First implementation of capture polymorphism

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

@@ -521,12 +521,17 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def captureRoot(using Context): Select =
     Select(scalaDot(nme.caps), nme.CAPTURE_ROOT)
 
-  def captureRootIn(using Context): Select =
-    Select(scalaDot(nme.caps), nme.capIn)
-
   def makeRetaining(parent: Tree, refs: List[Tree], annotName: TypeName)(using Context): Annotated =
     Annotated(parent, New(scalaAnnotationDot(annotName), List(refs)))
 
+  def makeCapsOf(id: Ident)(using Context): Tree =
+    TypeApply(Select(scalaDot(nme.caps), nme.capsOf), id :: Nil)
+
+  def makeCapsBound()(using Context): Tree =
+    makeRetaining(
+      Select(scalaDot(nme.caps), tpnme.CapSet),
+      Nil, tpnme.retainsCap)
+
   def makeConstructor(tparams: List[TypeDef], vparamss: List[List[ValDef]], rhs: Tree = EmptyTree)(using Context): DefDef =
     DefDef(nme.CONSTRUCTOR, joinParams(tparams, vparamss), TypeTree(), rhs)
 

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -131,6 +131,8 @@ extension (tree: Tree)
   def toCaptureRef(using Context): CaptureRef = tree match
     case ReachCapabilityApply(arg) =>
       arg.toCaptureRef.reach
+    case CapsOfApply(arg) =>
+      arg.toCaptureRef
     case _ => tree.tpe match
       case ref: CaptureRef if ref.isTrackableRef =>
         ref
@@ -145,7 +147,7 @@ extension (tree: Tree)
       case Some(refs) => refs
       case None =>
         val refs = CaptureSet(tree.retainedElems.map(_.toCaptureRef)*)
-          .showing(i"toCaptureSet $tree --> $result", capt)
+          //.showing(i"toCaptureSet $tree --> $result", capt)
         tree.putAttachment(Captures, refs)
         refs
 
@@ -526,6 +528,14 @@ object ReachCapabilityApply:
     case Apply(reach, arg :: Nil) if reach.symbol == defn.Caps_reachCapability => Some(arg)
     case _ => None
 
+/** An extractor for `caps.capsOf[X]`, which is used to express a generic capture set
+ *  as a tree in a @retains annotation.
+ */
+object CapsOfApply:
+  def unapply(tree: TypeApply)(using Context): Option[Tree] = tree match
+    case TypeApply(capsOf, arg :: Nil) if capsOf.symbol == defn.Caps_capsOf => Some(arg)
+    case _ => None
+
 class AnnotatedCapability(annot: Context ?=> ClassSymbol):
   def apply(tp: Type)(using Context) =
     AnnotatedType(tp, Annotation(annot, util.Spans.NoSpan))

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -879,7 +879,9 @@ object CaptureSet:
     val r1 = tm(r)
     val upper = r1.captureSet
     def isExact =
-      upper.isAlwaysEmpty || upper.isConst && upper.elems.size == 1 && upper.elems.contains(r1)
+      upper.isAlwaysEmpty
+      || upper.isConst && upper.elems.size == 1 && upper.elems.contains(r1)
+      || r.derivesFrom(defn.Caps_CapSet)
     if variance > 0 || isExact then upper
     else if variance < 0 then CaptureSet.empty
     else upper.maybe

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -123,16 +123,24 @@ object CheckCaptures:
       case _: SingletonType =>
         report.error(em"Singleton type $parent cannot have capture set", parent.srcPos)
       case _ =>
+    def check(elem: Tree, pos: SrcPos): Unit = elem.tpe match
+      case ref: CaptureRef =>
+        if !ref.isTrackableRef then
+          report.error(em"$elem cannot be tracked since it is not a parameter or local value", pos)
+      case tpe =>
+        report.error(em"$elem: $tpe is not a legal element of a capture set", pos)
     for elem <- ann.retainedElems do
-      val elem1 = elem match
-        case ReachCapabilityApply(arg) => arg
-        case _ => elem
-      elem1.tpe match
-        case ref: CaptureRef =>
-          if !ref.isTrackableRef then
-            report.error(em"$elem cannot be tracked since it is not a parameter or local value", elem.srcPos)
-        case tpe =>
-          report.error(em"$elem: $tpe is not a legal element of a capture set", elem.srcPos)
+      elem match
+        case CapsOfApply(arg) =>
+          def isLegalCapsOfArg =
+            arg.symbol.isAbstractOrParamType && arg.symbol.info.derivesFrom(defn.Caps_CapSet)
+          if !isLegalCapsOfArg then
+            report.error(
+              em"""$arg is not a legal prefix for `^` here,
+                  |is must be a type parameter or abstract type with a caps.CapSet upper bound.""",
+              elem.srcPos)
+        case ReachCapabilityApply(arg) => check(arg, elem.srcPos)
+        case _ => check(elem, elem.srcPos)
 
