Improve opaque types

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

@@ -818,47 +818,6 @@ object desugar {
     }
   }
 
-  /** Expand
-   *
-   *    <mods> opaque type T = [Xs] =>> R
-   *
-   *  to
-   *
-   *    <mods> opaque type T = T.T
-   *    synthetic object T {
-   *      synthetic opaque type T >: [Xs] =>> R
-   *    }
-   *
-   *  The generated companion object will later (in Namer) be merged with the user-defined
-   *  companion object, and the synthetic opaque type member will go into the self type.
-   */
-  def opaqueAlias(tdef: TypeDef)(implicit ctx: Context): Tree =
-    if (lacksDefinition(tdef)) {
-      ctx.error(em"opaque type ${tdef.name} must be an alias type", tdef.sourcePos)
-      tdef.withFlags(tdef.mods.flags &~ Opaque)
-    }
-    else {
-      def completeForwarder(fwd: Tree) = tdef.rhs match {
-        case LambdaTypeTree(tparams, tpt) =>
-          val tparams1 =
-            for (tparam <- tparams)
-            yield tparam.withMods(tparam.mods | Synthetic)
-          lambdaAbstract(tparams1,
-            AppliedTypeTree(fwd, tparams.map(tparam => Ident(tparam.name))))
-        case _ =>
-          fwd
-      }
-      val moduleName = tdef.name.toTermName
-      val localRef = Select(Ident(moduleName), tdef.name).withAttachment(SuppressAccessCheck, ())
-      val aliasType = cpy.TypeDef(tdef)(rhs = completeForwarder(localRef)).withSpan(tdef.span.startPos)
-      val localType = tdef.withMods(Modifiers(Synthetic | Opaque).withPrivateWithin(tdef.name))
-
-      val companions = moduleDef(ModuleDef(
-        moduleName, Template(emptyConstructor, Nil, Nil, EmptyValDef, localType :: Nil))
-          .withFlags(Synthetic | Opaque))
-      Thicket(aliasType :: companions.toList)
-    }
-
   /** The normalized name of `mdef`. This means
    *   1. Check that the name does not redefine a Scala core class.
    *      If it does redefine, issue an error and return a mangled name instead of the original one.
@@ -1035,19 +994,51 @@ object desugar {
     Bind(name, Ident(nme.WILDCARD)).withSpan(tree.span)
   }
 
-  def defTree(tree: Tree)(implicit ctx: Context): Tree = tree match {
-    case tree: ValDef => valDef(tree)
-    case tree: TypeDef =>
-      if (tree.isClassDef) classDef(tree)
-      else if (tree.mods.is(Opaque, butNot = Synthetic)) opaqueAlias(tree)
-      else tree
-    case tree: DefDef =>
-      if (tree.name.isConstructorName) tree // was already handled by enclosing classDef
-      else defDef(tree)
-    case tree: ModuleDef => moduleDef(tree)
-    case tree: PatDef => patDef(tree)
+  /** The type of tests that check whether a MemberDef is OK for some flag.
+   *  The test succeeds if the partial function is defined and returns true.
+   */
+  type MemberDefTest = PartialFunction[MemberDef, Boolean]
+
+  val legalOpaque: MemberDefTest = {
+    case TypeDef(_, rhs) =>
+      def rhsOK(tree: Tree): Boolean = tree match {
+        case _: TypeBoundsTree | _: Template => false
+        case LambdaTypeTree(_, body) => rhsOK(body)
+        case _ => true
+      }
+      rhsOK(rhs)
   }
 
+  /** Check that modifiers are legal for the definition `tree`.
+   *  Right now, we only check for `opaque`. TODO: Move other modifier checks here.
+   */
+  def checkModifiers(tree: Tree)(implicit ctx: Context): Tree = tree match {
+    case tree: MemberDef =>
+      var tested: MemberDef = tree
+      def fail(msg: String) = ctx.error(msg, tree.sourcePos)
+      def checkApplicable(flag: FlagSet, test: MemberDefTest): Unit =
+        if (tested.mods.is(flag) && !test.applyOrElse(tree, _ => false)) {
+          fail(i"modifier `$flag` is not allowed for this definition")
+          tested = tested.withMods(tested.mods.withoutFlags(flag))
+        }
+      checkApplicable(Opaque, legalOpaque)
+      tested
+    case _ =>
+      tree
+  }
+
+  def defTree(tree: Tree)(implicit ctx: Context): Tree =
+    checkModifiers(tree) match {
+      case tree: ValDef => valDef(tree)
+      case tree: TypeDef =>
+        if (tree.isClassDef) classDef(tree) else tree
+      case tree: DefDef =>
+        if (tree.name.isConstructorName) tree // was already handled by enclosing classDef
+        else defDef(tree)
+      case tree: ModuleDef => moduleDef(tree)
+      case tree: PatDef => patDef(tree)
+    }
+
   /**     { stats; <empty > }
    *  ==>
    *      { stats; () }

