation.infoOrCompleter.isAlias)
+          s"@@@ ${lastDenotation.infoOrCompleter.asInstanceOf[TypeAlias].hi}"
+        else ""}"
+    */
   }
 
   abstract case class TermRef(override val prefix: Type, name: TermName) extends NamedType with SingletonType {
@@ -1756,8 +1792,8 @@ object Types {
 
   object TypeRef {
     def checkProjection(prefix: Type, name: TypeName)(implicit ctx: Context) =
-      if (name == tpnme.hkApply && prefix.noHK)
-        assert(false, s"bad type : $prefix.$name should not be $$applied")
+      if (name == tpnme.hkApply && prefix.classNotLambda)
+        assert(false, s"bad type : $prefix.$name does not allow $$Apply projection")
 
     /** Create type ref with given prefix and name */
     def apply(prefix: Type, name: TypeName)(implicit ctx: Context): TypeRef = {
@@ -1897,20 +1933,15 @@ object Types {
 
     override def underlying(implicit ctx: Context) = parent
 
-    private def checkInst(implicit ctx: Context): this.type = {
-      if (Config.checkLambdaVariance)
-        refinedInfo match {
-          case refinedInfo: TypeBounds if refinedInfo.variance != 0 && refinedName.isLambdaArgName =>
-            val cls = parent.LambdaClass(forcing = false)
-            if (cls.exists)
-              assert(refinedInfo.variance == cls.typeParams.apply(refinedName.LambdaArgIndex).variance,
-                  s"variance mismatch for $this, $cls, ${cls.typeParams}, ${cls.typeParams.apply(refinedName.LambdaArgIndex).variance}, ${refinedInfo.variance}")
-          case _ =>
+    private def badInst =
+      throw new AssertionError(s"bad instantiation: $this")
+
+    def checkInst(implicit ctx: Context): this.type = {
+      if (refinedName == tpnme.hkApply)
+        parent.stripTypeVar match {
+          case RefinedType(_, name) if name.isHkArgName => // ok
+          case _ => badInst
         }
-      if (Config.checkProjections &&
-          (refinedName == tpnme.hkApply || refinedName.isLambdaArgName) &&
-          parent.noHK)
-        assert(false, s"illegal refinement of first-order type: $this")
       this
     }
 
@@ -1932,7 +1963,7 @@ object Types {
         false
     }
     override def computeHash = doHash(refinedName, refinedInfo, parent)
-    override def toString = s"RefinedType($parent, $refinedName, $refinedInfo | $hashCode)" // !!! TODO: remove
+    override def toString = s"RefinedType($parent, $refinedName, $refinedInfo)"
   }
 
   class CachedRefinedType(parent: Type, refinedName: Name, infoFn: RefinedType => Type) extends RefinedType(parent, refinedName) {
@@ -2632,11 +2663,13 @@ object Types {
       def clsDenot = if (prefix eq cls.owner.thisType) cls.denot else cls.denot.copySymDenotation(info = this)
       if (typeRefCache == null)
         typeRefCache =
-          if ((cls is PackageClass) || cls.owner.isTerm) prefix select cls
-          else prefix select (cls.name, clsDenot)
+          if ((cls is PackageClass) || cls.owner.isTerm) symbolicTypeRef
+          else TypeRef(prefix, cls.name, clsDenot)
       typeRefCache
     }
 
+    def symbolicTypeRef(implicit ctx: Context): Type = TypeRef(prefix, cls)
+
     // cached because baseType needs parents
     private var parentsCache: List[TypeRef] = null
 
@@ -2701,8 +2734,12 @@ object Types {
       case _ => this
     }
 
-    def contains(tp: Type)(implicit ctx: Context) = tp match {
+    def contains(tp: Type)(implicit ctx: Context): Boolean = tp match {
       case tp: TypeBounds => lo <:< tp.lo && tp.hi <:< hi
+      case tp: ClassInfo =>
+        // Note: Taking a normal typeRef does not work here. A normal ref might contain
+        // also other information about the named type (e.g. bounds).
+        contains(tp.symbolicTypeRef)
       case _ => lo <:< tp && tp <:< hi
     }
 
