Add kind polymorphism

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -123,6 +123,9 @@ class ScalaSettings extends Settings.SettingGroup {
   val YprofileRunGcBetweenPhases = PhasesSetting("-Yprofile-run-gc", "Run a GC between phases - this allows heap size to be accurate at the expense of more time. Specify a list of phases, or *", "_")
       //.withPostSetHook( _ => YprofileEnabled.value = true )
 
+  // Extremely experimental language features
+  val YkindPolymorphism = BooleanSetting("-Ykind-polymorphism", "Enable kind polymorphism (see http://dotty.epfl.ch/docs/reference/kind-polymorphism.html). Potentially unsound.")
+
   /** Area-specific debug output */
   val YexplainLowlevel = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")
   val YnoDoubleBindings = BooleanSetting("-Yno-double-bindings", "Assert no namedtype is bound twice (should be enabled only if program is error-free).")

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -304,6 +304,16 @@ class Definitions {
     def ObjectMethods = List(Object_eq, Object_ne, Object_synchronized, Object_clone,
         Object_finalize, Object_notify, Object_notifyAll, Object_wait, Object_waitL, Object_waitLI)
 
+  lazy val AnyKindClass = {
+    val cls = ctx.newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyKind, AbstractFinal | Permanent, Nil)
+    if (ctx.settings.YkindPolymorphism.value) {
+      // Enable kind-polymorphism by exposing scala.AnyKind
+      cls.entered
+    }
+    cls
+  }
+  def AnyKindType = AnyKindClass.typeRef
+
   /** Marker method to indicate an argument to a call-by-name parameter.
    *  Created by byNameClosures and elimByName, eliminated by Erasure,
    */
@@ -1158,6 +1168,7 @@ class Definitions {
   lazy val syntheticScalaClasses = List(
     AnyClass,
     AnyRefAlias,
+    AnyKindClass,
     RepeatedParamClass,
     ByNameParamClass2x,
     AnyValClass,

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -196,6 +196,7 @@ object StdNames {
     final val REIFY_TYPECREATOR_PREFIX: N       = "$typecreator"
 
     final val Any: N                 = "Any"
+    final val AnyKind: N             = "AnyKind"
     final val AnyVal: N              = "AnyVal"
     final val ExprApi: N             = "ExprApi"
     final val Mirror: N              = "Mirror"

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -667,7 +667,7 @@ object SymDenotations {
 
     /** Is this symbol a class references to which that are supertypes of null? */
     final def isNullableClass(implicit ctx: Context): Boolean =
-      isClass && !isValueClass && !(this is ModuleClass) && symbol != defn.NothingClass
+      isClass && !isValueClass && !is(ModuleClass) && symbol != defn.NothingClass
 
     /** Is this definition accessible as a member of tree with type `pre`?
      *  @param pre          The type of the tree from which the selection is made

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

@@ -197,7 +197,7 @@ class TypeApplications(val self: Type) extends AnyVal {
 
   /** If `self` is a higher-kinded type, its type parameters, otherwise Nil */
   final def hkTypeParams(implicit ctx: Context): List[TypeParamInfo] =
-    if (isHK) typeParams else Nil
+    if (isLambdaSub) typeParams else Nil
 
   /** If `self` is a generic class, its type parameter symbols, otherwise Nil */
   final def typeParamSymbols(implicit ctx: Context): List[TypeSymbol] = typeParams match {
@@ -207,12 +207,19 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => Nil
   }
 
-  /** Is self type higher-kinded (i.e. of kind != "*")? */
-  def isHK(implicit ctx: Context): Boolean = hkResult.exists
+  /** Is self type bounded by a type lambda or AnyKind? */
+  def isLambdaSub(implicit ctx: Context): Boolean = hkResult.exists
 