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

@@ -818,6 +818,8 @@ object Trees {
     extends ProxyTree[T] {
     type ThisTree[-T >: Untyped] = Annotated[T]
     def forwardTo: Tree[T] = arg
+    override def disableOverlapChecks = true
+      // disable overlaps checks since the WithBounds annotation swaps type and annotation.
   }
 
   trait WithoutTypeOrPos[-T >: Untyped] extends Tree[T] {

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

@@ -206,8 +206,12 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def DefDef(sym: TermSymbol, tparams: List[TypeSymbol], vparamss: List[List[TermSymbol]],
              resultType: Type, rhs: Tree)(implicit ctx: Context): DefDef =
     ta.assignType(
-      untpd.DefDef(sym.name, tparams map TypeDef, vparamss.nestedMap(ValDef(_)),
-                   TypeTree(resultType), rhs),
+      untpd.DefDef(
+        sym.name,
+        tparams.map(tparam => TypeDef(tparam).withSpan(tparam.span)),
+        vparamss.nestedMap(vparam => ValDef(vparam).withSpan(vparam.span)),
+        TypeTree(resultType),
+        rhs),
       sym)
 
   def DefDef(sym: TermSymbol, rhs: Tree = EmptyTree)(implicit ctx: Context): DefDef =

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

@@ -198,6 +198,11 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       if (this.flags == flags) this
       else copy(flags = flags)
 
+    def withoutFlags(flags: FlagSet): Modifiers =
+      if (this.is(flags))
+        Modifiers(this.flags &~ flags, this.privateWithin, this.annotations, this.mods.filterNot(_.flags.is(flags)))
+      else this
+
     def withAddedMod(mod: Mod): Modifiers =
       if (mods.exists(_ eq mod)) this
       else withMods(mods :+ mod)

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

@@ -171,12 +171,27 @@ object Annotations {
 
       def unapply(ann: Annotation)(implicit ctx: Context): Option[Symbol] =
         if (ann.symbol == defn.ChildAnnot) {
-          val AppliedType(tycon, (arg: NamedType) :: Nil) = ann.tree.tpe
+          val AppliedType(_, (arg: NamedType) :: Nil) = ann.tree.tpe
           Some(arg.symbol)
         }
         else None
     }
 
+    /** Extractor for WithBounds[T] annotations */
+    object WithBounds {
+      def unapply(ann: Annotation)(implicit ctx: Context): Option[TypeBounds] =
+        if (ann.symbol == defn.WithBoundsAnnot) {
+          // We need to extract the type of the type tree in the New itself.
+          // The annotation's type has been simplified as the type of an expression,
+          // which means that `&` or `|` might have been lost.
+          // Test in pos/reference/opaque.scala
+          val Apply(TypeApply(Select(New(tpt), nme.CONSTRUCTOR), _), Nil) = ann.tree
+          val AppliedType(_, lo :: hi :: Nil) = tpt.tpe
+          Some(TypeBounds(lo, hi))
+        }
+        else None
+    }
+
     def makeSourceFile(path: String)(implicit ctx: Context): Annotation =
       apply(defn.SourceFileAnnot, Literal(Constant(path)))
   }

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

@@ -145,25 +145,16 @@ class CheckRealizable(implicit ctx: Context) {
    */
   private def boundsRealizability(tp: Type) = {
 
-    def isOpaqueCompanionThis = tp match {
-      case tp: ThisType => tp.cls.isOpaqueCompanion
-      case _ => false
-    }
-
     val memberProblems =
-      for {
-        mbr <- tp.nonClassTypeMembers
-        if !(mbr.info.loBound <:< mbr.info.hiBound) && !mbr.symbol.isOpaqueHelper
-      }
+      for mbr <- tp.nonClassTypeMembers if !(mbr.info.loBound <:< mbr.info.hiBound)
       yield new HasProblemBounds(mbr.name, mbr.info)
 
     val refinementProblems =
-      for {
+      for
         name <- refinedNames(tp)
         if (name.isTypeName)
         mbr <- tp.member(name).alternatives
-        if !(mbr.info.loBound <:< mbr.info.hiBound) && !isOpaqueCompanionThis
-      }
+        if !(mbr.info.loBound <:< mbr.info.hiBound)
       yield new HasProblemBounds(name, mbr.info)
 
