Merge pull request #8082 from dotty-staging/change-lambda-variance

Rework variances of higher-kinded types

Author: odersky

compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala

@@ -49,13 +49,13 @@ object DesugarEnums {
     val tparams = enumClass.typeParams
     def isGround(tp: Type) = tp.subst(tparams, tparams.map(_ => NoType)) eq tp
     val targs = tparams map { tparam =>
-      if (tparam.variance > 0 && isGround(tparam.info.bounds.lo))
+      if (tparam.is(Covariant) && isGround(tparam.info.bounds.lo))
         tparam.info.bounds.lo
-      else if (tparam.variance < 0 && isGround(tparam.info.bounds.hi))
+      else if (tparam.is(Contravariant) && isGround(tparam.info.bounds.hi))
         tparam.info.bounds.hi
       else {
         def problem =
-          if (tparam.variance == 0) "is non variant"
+          if (!tparam.isOneOf(VarianceFlags)) "is non variant"
           else "has bounds that depend on a type parameter in the same parameter list"
         errorType(i"""cannot determine type argument for enum parent $enumClass,
                      |type parameter $tparam $problem""", ctx.source.atSpan(span))

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

@@ -379,8 +379,9 @@ trait ConstraintHandling[AbstractContext] {
           case bounds: TypeBounds =>
             val lower = constraint.lower(param)
             val upper = constraint.upper(param)
-            if (lower.nonEmpty && !bounds.lo.isRef(defn.NothingClass) ||
-              upper.nonEmpty && !bounds.hi.isRef(defn.AnyClass)) constr_println(i"INIT*** $tl")
+            if lower.nonEmpty && !bounds.lo.isRef(defn.NothingClass)
+               || upper.nonEmpty && !bounds.hi.isAny
+            then constr_println(i"INIT*** $tl")
             lower.forall(addOneBound(_, bounds.hi, isUpper = true)) &&
               upper.forall(addOneBound(_, bounds.lo, isUpper = false))
           case _ =>

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

@@ -535,7 +535,7 @@ class Definitions {
   @tu lazy val StringModule: Symbol = StringClass.linkedClass
     @tu lazy val String_+ : TermSymbol = enterMethod(StringClass, nme.raw.PLUS, methOfAny(StringType), Final)
     @tu lazy val String_valueOf_Object: Symbol = StringModule.info.member(nme.valueOf).suchThat(_.info.firstParamTypes match {
-      case List(pt) => pt.isRef(AnyClass) || pt.isRef(ObjectClass)
+      case List(pt) => pt.isAny || pt.isAnyRef
       case _ => false
     }).symbol
 

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

@@ -775,7 +775,7 @@ object Denotations {
 
     def matchesImportBound(bound: Type)(implicit ctx: Context): Boolean =
       if bound.isRef(defn.NothingClass) then false
-      else if bound.isRef(defn.AnyClass) then true
+      else if bound.isAny then true
       else NoViewsAllowed.normalizedCompatible(info, bound, keepConstraint = false)
 
     // ------ Transformations -----------------------------------------

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

@@ -155,26 +155,8 @@ final class ProperGadtConstraint private(
           else if (isUpper) addLess(symTvar.origin, boundTvar.origin)
           else addLess(boundTvar.origin, symTvar.origin)
         case bound =>
-          val oldUpperBound = bounds(symTvar.origin)
-          // If we have bounds:
-          //     F >: [t] => List[t] <: [t] => Any
-          // and we want to record that:
-          //     F <: [+A] => List[A]
-          // we need to adapt the variance and instead record that:
-          //     F <: [A] => List[A]
-          // We cannot record the original bound, since it is false that:
-          //     [t] => List[t]  <:  [+A] => List[A]
-          //
-          // Note that the following code is accepted:
-          //     class Foo[F[t] >: List[t]]
-          //     type T = Foo[List]
-          // precisely because Foo[List] is desugared to Foo[[A] => List[A]].
-          //
-          // Ideally we'd adapt the bound in ConstraintHandling#addOneBound,
-          // but doing it there actually interferes with type inference.
-          val bound1 = bound.adaptHkVariances(oldUpperBound)
-          if (isUpper) addUpperBound(symTvar.origin, bound1)
-          else addLowerBound(symTvar.origin, bound1)
+          if (isUpper) addUpperBound(symTvar.origin, bound)
+          else addLowerBound(symTvar.origin, bound)
       }
     ).reporting({
       val descr = if (isUpper) "upper" else "lower"

