Fix of #34

The root cause of #34 was that we took a type argument which was an existential type. These are returned as type bounds, which make no sense in the calling context. To avoid that problem in the future, `typeArgs`
got renamed to `argInfos`, so it is clear we get an info, not necessarily a value type. There are
also added method `argTypes`, `argTypesLo`, `argTypesHi`, which return a type, but either throw an exception or return a lower/upper approximation of the argument is an existential type.

There's another issue that the existential type only arose when compiling the same couple fo files the seciond time. We need to chase that one down separately.

11 files changed, 88 insertions, 33 deletions

diff --git a/src/dotty/tools/dotc/ast/CheckTrees.scala b/src/dotty/tools/dotc/ast/CheckTrees.scala
index 6152a2880..ad3764450 100644
--- a/src/dotty/tools/dotc/ast/CheckTrees.scala
+++ b/src/dotty/tools/dotc/ast/CheckTrees.scala
@@ -135,7 +135,7 @@ object CheckTrees {
       check(finalizer.isTerm)
       check(handler.isTerm)
       check(handler.tpe derivesFrom defn.FunctionClass(1))
-      check(handler.tpe.baseTypeArgs(defn.FunctionClass(1)).head <:< defn.ThrowableType)
+      check(handler.tpe.baseArgInfos(defn.FunctionClass(1)).head <:< defn.ThrowableType)
     case Throw(expr) =>
       check(expr.isValue)
       check(expr.tpe.derivesFrom(defn.ThrowableClass))
@@ -210,9 +210,9 @@ object CheckTrees {
             check(args.isEmpty)
           else {
             check(rtp isRef defn.OptionClass)
-            val normArgs = rtp.typeArgs match {
+            val normArgs = rtp.argTypesHi match {
               case optionArg :: Nil =>
-                optionArg.typeArgs match {
+                optionArg.argTypesHi match {
                   case Nil =>
                     optionArg :: Nil
                   case tupleArgs if defn.isTupleType(optionArg) =>
diff --git a/src/dotty/tools/dotc/ast/tpd.scala b/src/dotty/tools/dotc/ast/tpd.scala
index 8b9e0c12e..03c0f83a9 100644
--- a/src/dotty/tools/dotc/ast/tpd.scala
+++ b/src/dotty/tools/dotc/ast/tpd.scala
@@ -336,7 +336,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
   /** new C(args) */
   def New(tp: Type, args: List[Tree])(implicit ctx: Context): Apply = {
-    val targs = tp.typeArgs
+    val targs = tp.argTypes
     Apply(
       Select(
         New(tp withoutArgs targs),
diff --git a/src/dotty/tools/dotc/ast/untpd.scala b/src/dotty/tools/dotc/ast/untpd.scala
index 4aff9da5d..fc2b07b02 100644
--- a/src/dotty/tools/dotc/ast/untpd.scala
+++ b/src/dotty/tools/dotc/ast/untpd.scala
@@ -124,7 +124,7 @@ object untpd extends Trees.Instance[Untyped] with TreeInfo[Untyped] {
       case TypedSplice(AppliedTypeTree(tycon, targs)) =>
         (TypedSplice(tycon), targs map TypedSplice)
       case TypedSplice(tpt1: Tree) =>
-        val argTypes = tpt1.tpe.typeArgs
+        val argTypes = tpt1.tpe.argTypes
         val tycon = tpt1.tpe.withoutArgs(argTypes)
         def wrap(tpe: Type) = TypeTree(tpe) withPos tpt.pos
         (wrap(tycon), argTypes map wrap)
diff --git a/src/dotty/tools/dotc/core/Definitions.scala b/src/dotty/tools/dotc/core/Definitions.scala
index 0ee0ea1e2..ef16a970d 100644
--- a/src/dotty/tools/dotc/core/Definitions.scala
+++ b/src/dotty/tools/dotc/core/Definitions.scala
@@ -286,7 +286,7 @@ class Definitions {
     def unapply(ft: Type): Option[(List[Type], Type)] = { // Dotty deviation: Type annotation needed because inferred type
                                                           // is Some[(List[Type], Type)] | None, which is not a legal unapply type.
       val tsym = ft.typeSymbol
-      lazy val targs = ft.typeArgs
+      lazy val targs = ft.argInfos
       if ((FunctionClasses contains tsym) &&
           (targs.length - 1 <= MaxFunctionArity) &&
           (FunctionClass(targs.length - 1) == tsym)) Some((targs.init, targs.last)) // Dotty deviation: no auto-tupling
@@ -317,7 +317,7 @@ class Definitions {
   lazy val RootImports = List[Symbol](JavaLangPackageVal, ScalaPackageVal, ScalaPredefModule, DottyPredefModule)
 