     def baseTypeProblems(base: Type) = base match {

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

@@ -877,6 +877,8 @@ class Definitions {
   def BodyAnnot(implicit ctx: Context): ClassSymbol = BodyAnnotType.symbol.asClass
   @threadUnsafe lazy val ChildAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.internal.Child")
   def ChildAnnot(implicit ctx: Context): ClassSymbol = ChildAnnotType.symbol.asClass
+  @threadUnsafe lazy val WithBoundsAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.internal.WithBounds")
+  def WithBoundsAnnot(implicit ctx: Context): ClassSymbol = WithBoundsAnnotType.symbol.asClass
   @threadUnsafe lazy val CovariantBetweenAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.internal.CovariantBetween")
   def CovariantBetweenAnnot(implicit ctx: Context): ClassSymbol = CovariantBetweenAnnotType.symbol.asClass
   @threadUnsafe lazy val ContravariantBetweenAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.internal.ContravariantBetween")

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

@@ -268,7 +268,7 @@ object Flags {
   /** A mutable var */
   final val Mutable: FlagSet = termFlag(12, "mutable")
 
-  /** An opqaue type */
+  /** An opaque type or a class containing one */
   final val Opaque: FlagSet = typeFlag(12, "opaque")
 
   final val MutableOrOpaque: FlagSet = Mutable.toCommonFlags
@@ -550,13 +550,7 @@ object Flags {
     Accessor | AbsOverride | StableRealizable | Captured | Synchronized | Erased
 
   /** Flags that can apply to a module class */
-  final val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags |
-    Enum | Opaque
-
-  /** Flags that are copied from a synthetic companion to a user-defined one
-   *  when the two are merged. See: Namer.mergeCompanionDefs
-   */
-  final val RetainedSyntheticCompanionFlags: FlagSet = Opaque
+  final val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags | Enum
 
   /** Packages and package classes always have these flags set */
   final val PackageCreationFlags: FlagSet =
@@ -687,9 +681,6 @@ object Flags {
   /** A Java companion object */
   final val JavaModule: FlagConjunction = allOf(JavaDefined, Module)
 
-  /** An opaque companion object */
-  final val OpaqueModule: FlagConjunction = allOf(Opaque, Module)
-
   /** A Java companion object */
   final val JavaProtected: FlagConjunction = allOf(JavaDefined, Protected)
 
@@ -739,7 +730,6 @@ object Flags {
   final val SyntheticTermParam: FlagConjunction = allOf(Synthetic, TermParam)
   final val SyntheticTypeParam: FlagConjunction = allOf(Synthetic, TypeParam)
   final val SyntheticCase: FlagConjunction = allOf(Synthetic, Case)
-  final val SyntheticOpaque: FlagConjunction = allOf(Synthetic, Opaque)
 
   implicit def conjToFlagSet(conj: FlagConjunction): FlagSet =
     FlagSet(conj.bits)

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

@@ -372,18 +372,38 @@ object SymDenotations {
       case _ => unforcedDecls.openForMutations
     }
 
-    /** If this is a synthetic opaque type alias, mark it as Deferred with empty bounds
+    /** If this is a synthetic opaque type alias, mark it as Deferred with bounds
+     *  as given by the right hand side's `WithBounds` annotation, if one is present,
+     *  or with empty bounds of the right kind, otherwise.
+     *  At the same time, integrate the original alias as a refinement of the
+     *  self type of the enclosing class.
      */
     final def normalizeOpaque()(implicit ctx: Context) = {
       def abstractRHS(tp: Type): Type = tp match {
         case tp: HKTypeLambda => tp.derivedLambdaType(resType = abstractRHS(tp.resType))
         case _ => defn.AnyType
       }
-      if (isOpaqueHelper) {
+      if (isOpaqueAlias) {
         info match {
           case TypeAlias(alias) =>
-            info = TypeBounds(defn.NothingType, abstractRHS(alias))
+            val (refiningAlias, bounds) = alias match {
+              case AnnotatedType(alias1, Annotation.WithBounds(bounds)) =>
+              	(alias1, bounds)
+              case _ =>
+              	(alias, TypeBounds(defn.NothingType, abstractRHS(alias)))
+            }
+            def refineSelfType(selfType: Type) =
+              RefinedType(selfType, name, TypeAlias(refiningAlias))
+            val enclClassInfo = owner.asClass.classInfo
+            enclClassInfo.selfInfo match {
+              case self: Type =>
+                owner.info = enclClassInfo.derivedClassInfo(selfInfo = refineSelfType(self))
+              case self: Symbol =>
+                self.info = refineSelfType(self.info)
+            }
+            info = bounds
             setFlag(Deferred)
+            typeRef.recomputeDenot()
           case _ =>
         }
       }
@@ -553,18 +573,14 @@ object SymDenotations {
     final def isAbstractOrParamType(implicit ctx: Context): Boolean = this is DeferredOrTypeParam
 