-  /** If self type is higher-kinded, its result type, otherwise NoType */
+  /** Is self type of kind != "*"? */
+  def hasHigherKind(implicit ctx: Context): Boolean =
+    typeParams.nonEmpty || self.isRef(defn.AnyKindClass)
+
+  /** If self type is higher-kinded, its result type, otherwise NoType.
+   *  Note: The hkResult of an any-kinded type is again AnyKind.
+   */
   def hkResult(implicit ctx: Context): Type = self.dealias match {
-    case self: TypeRef => self.info.hkResult
+    case self: TypeRef =>
+      if (self.symbol == defn.AnyKindClass) self else self.info.hkResult
     case self: AppliedType =>
       if (self.tycon.typeSymbol.isClass) NoType else self.superType.hkResult
     case self: HKTypeLambda => self.resultType
@@ -226,17 +233,24 @@ class TypeApplications(val self: Type) extends AnyVal {
     case _ => NoType
   }
 
-  /** Do self and other have the same kinds (not counting bounds and variances) */
+  /** Do self and other have the same kinds (not counting bounds and variances)?
+   *  Note: An any-kinded type "has the same kind" as any other type.
+   */
   def hasSameKindAs(other: Type)(implicit ctx: Context): Boolean = {
-    // println(i"check kind $self $other") // DEBUG
+    def isAnyKind(tp: Type) = tp match {
+      case tp: TypeRef => tp.symbol == defn.AnyKindClass
+      case _ => false
+    }
     val selfResult = self.hkResult
     val otherResult = other.hkResult
-    if (selfResult.exists)
-      otherResult.exists &&
-      selfResult.hasSameKindAs(otherResult) &&
-      self.typeParams.corresponds(other.typeParams)((sparam, oparam) =>
-        sparam.paramInfo.hasSameKindAs(oparam.paramInfo))
-    else !otherResult.exists
+    isAnyKind(selfResult) || isAnyKind(otherResult) ||
+    { if (selfResult.exists)
+        otherResult.exists &&
+        selfResult.hasSameKindAs(otherResult) &&
+        self.typeParams.corresponds(other.typeParams)((sparam, oparam) =>
+          sparam.paramInfo.hasSameKindAs(oparam.paramInfo))
+      else !otherResult.exists
+    }
   }
 
   /** Dealias type if it can be done without forcing the TypeRef's info */
@@ -256,9 +270,9 @@ class TypeApplications(val self: Type) extends AnyVal {
       //.ensuring(res => res.EtaReduce =:= self, s"res = $res, core = ${res.EtaReduce}, self = $self, hc = ${res.hashCode}")
   }
 
-  /** If self is not higher-kinded, eta expand it. */
-  def ensureHK(implicit ctx: Context): Type =
-    if (isHK) self else EtaExpansion(self)
+  /** If self is not lambda-bound, eta expand it. */
+  def ensureLambdaSub(implicit ctx: Context): Type =
+    if (isLambdaSub) self else EtaExpansion(self)
 
   /** 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 = {
@@ -355,7 +369,9 @@ class TypeApplications(val self: Type) extends AnyVal {
                 case _ => false
               }
             }
-            if ((dealiased eq stripped) || followAlias) dealiased.instantiate(args)
+            if ((dealiased eq stripped) || followAlias)
+              try dealiased.instantiate(args)
+              catch { case ex: IndexOutOfBoundsException => AppliedType(self, args) }
             else AppliedType(self, args)
           }
           else dealiased.resType match {

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

@@ -48,16 +48,22 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
   private[this] var totalCount = 0
 
   private[this] var myAnyClass: ClassSymbol = null
+  private[this] var myAnyKindClass: ClassSymbol = null
   private[this] var myNothingClass: ClassSymbol = null
   private[this] var myNullClass: ClassSymbol = null
   private[this] var myObjectClass: ClassSymbol = null
   private[this] var myAnyType: TypeRef = null
+  private[this] var myAnyKindType: TypeRef = null
   private[this] var myNothingType: TypeRef = null
 