   def isTupleType(tp: Type) = {
-    val arity = tp.dealias.typeArgs.length
+    val arity = tp.dealias.argInfos.length
     arity <= MaxTupleArity && (tp isRef TupleClass(arity))
   }
 
@@ -329,7 +329,7 @@ class Definitions {
     0 <= arity && arity <= MaxFunctionArity && (tp isRef FunctionClass(arity))
   }
 
-  def functionArity(tp: Type) = tp.dealias.typeArgs.length - 1
+  def functionArity(tp: Type) = tp.dealias.argInfos.length - 1
 
   // ----- Higher kinds machinery ------------------------------------------
 
diff --git a/src/dotty/tools/dotc/core/TypeApplications.scala b/src/dotty/tools/dotc/core/TypeApplications.scala
index e854e672f..b4c30d902 100644
--- a/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -12,6 +12,29 @@ import Flags._
 import util.Positions.Position
 import collection.mutable
 
+object TypeApplications {
+
+  /** Assert type is not a TypeBounds instance and return it unchanged */
+  val noBounds = (tp: Type) => tp match {
+    case tp: TypeBounds => throw new AssertionError("no TypeBounds allowed")
+    case _ => tp
+  }
+
+  /** If `tp` is a TypeBounds instance return its lower bound else return `tp` */
+  val boundsToLo = (tp: Type) => tp match {
+    case tp: TypeBounds => tp.lo
+    case _ => tp
+  }
+
+  /** If `tp` is a TypeBounds instance return its upper bound else return `tp` */
+  val boundsToHi = (tp: Type) => tp match {
+    case tp: TypeBounds => tp.hi
+    case _ => tp
+  }
+}
+
+import TypeApplications._
+
 /** A decorator that provides methods for modeling type application */
 class TypeApplications(val self: Type) extends AnyVal {
 
@@ -135,24 +158,43 @@ class TypeApplications(val self: Type) extends AnyVal {
       else TypeAlias(self, v)
   }
 
-  /** The type arguments of the base type instance wrt `base` of this type */
-  final def baseTypeArgs(base: Symbol)(implicit ctx: Context): List[Type] =
+  /** 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] =
     if (self derivesFrom base)
-      base.typeParams map (param => self.member(param.name).info.argType(param))
+      base.typeParams map (param => self.member(param.name).info.argInfo(param))
     else
       Nil
 
+  /** The type arguments of this type's base type instance wrt.`base`.
+   *  Existential types in arguments are disallowed.
+   */
+  final def baseArgTypes(base: Symbol)(implicit ctx: Context): List[Type] = baseArgInfos(base) mapConserve noBounds
+
+  /** The type arguments of this type's base type instance wrt.`base`.
+   *  Existential types in arguments are approximanted by their lower bound.
+   */
+  final def baseArgTypesLo(base: Symbol)(implicit ctx: Context): List[Type] = baseArgInfos(base) mapConserve boundsToLo
+
+  /** The type arguments of this type's base type instance wrt.`base`.
+   *  Existential types in arguments are approximanted by their upper bound.
+   */
+  final def baseArgTypesHi(base: Symbol)(implicit ctx: Context): List[Type] = baseArgInfos(base) mapConserve boundsToHi
+
   /** The first type argument of the base type instance wrt `base` of this type */
-  final def firstBaseTypeArg(base: Symbol)(implicit ctx: Context): Type = base.typeParams match {
+  final def firstBaseArgInfo(base: Symbol)(implicit ctx: Context): Type = base.typeParams match {
     case param :: _ if self derivesFrom base =>
-      self.member(param.name).info.argType(param)
+      self.member(param.name).info.argInfo(param)
     case _ =>
       NoType
   }
 