     /** Is this symbol a user-defined opaque alias type? */
-    def isOpaqueAlias(implicit ctx: Context): Boolean = is(Opaque, butNot = Synthetic)
-
-    /** Is this symbol the companion of an opaque alias type? */
-    def isOpaqueCompanion(implicit ctx: Context): Boolean = is(OpaqueModule)
+    def isOpaqueAlias(implicit ctx: Context): Boolean = is(Opaque) && !isClass
 
-    /** Is this symbol a synthetic opaque type inside an opaque companion object? */
-    def isOpaqueHelper(implicit ctx: Context): Boolean = is(SyntheticOpaque, butNot = Module)
+    /** Is this symbol a module that contains opaque aliases? */
+    def containsOpaques(implicit ctx: Context): Boolean = is(Opaque) && isClass
 
-    /** Can this symbol have a companion module?
-     *  This is the case if it is a class or an opaque type alias.
-     */
-    final def canHaveCompanion(implicit ctx: Context) = isClass || isOpaqueAlias
+    def seesOpaques(implicit ctx: Context): Boolean =
+      containsOpaques ||
+      is(Module, butNot = Package) && owner.containsOpaques
 
     /** Is this the denotation of a self symbol of some class?
      *  This is the case if one of two conditions holds:
@@ -789,7 +805,7 @@ object SymDenotations {
      */
     def membersNeedAsSeenFrom(pre: Type)(implicit ctx: Context): Boolean =
       !(  this.isTerm
-       || this.isStaticOwner && !this.isOpaqueCompanion
+       || this.isStaticOwner && !this.seesOpaques
        || ctx.erasedTypes
        || (pre eq NoPrefix)
        || (pre eq thisType)
@@ -1029,16 +1045,6 @@ object SymDenotations {
      */
     final def companionModule(implicit ctx: Context): Symbol =
       if (is(Module)) sourceModule
-      else if (isOpaqueAlias) {
-        def reference(tp: Type): Symbol = tp match {
-          case TypeRef(prefix: TermRef, _) => prefix.termSymbol
-          case tp: HKTypeLambda => reference(tp.resType)
-          case tp: AppliedType => reference(tp.tycon)
-          case tp: ErrorType => registeredCompanion.sourceModule
-        }
-        val TypeAlias(alias) = info
-        reference(alias)
-      }
       else registeredCompanion.sourceModule
 
     private def companionType(implicit ctx: Context): Symbol =
@@ -1053,13 +1059,6 @@ object SymDenotations {
     final def companionClass(implicit ctx: Context): Symbol =
       companionType.suchThat(_.isClass).symbol
 
-    /** The opaque type with the same (type-) name as this module or module class,
-     *  and which is also defined in the same scope and compilation unit.
-     *  NoSymbol if this type does not exist.
-     */
-    final def companionOpaqueType(implicit ctx: Context): Symbol =
-      companionType.suchThat(_.isOpaqueAlias).symbol
-
     final def scalacLinkedClass(implicit ctx: Context): Symbol =
       if (this is ModuleClass) companionNamed(effectiveName.toTypeName)
       else if (this.isClass) companionNamed(effectiveName.moduleClassName).sourceModule.moduleClass
@@ -1109,15 +1108,17 @@ object SymDenotations {
     final def enclosingSubClass(implicit ctx: Context): Symbol =
       ctx.owner.ownersIterator.findSymbol(_.isSubClass(symbol))
 
-    /** The alias of a synthetic opaque type that's stored in the self type of the
+    /** The alias of an opaque type alias that's stored in the self type of the
      *  containing object.
      */
     def opaqueAlias(implicit ctx: Context): Type = {
-      if (isOpaqueHelper)
-        owner.asClass.classInfo.selfType match {
-          case RefinedType(_, _, bounds) => bounds.extractOpaqueAlias
-        }
-      else NoType
+      def recur(tp: Type): Type = tp match {
+        case RefinedType(parent, rname, TypeAlias(alias)) =>
+          if (rname == name) alias else recur(parent)
+        case _ =>
+          NoType
+      }
+      recur(owner.asClass.classInfo.selfType)
     }
 
     /** The non-private symbol whose name and type matches the type of this symbol
@@ -1974,7 +1975,7 @@ object SymDenotations {
 
     /** Register companion class */
     override def registerCompanion(companion: Symbol)(implicit ctx: Context) =
-      if (companion.canHaveCompanion && !unforcedIsAbsent && !companion.unforcedIsAbsent)
+      if (companion.isClass && !unforcedIsAbsent && !companion.unforcedIsAbsent)
         myCompanion = companion
 
     override def registeredCompanion(implicit ctx: Context) = { ensureCompleted(); myCompanion }