@@ -2746,9 +2783,9 @@ object Types {
 
   abstract class TypeAlias(val alias: Type, override val variance: Int) extends TypeBounds(alias, alias) {
     /** pre: this is a type alias */
-    def derivedTypeAlias(tp: Type, variance: Int = this.variance)(implicit ctx: Context) =
-      if ((lo eq tp) && (variance == this.variance)) this
-      else TypeAlias(tp, variance)
+    def derivedTypeAlias(alias: Type, variance: Int = this.variance)(implicit ctx: Context) =
+      if ((alias eq this.alias) && (variance == this.variance)) this
+      else TypeAlias(alias, variance)
 
     override def & (that: TypeBounds)(implicit ctx: Context): TypeBounds = {
       val v = this commonVariance that
@@ -3287,7 +3324,7 @@ object Types {
     }
   }
 
-  class MergeError(msg: String) extends TypeError(msg)
+  class MergeError(msg: String, val tp1: Type, val tp2: Type) extends TypeError(msg)
 
   // ----- Debug ---------------------------------------------------------
 
diff --git a/src/dotty/tools/dotc/core/tasty/TreePickler.scala b/src/dotty/tools/dotc/core/tasty/TreePickler.scala
index c0136538b..d11d6f4b7 100644
--- a/src/dotty/tools/dotc/core/tasty/TreePickler.scala
+++ b/src/dotty/tools/dotc/core/tasty/TreePickler.scala
@@ -9,6 +9,7 @@ import Contexts._, Symbols._, Types._, Names._, Constants._, Decorators._, Annot
 import collection.mutable
 import NameOps._
 import TastyBuffer._
+import TypeApplications._
 
 class TreePickler(pickler: TastyPickler) {
   val buf = new TreeBuffer
@@ -142,6 +143,9 @@ class TreePickler(pickler: TastyPickler) {
     }
 