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

@@ -6,7 +6,7 @@ import scala.util.hashing.{ MurmurHash3 => hashing }
 import annotation.tailrec
 
 object Hashable {
- 
+
   /** A null terminated list of BindingTypes. We use `null` here for efficiency */
   class Binders(val tp: BindingType, val next: Binders)
 
@@ -44,7 +44,7 @@ trait Hashable {
     avoidSpecialHashes(hashing.finalizeHash(hashCode, arity))
 
   final def typeHash(bs: Binders, tp: Type): Int =
-    if (bs == null || tp.stableHash) tp.hash else tp.computeHash(bs)
+    if (bs == null || tp.hashIsStable) tp.hash else tp.computeHash(bs)
 
   def identityHash(bs: Binders): Int = avoidSpecialHashes(System.identityHashCode(this))
 
@@ -80,7 +80,7 @@ trait Hashable {
     finishHash(bs, hashing.mix(seed, elemHash), arity + 1, tps)
   }
 
-  
+
   protected final def doHash(x: Any): Int =
     finishHash(hashing.mix(hashSeed, x.hashCode), 1)
 

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

@@ -338,11 +338,6 @@ object NameKinds {
     }
   }
 
-  /** The kind of names that also encode a variance: 0 for contravariance, 1 for covariance. */
-  val VariantName: NumberedNameKind = new NumberedNameKind(VARIANT, "Variant") {
-    def mkString(underlying: TermName, info: ThisInfo) = "-+"(info.num).toString + underlying
-  }
-
   /** Names of the form N_<outer>. Emitted by inliner, replaced by outer path
    *  in ExplicitOuter.
    */

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

@@ -139,32 +139,8 @@ object NameOps {
       name.replace { case ExpandedName(_, unexp) => unexp }
     }
 
-    /** Remove the variance from the name. */
-    def invariantName: N = likeSpacedN {
-      name.replace { case VariantName(invariant, _) => invariant }
-    }
-
     def errorName: N = likeSpacedN(name ++ nme.ERROR)
 
-    /** Map variance value -1, +1 to 0, 1 */
-    private def varianceToNat(v: Int) = (v + 1) / 2
-
-    /** Map 0, 1 to variance value -1, +1 */
-    private def natToVariance(n: Int) = n * 2 - 1
-
-    /** Name with variance prefix: `+` for covariant, `-` for contravariant */
-    def withVariance(v: Int): N = {
-      val underlying = name.exclude(VariantName)
-      likeSpacedN(
-          if (v == 0) underlying
-          else VariantName(underlying.toTermName, varianceToNat(v)))
-    }
-
-    /** The variance as implied by the variance prefix, or 0 if there is
-     *  no variance prefix.
-     */
-    def variance: Int = name.collect { case VariantName(_, n) => natToVariance(n) }.getOrElse(0)
-
     def freshened(implicit ctx: Context): N = likeSpacedN {
       name.toTermName match {
         case ModuleClassName(original) => ModuleClassName(original.freshened)

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

@@ -45,7 +45,6 @@ object NameTags extends TastyFormat.NameTags {
     case TRAITSETTER => "TRAITSETTER"
     case UNIQUE => "UNIQUE"
     case DEFAULTGETTER => "DEFAULTGETTER"
-    case VARIANT => "VARIANT"
     case OUTERSELECT => "OUTERSELECT"
 
     case SUPERACCESSOR => "SUPERACCESSOR"