-  /** The base type including all type arguments of this type */
+  /** The base type including all type arguments of this type.
+   *  Existential types in arguments are returned as TypeBounds instances.
+   */
   final def baseTypeWithArgs(base: Symbol)(implicit ctx: Context): Type =
-    self.baseTypeRef(base).appliedTo(baseTypeArgs(base))
+    self.baseTypeRef(base).appliedTo(baseArgInfos(base))
 
   /** Translate a type of the form From[T] to To[T], keep other types as they are.
    *  `from` and `to` must be static classes, both with one type parameter, and the same variance.
@@ -163,9 +205,10 @@ class TypeApplications(val self: Type) extends AnyVal {
     else self
 
   /** If this is an encoding of a (partially) applied type, return its arguments,
-   *  otherwise return Nil
+   *  otherwise return Nil. 
+   *  Existential types in arguments are returned as TypeBounds instances.
    */
-  final def typeArgs(implicit ctx: Context): List[Type] = {
+  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) =>
@@ -175,7 +218,7 @@ class TypeApplications(val self: Type) extends AnyVal {
           if (tparams == null) tparams = tycon.typeParams
           if (buf.size < tparams.length) {
             val tparam = tparams(buf.size)
-            if (name == tparam.name) buf += tp.refinedInfo.argType(tparam)
+            if (name == tparam.name) buf += tp.refinedInfo.argInfo(tparam)
             else null
           } else null
         }
@@ -187,6 +230,15 @@ class TypeApplications(val self: Type) extends AnyVal {
     if (buf == null) Nil else buf.toList
   }
 
+  /** Argument types where existential types in arguments are disallowed */
+  def argTypes(implicit ctx: Context) = argInfos mapConserve noBounds
+
+  /** Argument types where existential types in arguments are approximated by their lower bound */
+  def argTypesLo(implicit ctx: Context) = argInfos mapConserve boundsToLo
+
+  /** Argument types where existential types in arguments are approximated by their upper bound  */
+  def argTypesHi(implicit ctx: Context) = argInfos mapConserve boundsToHi
+
   /** The core type without any type arguments.
    *  @param `typeArgs` must be the type arguments of this type.
    */
@@ -201,7 +253,7 @@ class TypeApplications(val self: Type) extends AnyVal {
   /** If this is the image of a type argument to type parameter `tparam`,
    *  recover the type argument, otherwise NoType.
    */
-  final def argType(tparam: Symbol)(implicit ctx: Context): Type = self match {
+  final def argInfo(tparam: Symbol)(implicit ctx: Context): Type = self match {
     case TypeBounds(lo, hi) =>
       if (lo eq hi) hi
       else {
@@ -216,7 +268,7 @@ class TypeApplications(val self: Type) extends AnyVal {
 
   /** The element type of a sequence or array */
   def elemType(implicit ctx: Context): Type =
-    firstBaseTypeArg(defn.SeqClass) orElse firstBaseTypeArg(defn.ArrayClass)
+    firstBaseArgInfo(defn.SeqClass) orElse firstBaseArgInfo(defn.ArrayClass)
 
   /** If this type is of the normalized form Array[...[Array[T]...]
    *  return the number of Array wrappers and T.
@@ -225,7 +277,7 @@ class TypeApplications(val self: Type) extends AnyVal {
   final def splitArray(implicit ctx: Context): (Int, Type) = {
     def recur(n: Int, tp: Type): (Int, Type) = tp.stripTypeVar match {
       case RefinedType(tycon, _) if tycon isRef defn.ArrayClass =>
-        tp.typeArgs match {
+        tp.argInfos match {
           case arg :: Nil => recur(n + 1, arg)
           case _ => (n, tp)
         }
@@ -274,7 +326,7 @@ class TypeApplications(val self: Type) extends AnyVal {
 