     def pickleNewType(tpe: Type, richTypes: Boolean): Unit = try { tpe match {
+      case AppliedType(tycon, args) =>
+        writeByte(APPLIEDtype)
+        withLength { pickleType(tycon); args.foreach(pickleType(_)) }
       case ConstantType(value) =>
         pickleConstant(value)
       case tpe: TypeRef if tpe.info.isAlias && tpe.symbol.is(Flags.AliasPreferred) =>
@@ -181,15 +185,10 @@ class TreePickler(pickler: TastyPickler) {
           pickleNameAndSig(tpe.name, tpe.signature); pickleType(tpe.prefix)
         }
       case tpe: NamedType =>
-        if (tpe.name == tpnme.hkApply && tpe.prefix.argInfos.nonEmpty && tpe.prefix.isInstantiatedLambda)
-          // instantiated lambdas are pickled as APPLIEDTYPE; #Apply will
-          // be reconstituted when unpickling.
-          pickleType(tpe.prefix)
-        else if (isLocallyDefined(tpe.symbol)) {
+        if (isLocallyDefined(tpe.symbol)) {
           writeByte(if (tpe.isType) TYPEREFsymbol else TERMREFsymbol)
           pickleSymRef(tpe.symbol); pickleType(tpe.prefix)
-        }
-        else {
+        } else {
           writeByte(if (tpe.isType) TYPEREF else TERMREF)
           pickleName(tpe.name); pickleType(tpe.prefix)
         }
@@ -211,18 +210,11 @@ class TreePickler(pickler: TastyPickler) {
       case tpe: SkolemType =>
         pickleType(tpe.info)
       case tpe: RefinedType =>
-        val args = tpe.argInfos
-        if (args.isEmpty) {
-          writeByte(REFINEDtype)
-          withLength {
-            pickleType(tpe.parent)
-            pickleName(tpe.refinedName)
-            pickleType(tpe.refinedInfo, richTypes = true)
-          }
-        }
-        else {
-          writeByte(APPLIEDtype)
-          withLength { pickleType(tpe.withoutArgs(args)); args.foreach(pickleType(_)) }
+        writeByte(REFINEDtype)
+        withLength {
+          pickleType(tpe.parent)
+          pickleName(tpe.refinedName)
+          pickleType(tpe.refinedInfo, richTypes = true)
         }
       case tpe: TypeAlias =>
         writeByte(TYPEALIAS)
diff --git a/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala b/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
index cf7b487bb..618e3ceea 100644
--- a/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
+++ b/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
@@ -21,6 +21,7 @@ import typer.Mode
 import PickleBuffer._
 import scala.reflect.internal.pickling.PickleFormat._
 import Decorators._
+import TypeApplications._
 import classfile.ClassfileParser
 import scala.collection.{ mutable, immutable }
 import scala.collection.mutable.ListBuffer
@@ -52,7 +53,7 @@ object Scala2Unpickler {
     case TempPolyType(tparams, restpe) =>
       if (denot.isType) {
         assert(!denot.isClass)
-        restpe.LambdaAbstract(tparams, cycleParanoid = true)
+        restpe.LambdaAbstract(tparams)
       }
       else
         PolyType.fromSymbols(tparams, restpe)
@@ -142,51 +143,6 @@ object Scala2Unpickler {
     denot.info = ClassInfo( // final info
       denot.owner.thisType, denot.classSymbol, parentRefs, declsInRightOrder, ost)
   }
-
-  /** Adapt arguments to type parameters so that variance of type lambda arguments
-   *  agrees with variance of corresponding higherkinded type parameters. Example:
-   *
-   *     class Companion[+CC[X]]
-   *     Companion[List]
-   *
-   *  with adaptArgs, this will expand to
-   *
-   *     Companion[[X] => List[X]]
-   *
-   *  instead of
-   *
-   *      Companion[[+X] => List[X]]
-   *
-   *  even though `List` is covariant. This adaptation is necessary to ignore conflicting
-   *  variances in overriding members that have types of hk-type parameters such as `Companion[GenTraversable]`
-   *  or `Companion[ListBuffer]`. Without the adaptation we would end up with
-   *
-   *      Companion[[+X] => GenTraversable[X]]
-   *      Companion[[X] => List[X]]
-   *
-   *  and the second is not a subtype of the first. So if we have overridding memebrs of the two
-   *  types we get an error.
-   */
-  def adaptArgs(tparams: List[Symbol], args: List[Type])(implicit ctx: Context): List[Type] = tparams match {
-    case tparam :: tparams1 =>
-      val boundLambda = tparam.infoOrCompleter match {
-        case TypeBounds(_, hi) => hi.LambdaClass(forcing = false)
-        case _ => NoSymbol
-      }
-      def adaptArg(arg: Type): Type = arg match {
-        case arg: TypeRef if arg.symbol.isLambdaTrait =>
-          assert(arg.symbol.typeParams.length == boundLambda.typeParams.length)
-          arg.prefix.select(boundLambda)
-        case arg: RefinedType =>
-          arg.derivedRefinedType(adaptArg(arg.parent), arg.refinedName, arg.refinedInfo)
-        case _ =>
-          arg
-      }
-      val arg = args.head
-      val adapted = if (boundLambda.exists) adaptArg(arg) else arg
-      adapted :: adaptArgs(tparams1, args.tail)
-    case nil => args
-  }
 }
 
 /** Unpickle symbol table information descending from a class and/or module root
@@ -759,8 +715,9 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
           else TypeRef(pre, sym.name.asTypeName)
         val args = until(end, readTypeRef)
         if (sym == defn.ByNameParamClass2x) ExprType(args.head)
-        else if (args.isEmpty && sym.typeParams.nonEmpty) tycon.EtaExpand(sym.typeParams)
-        else tycon.appliedTo(adaptArgs(sym.typeParams, args))
+        else if (args.nonEmpty) tycon.safeAppliedTo(etaExpandIfHK(sym.typeParams, args))
+        else if (sym.typeParams.nonEmpty) tycon.EtaExpand(sym.typeParams)
+        else tycon
       case TYPEBOUNDStpe =>
         TypeBounds(readTypeRef(), readTypeRef())
       case REFINEDtpe =>