   /** Report an error if some part of `tp` contains the root capability in its capture set
    *  or if it refers to an unsealed type parameter that could possibly be instantiated with

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -384,7 +384,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
     sym.updateInfo(thisPhase, info, newFlagsFor(sym))
     toBeUpdated -= sym
     sym.namedType match
-      case ref: CaptureRef => ref.invalidateCaches() // TODO: needed?
+      case ref: CaptureRef if ref.isTrackableRef => ref.invalidateCaches() // TODO: needed?
       case _ =>
 
   extension (sym: Symbol) def nextInfo(using Context): Type =

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

@@ -991,8 +991,10 @@ class Definitions {
 
   @tu lazy val CapsModule: Symbol = requiredModule("scala.caps")
     @tu lazy val captureRoot: TermSymbol = CapsModule.requiredValue("cap")
-    @tu lazy val Caps_Capability: ClassSymbol = requiredClass("scala.caps.Capability")
+    @tu lazy val Caps_Capability: TypeSymbol = CapsModule.requiredType("Capability")
+    @tu lazy val Caps_CapSet = requiredClass("scala.caps.CapSet")
     @tu lazy val Caps_reachCapability: TermSymbol = CapsModule.requiredMethod("reachCapability")
+    @tu lazy val Caps_capsOf: TermSymbol = CapsModule.requiredMethod("capsOf")
     @tu lazy val Caps_Exists = requiredClass("scala.caps.Exists")
     @tu lazy val CapsUnsafeModule: Symbol = requiredModule("scala.caps.unsafe")
     @tu lazy val Caps_unsafeAssumePure: Symbol = CapsUnsafeModule.requiredMethod("unsafeAssumePure")

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

@@ -358,6 +358,7 @@ object StdNames {
     val AppliedTypeTree: N      = "AppliedTypeTree"
     val ArrayAnnotArg: N        = "ArrayAnnotArg"
     val CAP: N                  = "CAP"
+    val CapSet: N               = "CapSet"
     val Constant: N             = "Constant"
     val ConstantType: N         = "ConstantType"
     val Eql: N                  = "Eql"
@@ -441,8 +442,8 @@ object StdNames {
     val bytes: N                = "bytes"
     val canEqual_ : N           = "canEqual"
     val canEqualAny : N         = "canEqualAny"
-    val capIn: N                = "capIn"
     val caps: N                 = "caps"
+    val capsOf: N               = "capsOf"
     val captureChecking: N      = "captureChecking"
     val checkInitialized: N     = "checkInitialized"
     val classOf: N              = "classOf"

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

@@ -2839,7 +2839,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   private def existentialVarsConform(tp1: Type, tp2: Type) =
     tp2 match
       case tp2: TermParamRef => tp1 match
-        case tp1: CaptureRef => subsumesExistentially(tp2, tp1)
+        case tp1: CaptureRef if tp1.isTrackableRef => subsumesExistentially(tp2, tp1)
         case _ => false
       case _ => false
 

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

@@ -2313,7 +2313,11 @@ object Types extends TypeUtils {
 
     override def captureSet(using Context): CaptureSet =
       val cs = captureSetOfInfo
-      if isTrackableRef && !cs.isAlwaysEmpty then singletonCaptureSet else cs
+      if isTrackableRef then
+        if cs.isAlwaysEmpty then cs else singletonCaptureSet
+      else dealias match
+        case _: (TypeRef | TypeParamRef) => CaptureSet.empty
+        case _ => cs
 
   end CaptureRef
 
@@ -3032,7 +3036,7 @@ object Types extends TypeUtils {
 
   abstract case class TypeRef(override val prefix: Type,
                               private var myDesignator: Designator)
-    extends NamedType {
+    extends NamedType, CaptureRef {
 
     type ThisType = TypeRef
     type ThisName = TypeName
@@ -3081,6 +3085,9 @@ object Types extends TypeUtils {
     /** Hook that can be called from creation methods in TermRef and TypeRef */
     def validated(using Context): this.type =
       this
+
+    override def isTrackableRef(using Context) =
+      symbol.isAbstractOrParamType && derivesFrom(defn.Caps_CapSet)
   }
 