       val correspondingParamName: Map[Symbol, TypeName] = {
         for {
-          (tparam, targ: TypeRef) <- cls.typeParams zip typeArgs
+          (tparam, targ: TypeRef) <- cls.typeParams zip argInfos
           if boundSyms contains targ.symbol
         } yield targ.symbol -> tparam.name
       }.toMap
diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index 0af6ebf97..d46a8387f 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -616,7 +616,7 @@ class TypeComparer(initctx: Context) extends DotClass {
    */
   def isSubTypeHK(tp1: Type, tp2: Type): Boolean = {
     val tparams = tp1.typeParams
-    val hkArgs = tp2.typeArgs
+    val hkArgs = tp2.argInfos
     (hkArgs.length == tparams.length) && {
       val base = tp1.narrow
       (tparams, hkArgs).zipped.forall { (tparam, hkArg) =>
@@ -752,7 +752,7 @@ class TypeComparer(initctx: Context) extends DotClass {
   /** The least upper bound of two types
    *  @note  We do not admit singleton types in or-types as lubs.
    */
-  def lub(tp1: Type, tp2: Type): Type =
+  def lub(tp1: Type, tp2: Type): Type = /*>|>*/ ctx.traceIndented(s"lub(${tp1.show}, ${tp2.show})", typr, show = true) /*<|<*/ {
     if (tp1 eq tp2) tp1
     else if (!tp1.exists) tp1
     else if (!tp2.exists) tp2
@@ -772,6 +772,7 @@ class TypeComparer(initctx: Context) extends DotClass {
         }
       }
     }
+  }
 
   /** The least upper bound of a list of types */
   final def lub(tps: List[Type]): Type =
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index bc8d7dfd2..4d26a6735 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -96,7 +96,7 @@ object Types {
         else thissym eq sym
       case this1: RefinedType =>
         // make sure all refinements are type arguments
-        this1.parent.isRef(sym) && this.typeArgs.nonEmpty
+        this1.parent.isRef(sym) && this.argInfos.nonEmpty
       case _ =>
         false
     }
diff --git a/src/dotty/tools/dotc/core/pickling/UnPickler.scala b/src/dotty/tools/dotc/core/pickling/UnPickler.scala
index c48e71052..c69f60758 100644
--- a/src/dotty/tools/dotc/core/pickling/UnPickler.scala
+++ b/src/dotty/tools/dotc/core/pickling/UnPickler.scala
@@ -76,7 +76,7 @@ object UnPickler {
     case tp @ MethodType(paramNames, paramTypes) =>
       val lastArg = paramTypes.last
       assert(lastArg isRef defn.ArrayClass)
-      val elemtp0 :: Nil = lastArg.baseTypeArgs(defn.ArrayClass)
+      val elemtp0 :: Nil = lastArg.baseArgInfos(defn.ArrayClass)
       val elemtp = elemtp0 match {
         case AndType(t1, t2) if t1.typeSymbol.isAbstractType && (t2 isRef defn.ObjectClass) =>
           t1 // drop intersection with Object for abstract types in varargs. UnCurry can handle them.
diff --git a/src/dotty/tools/dotc/printing/RefinedPrinter.scala b/src/dotty/tools/dotc/printing/RefinedPrinter.scala
index 6fad6738e..cb0780ae3 100644
--- a/src/dotty/tools/dotc/printing/RefinedPrinter.scala
+++ b/src/dotty/tools/dotc/printing/RefinedPrinter.scala
@@ -96,7 +96,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       }
     tp match {
       case tp: RefinedType =>
-        val args = tp.typeArgs
+        val args = tp.argInfos
         if (args.nonEmpty) {
           val tycon = tp.unrefine
           val cls = tycon.typeSymbol
diff --git a/src/dotty/tools/dotc/typer/Applications.scala b/src/dotty/tools/dotc/typer/Applications.scala
index 4b43aa8b7..4d7bc8976 100644
--- a/src/dotty/tools/dotc/typer/Applications.scala
+++ b/src/dotty/tools/dotc/typer/Applications.scala
@@ -24,6 +24,7 @@ import EtaExpansion._
 import collection.mutable
 import reflect.ClassTag
 import config.Printers._
+import TypeApplications._
 import language.implicitConversions
 