   def AnyClass = {
     if (myAnyClass == null) myAnyClass = defn.AnyClass
     myAnyClass
   }
+  def AnyKindClass = {
+    if (myAnyKindClass == null) myAnyKindClass = defn.AnyKindClass
+    myAnyKindClass
+  }
   def NothingClass = {
     if (myNothingClass == null) myNothingClass = defn.NothingClass
     myNothingClass
@@ -74,6 +80,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     if (myAnyType == null) myAnyType = AnyClass.typeRef
     myAnyType
   }
+  def AnyKindType = {
+    if (myAnyKindType == null) myAnyKindType = AnyKindClass.typeRef
+    myAnyKindType
+  }
   def NothingType = {
     if (myNothingType == null) myNothingType = NothingClass.typeRef
     myNothingType
@@ -367,19 +377,25 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       case _ =>
         val cls2 = tp2.symbol
         if (cls2.isClass) {
-          if (cls2.typeParams.nonEmpty && tp1.isHK)
-            recur(tp1, EtaExpansion(cls2.typeRef))
-          else {
+          if (cls2.typeParams.isEmpty) {
+            if (cls2 eq AnyKindClass) return true
+            if (tp1.isRef(defn.NothingClass)) return true
+            if (tp1.isLambdaSub) return false
+              // Note: We would like to replace this by `if (tp1.hasHigherKind)`
+              // but right now we cannot since some parts of the standard library rely on the
+              // idiom that e.g. `List <: Any`. We have to bootstrap without scalac first.
             val base = tp1.baseType(cls2)
-            if (base.exists) {
-              if (cls2.is(JavaDefined))
-                // If `cls2` is parameterized, we are seeing a raw type, so we need to compare only the symbol
-                return base.typeSymbol == cls2
-              if (base ne tp1)
-                return isSubType(base, tp2, if (tp1.isRef(cls2)) approx else approx.addLow)
-            }
+            if (base.exists && base.ne(tp1))
+              return isSubType(base, tp2, if (tp1.isRef(cls2)) approx else approx.addLow)
             if (cls2 == defn.SingletonClass && tp1.isStable) return true
           }
+          else if (cls2.is(JavaDefined)) {
+            // If `cls2` is parameterized, we are seeing a raw type, so we need to compare only the symbol
+            val base = tp1.baseType(cls2)
+            if (base.typeSymbol == cls2) return true
+          }
+          else if (tp1.isLambdaSub && !tp1.isRef(defn.AnyKindClass))
+            return recur(tp1, EtaExpansion(cls2.typeRef))
         }
         fourthTry
     }
@@ -475,13 +491,11 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
               isSubType(tp1.resType, tp2.resType.subst(tp2, tp1))
             finally comparedTypeLambdas = saved
           case _ =>
-            if (tp1.isHK) {
-              val tparams1 = tp1.typeParams
+            val tparams1 = tp1.typeParams
+            if (tparams1.nonEmpty)
               return recur(
                 HKTypeLambda.fromParams(tparams1, tp1.appliedTo(tparams1.map(_.paramRef))),
-                tp2
-              )
-            }
+                tp2)
             else tp2 match {
               case EtaExpansion(tycon2) if tycon2.symbol.isClass =>
                 return recur(tp1, tycon2)
@@ -540,7 +554,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         def compareTypeBounds = tp1 match {
           case tp1 @ TypeBounds(lo1, hi1) =>
             ((lo2 eq NothingType) || isSubType(lo2, lo1)) &&
-            ((hi2 eq AnyType) || isSubType(hi1, hi2))
+            ((hi2 eq AnyType) && !hi1.isLambdaSub || (hi2 eq AnyKindType) || isSubType(hi1, hi2))
           case tp1: ClassInfo =>
             tp2 contains tp1
           case _ =>
@@ -610,7 +624,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
           case EtaExpansion(tycon1) => recur(tycon1, tp2)
           case _ => tp2 match {
             case tp2: HKTypeLambda => false // this case was covered in thirdTry
-            case _ => tp2.isHK && isSubType(tp1.resultType, tp2.appliedTo(tp1.paramRefs))
+            case _ => tp2.isLambdaSub && isSubType(tp1.resultType, tp2.appliedTo(tp1.paramRefs))
           }
         }
         compareHKLambda
@@ -718,7 +732,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
                     tl => tp1base.tycon.appliedTo(args1.take(lengthDiff) ++
                             tparams1.indices.toList.map(tl.paramRefs(_))))
                 (ctx.mode.is(Mode.TypevarsMissContext) ||
-                  tryInstantiate(tycon2, tycon1.ensureHK)) &&
+                  tryInstantiate(tycon2, tycon1.ensureLambdaSub)) &&
                   recur(tp1, tycon1.appliedTo(args2))
               }
             }
@@ -801,7 +815,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         case param1: TypeParamRef =>
           def canInstantiate = tp2 match {
             case AppliedType(tycon2, args2) =>
-              tryInstantiate(param1, tycon2.ensureHK) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
+              tryInstantiate(param1, tycon2.ensureLambdaSub) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
             case _ =>
               false
           }
@@ -1216,8 +1230,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     if (tp1 eq tp2) tp1
     else if (!tp1.exists) tp2
     else if (!tp2.exists) tp1
-    else if ((tp1 isRef AnyClass) || (tp2 isRef NothingClass)) tp2
-    else if ((tp2 isRef AnyClass) || (tp1 isRef NothingClass)) tp1
+    else if ((tp1 isRef AnyClass) && !tp2.isLambdaSub || (tp1 isRef AnyKindClass) || (tp2 isRef NothingClass)) tp2
+    else if ((tp2 isRef AnyClass) && !tp1.isLambdaSub || (tp2 isRef AnyKindClass) || (tp1 isRef NothingClass)) tp1
     else tp2 match {  // normalize to disjunctive normal form if possible.
       case OrType(tp21, tp22) =>
         tp1 & tp21 | tp1 & tp22
@@ -1265,8 +1279,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     if (tp1 eq tp2) tp1
     else if (!tp1.exists) tp1
     else if (!tp2.exists) tp2
-    else if ((tp1 isRef AnyClass) || (tp2 isRef NothingClass)) tp1
-    else if ((tp2 isRef AnyClass) || (tp1 isRef NothingClass)) tp2
+    else if ((tp1 isRef AnyClass) || (tp1 isRef AnyKindClass) || (tp2 isRef NothingClass)) tp1
+    else if ((tp2 isRef AnyClass) || (tp2 isRef AnyKindClass) || (tp1 isRef NothingClass)) tp2
     else {
       val t1 = mergeIfSuper(tp1, tp2, canConstrain)
       if (t1.exists) t1