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

@@ -3,6 +3,7 @@ package dotty.tools.dotc.core
 import Names.Name
 import Contexts.Context
 import Types.Type
+import Variances.{Variance, varianceToInt}
 
 /** A common super trait of Symbol and LambdaParam.
  *  Used to capture the attributes of type parameters which can be implemented as either.
@@ -35,7 +36,13 @@ trait ParamInfo {
   def paramInfoOrCompleter(implicit ctx: Context): Type
 
   /** The variance of the type parameter */
-  def paramVariance(implicit ctx: Context): Int
+  def paramVariance(implicit ctx: Context): Variance
+
+  /** The variance of the type parameter, as a number -1, 0, +1.
+   *  Bivariant is mapped to 1, i.e. it is treated like Covariant.
+   */
+  final def paramVarianceSign(implicit ctx: Context): Int =
+    varianceToInt(paramVariance)
 
   /** A type that refers to the parameter */
   def paramRef(implicit ctx: Context): Type

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

@@ -191,7 +191,7 @@ trait PatternTypeConstrainer { self: TypeComparer =>
       def apply(tp: Type) = mapOver(tp) match {
         case tp @ AppliedType(tycon, args) =>
           val args1 = args.zipWithConserve(tycon.typeParams)((arg, tparam) =>
-            if (tparam.paramVariance != 0) TypeBounds.empty else arg
+            if (tparam.paramVarianceSign != 0) TypeBounds.empty else arg
           )
           tp.derivedAppliedType(tycon, args1)
         case tp =>

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

@@ -12,6 +12,7 @@ import dotty.tools.io.AbstractFile
 import Decorators.SymbolIteratorDecorator
 import ast._
 import Trees.Literal
+import Variances.Variance
 import annotation.tailrec
 import util.SimpleIdentityMap
 import util.Stats
@@ -1371,13 +1372,13 @@ object SymDenotations {
     def namedType(implicit ctx: Context): NamedType =
       if (isType) typeRef else termRef
 
-    /** The variance of this type parameter or type member as an Int, with
-     *  +1 = Covariant, -1 = Contravariant, 0 = Nonvariant, or not a type parameter
+    /** The variance of this type parameter or type member as a subset of
+     *  {Covariant, Contravariant}
      */
-    final def variance(implicit ctx: Context): Int =
-      if (this.is(Covariant)) 1
-      else if (this.is(Contravariant)) -1
-      else 0
+    final def variance(implicit ctx: Context): Variance =
+      if is(Covariant) then Covariant
+      else if is(Contravariant) then Contravariant
+      else EmptyFlags
 
     /** The flags to be used for a type parameter owned by this symbol.
      *  Overridden by ClassDenotation.

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

@@ -22,6 +22,7 @@ import ast.tpd
 import tpd.{Tree, TreeProvider, TreeOps}
 import ast.TreeTypeMap
 import Constants.Constant
+import Variances.{Variance, varianceFromInt}
 import reporting.diagnostic.Message
 import collection.mutable
 import io.AbstractFile
@@ -699,7 +700,7 @@ object Symbols {
     def paramInfo(implicit ctx: Context): Type = denot.info
     def paramInfoAsSeenFrom(pre: Type)(implicit ctx: Context): Type = pre.memberInfo(this)
     def paramInfoOrCompleter(implicit ctx: Context): Type = denot.infoOrCompleter
-    def paramVariance(implicit ctx: Context): Int = denot.variance
+    def paramVariance(implicit ctx: Context): Variance = denot.variance
     def paramRef(implicit ctx: Context): TypeRef = denot.typeRef
 
 // -------- Printing --------------------------------------------------------

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

@@ -10,6 +10,7 @@ import Decorators._
 import util.Stats._
 import Names._
 import NameOps._
+import Variances.variancesConform
 import dotty.tools.dotc.config.Config
 
 object TypeApplications {
@@ -22,24 +23,6 @@ object TypeApplications {
     case _ => tp
   }
 