 object Applications {
@@ -266,7 +267,7 @@ trait Applications extends Compatibility { self: Typer =>
               case arg :: Nil if isVarArg(arg) =>
                 addTyped(arg, formal)
               case _ =>
-                val elemFormal = formal.typeArgs.head
+                val elemFormal = formal.argTypesLo.head
                 args foreach (addTyped(_, elemFormal))
                 makeVarArg(args.length, elemFormal)
             }
@@ -609,7 +610,7 @@ trait Applications extends Compatibility { self: Typer =>
       if (extractorMemberType(unapplyResult, nme.isDefined) isRef defn.BooleanClass) {
         if (getTp.exists)
           if (unapply.symbol.name == nme.unapplySeq) {
-            val seqArg = getTp.firstBaseTypeArg(defn.SeqClass)
+            val seqArg = boundsToHi(getTp.firstBaseArgInfo(defn.SeqClass))
             if (seqArg.exists) return args map Function.const(seqArg)
           }
           else return getSelectors(getTp)
@@ -683,6 +684,7 @@ trait Applications extends Compatibility { self: Typer =>
         }
 
         var argTypes = unapplyArgs(unapplyApp.tpe)
+        for (argType <- argTypes) assert(!argType.isInstanceOf[TypeBounds], unapplyApp.tpe.show)
         val bunchedArgs = argTypes match {
           case argType :: Nil if argType.isRepeatedParam => untpd.SeqLiteral(args) :: Nil
           case _ => args
@@ -770,7 +772,7 @@ trait Applications extends Compatibility { self: Typer =>
         val tparams = ctx.newTypeParams(alt1.symbol.owner, tp1.paramNames, EmptyFlags, bounds)
         isAsSpecific(alt1, tp1.instantiate(tparams map (_.typeRef)), alt2, tp2)
       case tp1: MethodType =>
-        def repeatedToSingle(tp: Type) = if (tp.isRepeatedParam) tp.typeArgs.head else tp
+        def repeatedToSingle(tp: Type) = if (tp.isRepeatedParam) tp.argTypesHi.head else tp
         isApplicable(alt2, tp1.paramTypes map repeatedToSingle, WildcardType) ||
         tp1.paramTypes.isEmpty && tp2.isInstanceOf[MethodOrPoly]
       case _ =>
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 400a1407a..96fe73e7d 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -438,7 +438,7 @@ class Typer extends Namer with Applications with Implicits {
   }
 
   def typedPair(tree: untpd.Pair, pt: Type)(implicit ctx: Context) = track("typedPair") {
-    val (leftProto, rightProto) = pt.typeArgs match {
+    val (leftProto, rightProto) = pt.argTypesLo match {
       case l :: r :: Nil if pt isRef defn.PairClass => (l, r)
       case _ => (WildcardType, WildcardType)
     }
@@ -561,7 +561,7 @@ class Typer extends Namer with Applications with Implicits {
       val params = args.asInstanceOf[List[untpd.ValDef]]
       val (protoFormals, protoResult): (List[Type], Type) = pt match {
         case _ if defn.isFunctionType(pt) =>
-          (pt.dealias.typeArgs.init, pt.dealias.typeArgs.last)
+          (pt.dealias.argInfos.init, pt.dealias.argInfos.last)
         case SAMType(meth) =>
           val mt @ MethodType(_, paramTypes) = meth.info
           (paramTypes, mt.resultType)
@@ -750,7 +750,7 @@ class Typer extends Namer with Applications with Implicits {
     val expr1 = typed(tree.expr, pt)
     val handler1 = typed(tree.handler, defn.FunctionType(defn.ThrowableType :: Nil, pt))
     val finalizer1 = typed(tree.finalizer, defn.UnitType)
-    val handlerTypeArgs = handler1.tpe.baseTypeArgs(defn.FunctionClass(1))
+    val handlerTypeArgs = handler1.tpe.baseArgTypesHi(defn.FunctionClass(1))
     val ownType = if (handlerTypeArgs.nonEmpty) expr1.tpe | handlerTypeArgs(1) else expr1.tpe
     cpy.Try(tree, expr1, handler1, finalizer1) withType ownType
   }