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1462,7 +1462,9 @@ object messages {
 
   case class MissingTypeParameterFor(tpe: Type)(implicit ctx: Context)
     extends Message(MissingTypeParameterForID) {
-    val msg = hl"missing type parameter for ${tpe}"
+    val msg =
+      if (tpe.derivesFrom(defn.AnyKindClass)) hl"${tpe} cannot be used as a value type"
+      else hl"missing type parameter for ${tpe}"
     val kind = "Syntax"
     val explanation = ""
   }

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -357,7 +357,7 @@ private class ExtractAPICollector(implicit val ctx: Context) extends ThunkHolder
     // TODO: Never dealias. We currently have to dealias because
     // sbt main class discovery relies on the signature of the main
     // method being fully dealiased. See https://github.com/sbt/zinc/issues/102
-    val tp2 = if (!tp.isHK) tp.dealiasKeepAnnots else tp
+    val tp2 = if (!tp.isLambdaSub) tp.dealiasKeepAnnots else tp
     tp2 match {
       case NoPrefix | NoType =>
         Constants.emptyType

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

@@ -45,7 +45,7 @@ object Checking {
    */
   def checkBounds(args: List[tpd.Tree], boundss: List[TypeBounds], instantiate: (Type, List[Type]) => Type)(implicit ctx: Context): Unit = {
     (args, boundss).zipped.foreach { (arg, bound) =>
-      if (!bound.isHK && arg.tpe.isHK)
+      if (!bound.isLambdaSub && arg.tpe.isLambdaSub)
         // see MissingTypeParameterFor
         ctx.error(ex"missing type parameter(s) for $arg", arg.pos)
     }
@@ -657,7 +657,7 @@ trait Checking {
 