-  /** Does variance `v1` conform to variance `v2`?
-   *  This is the case if the variances are the same or `sym` is nonvariant.
-   */
-  def varianceConforms(v1: Int, v2: Int): Boolean =
-    v1 == v2 || v2 == 0
-
-  /** Does the variance of type parameter `tparam1` conform to the variance of type parameter `tparam2`?
-   */
-  def varianceConforms(tparam1: TypeParamInfo, tparam2: TypeParamInfo)(implicit ctx: Context): Boolean =
-    varianceConforms(tparam1.paramVariance, tparam2.paramVariance)
-
-  /** Do the variances of type parameters `tparams1` conform to the variances
-   *  of corresponding type parameters `tparams2`?
-   *  This is only the case of `tparams1` and `tparams2` have the same length.
-   */
-  def variancesConform(tparams1: List[TypeParamInfo], tparams2: List[TypeParamInfo])(implicit ctx: Context): Boolean =
-    tparams1.corresponds(tparams2)(varianceConforms)
-
   /** Extractor for
    *
    *    [v1 X1: B1, ..., vn Xn: Bn] -> C[X1, ..., Xn]
@@ -156,8 +139,6 @@ class TypeApplications(val self: Type) extends AnyVal {
   /** The type parameters of this type are:
    *  For a ClassInfo type, the type parameters of its class.
    *  For a typeref referring to a class, the type parameters of the class.
-   *  For a typeref referring to a Lambda class, the type parameters of
-   *    its right hand side or upper bound.
    *  For a refinement type, the type parameters of its parent, dropping
    *  any type parameter that is-rebound by the refinement.
    */
@@ -269,11 +250,9 @@ class TypeApplications(val self: Type) extends AnyVal {
   /** Convert a type constructor `TC` which has type parameters `X1, ..., Xn`
    *  to `[X1, ..., Xn] -> TC[X1, ..., Xn]`.
    */
-  def EtaExpand(tparams: List[TypeSymbol])(implicit ctx: Context): Type = {
-    val tparamsToUse = if (variancesConform(typeParams, tparams)) tparams else typeParamSymbols
-    HKTypeLambda.fromParams(tparamsToUse, self.appliedTo(tparams.map(_.typeRef)))
+  def EtaExpand(tparams: List[TypeSymbol])(implicit ctx: Context): Type =
+    HKTypeLambda.fromParams(tparams, self.appliedTo(tparams.map(_.typeRef)))
       //.ensuring(res => res.EtaReduce =:= self, s"res = $res, core = ${res.EtaReduce}, self = $self, hc = ${res.hashCode}")
-  }
 
   /** If self is not lambda-bound, eta expand it. */
   def ensureLambdaSub(implicit ctx: Context): Type =
@@ -290,62 +269,6 @@ class TypeApplications(val self: Type) extends AnyVal {
     }
   }
 
-  /** 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 hkParams = bound.hkTypeParams
-    if (hkParams.isEmpty) self
-    else {
-      def adaptArg(arg: Type): Type = arg match {
-        case arg @ HKTypeLambda(tparams, body) if
-             !tparams.corresponds(hkParams)(_.paramVariance == _.paramVariance) &&
-             tparams.corresponds(hkParams)(varianceConforms) =>
-          HKTypeLambda(
-            tparams.lazyZip(hkParams).map((tparam, hkparam) =>
-              tparam.paramName.withVariance(hkparam.paramVariance)))(
-            tl => arg.paramInfos.map(_.subst(arg, tl).bounds),
-            tl => arg.resultType.subst(arg, tl)
-          )
-        case arg: AliasingBounds =>
-          arg.derivedAlias(adaptArg(arg.alias))
-        case arg @ TypeBounds(lo, hi) =>
-          arg.derivedTypeBounds(adaptArg(lo), adaptArg(hi))
-        case _ =>
-          arg
-      }
-      adaptArg(self)
-    }
-  }
-
   /** The type representing
    *
    *     T[U1, ..., Un]