   /** Check that `tpt` does not define a higher-kinded type */
   def checkSimpleKinded(tpt: Tree)(implicit ctx: Context): Tree =
-    if (tpt.tpe.isHK && !ctx.compilationUnit.isJava) {
+    if (tpt.tpe.isLambdaSub && !ctx.compilationUnit.isJava) {
         // be more lenient with missing type params in Java,
         // needed to make pos/java-interop/t1196 work.
       errorTree(tpt, MissingTypeParameterFor(tpt.tpe))

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -255,7 +255,7 @@ trait TypeAssigner {
    */
   def accessibleSelectionType(tree: untpd.RefTree, qual1: Tree)(implicit ctx: Context): Type = {
     var qualType = qual1.tpe.widenIfUnstable
-    if (qualType.isHK) qualType = errorType(em"$qualType takes type parameters", qual1.pos)
+    if (qualType.isLambdaSub) qualType = errorType(em"$qualType takes type parameters", qual1.pos)
     val ownType = selectionType(qualType, tree.name, tree.pos)
     ensureAccessible(ownType, qual1.isInstanceOf[Super], tree.pos)
   }

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1233,7 +1233,24 @@ class Typer extends Namer
                 tparam.ensureCompleted() // This is needed to get the test `compileParSetSubset` to work
               case _ =>
             }
-          if (desugaredArg.isType) typed(desugaredArg, argPt)
+          if (desugaredArg.isType) {
+            var res = typed(desugaredArg, argPt)
+            arg match {
+              case TypeBoundsTree(EmptyTree, EmptyTree)
+              if tparam.paramInfo.isLambdaSub &&
+                 tpt1.tpe.typeParamSymbols.nonEmpty &&
+                 !ctx.mode.is(Mode.Pattern) =>
+                // An unbounded `_` automatically adapts to type parameter bounds. This means:
+                // If we have wildcard application C[_], where `C` is a class replace
+                // with C[_ >: L <: H] where `L` and `H` are the bounds of the corresponding
+                // type parameter in `C`, avoiding any referemces to parameters of `C`.
+                // The transform does not apply for patters, where empty bounds translate to
+                // wildcard identifiers `_` instead.
+                res = res.withType(avoid(tparam.paramInfo, tpt1.tpe.typeParamSymbols))
+              case _ =>
+            }
+            res
+          }
           else desugaredArg.withType(UnspecifiedErrorType)
         }
         args.zipWithConserve(tparams)(typedArg(_, _)).asInstanceOf[List[Tree]]
@@ -2147,7 +2164,7 @@ class Typer extends Namer
 
     def adaptNoArgsImplicitMethod(wtp: MethodType): Tree = {
       assert(wtp.isImplicitMethod)
-      val tvarsToInstantiate = tvarsInParams(tree, locked)
+      val tvarsToInstantiate = tvarsInParams(tree, locked).distinct
       wtp.paramInfos.foreach(instantiateSelected(_, tvarsToInstantiate))
       val constr = ctx.typerState.constraint
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -102,6 +102,7 @@ class CompilationTests extends ParallelTesting {
     compileFilesInDir("tests/pos", defaultOptions) +
     compileFilesInDir("tests/pos-no-optimise", defaultOptions) +
     compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes) +
+    compileFilesInDir("tests/pos-kind-polymorphism", defaultOptions and "-Ykind-polymorphism") +
     compileDir("tests/pos/i1137-1", defaultOptions and "-Yemit-tasty") +
     compileFile(
       // succeeds despite -Xfatal-warnings because of -nowarn
@@ -175,6 +176,7 @@ class CompilationTests extends ParallelTesting {
     compileFilesInDir("tests/neg", defaultOptions) +
     compileFilesInDir("tests/neg-tailcall", defaultOptions) +
     compileFilesInDir("tests/neg-no-optimise", defaultOptions) +
+    compileFilesInDir("tests/neg-kind-polymorphism", defaultOptions and "-Ykind-polymorphism") +
     compileFilesInDir("tests/neg-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")) +
     compileFilesInDir("tests/neg-custom-args/allow-double-bindings", allowDoubleBindings) +
     compileFile("tests/neg-custom-args/i3246.scala", scala2Mode) +