   final class CachedTermRef(prefix: Type, designator: Designator, hc: Int) extends TermRef(prefix, designator) {
@@ -4841,7 +4848,8 @@ object Types extends TypeUtils {
   /** Only created in `binder.paramRefs`. Use `binder.paramRefs(paramNum)` to
    *  refer to `TypeParamRef(binder, paramNum)`.
    */
-  abstract case class TypeParamRef(binder: TypeLambda, paramNum: Int) extends ParamRef {
+  abstract case class TypeParamRef(binder: TypeLambda, paramNum: Int)
+  extends ParamRef, CaptureRef {
     type BT = TypeLambda
     def kindString: String = "Type"
     def copyBoundType(bt: BT): Type = bt.paramRefs(paramNum)
@@ -4861,6 +4869,8 @@ object Types extends TypeUtils {
       case bound: OrType   => occursIn(bound.tp1, fromBelow) || occursIn(bound.tp2, fromBelow)
       case _ => false
     }
+
+    override def isTrackableRef(using Context) = derivesFrom(defn.Caps_CapSet)
   }
 
   private final class TypeParamRefImpl(binder: TypeLambda, paramNum: Int) extends TypeParamRef(binder, paramNum)

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1541,7 +1541,7 @@ object Parsers {
       case _ => None
     }
 
-    /** CaptureRef  ::=  ident | `this` | `cap` [`[` ident `]`]
+    /** CaptureRef  ::=  ident [`*` | `^`] | `this`
      */
     def captureRef(): Tree =
       if in.token == THIS then simpleRef()
@@ -1551,6 +1551,10 @@ object Parsers {
           in.nextToken()
           atSpan(startOffset(id)):
             PostfixOp(id, Ident(nme.CC_REACH))
+        else if isIdent(nme.UPARROW) then
+          in.nextToken()
+          atSpan(startOffset(id)):
+            makeCapsOf(cpy.Ident(id)(id.name.toTypeName))
         else id
 
     /**  CaptureSet ::=  `{` CaptureRef {`,` CaptureRef} `}`    -- under captureChecking
@@ -1968,7 +1972,7 @@ object Parsers {
       }
 
     /**  SimpleType      ::=  SimpleLiteral
-     *                     |  ‘?’ SubtypeBounds
+     *                     |  ‘?’ TypeBounds
      *                     |  SimpleType1
      *                     |  SimpleType ‘(’ Singletons ‘)’  -- under language.experimental.dependent, checked in Typer
      *   Singletons      ::=  Singleton {‘,’ Singleton}
@@ -2188,9 +2192,15 @@ object Parsers {
         inBraces(refineStatSeq())
 
     /** TypeBounds ::= [`>:' Type] [`<:' Type]
+     *              |  `^`                     -- under captureChecking
      */
     def typeBounds(): TypeBoundsTree =
-      atSpan(in.offset) { TypeBoundsTree(bound(SUPERTYPE), bound(SUBTYPE)) }
+      atSpan(in.offset):
+        if in.isIdent(nme.UPARROW) && Feature.ccEnabled then
+          in.nextToken()
+          TypeBoundsTree(EmptyTree, makeCapsBound())
+        else
+          TypeBoundsTree(bound(SUPERTYPE), bound(SUBTYPE))
 
     private def bound(tok: Int): Tree =
       if (in.token == tok) { in.nextToken(); toplevelTyp() }
@@ -3384,7 +3394,6 @@ object Parsers {
      *
      *  DefTypeParamClause::=  ‘[’ DefTypeParam {‘,’ DefTypeParam} ‘]’
      *  DefTypeParam      ::=  {Annotation}
-     *                         [`sealed`]                                    -- under captureChecking
      *                         id [HkTypeParamClause] TypeParamBounds
      *
      *  TypTypeParamClause::=  ‘[’ TypTypeParam {‘,’ TypTypeParam} ‘]’

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

@@ -2328,7 +2328,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val res = Throw(expr1).withSpan(tree.span)
     if Feature.ccEnabled && !cap.isEmpty && !ctx.isAfterTyper then
       // Record access to the CanThrow capabulity recovered in `cap` by wrapping
-      // the type of the `throw` (i.e. Nothing) in a `@requiresCapability` annotatoon.
+      // the type of the `throw` (i.e. Nothing) in a `@requiresCapability` annotation.
       Typed(res,
         TypeTree(
           AnnotatedType(res.tpe,

library/src/scala/caps.scala

@@ -1,6 +1,6 @@
 package scala
 
-import annotation.experimental
+import annotation.{experimental, compileTimeOnly}
 
 @experimental object caps:
 
@@ -16,6 +16,12 @@ import annotation.experimental
   @deprecated("Use `Capability` instead")
   type Cap = Capability
 
+  /** Carrier trait for capture set type parameters */
+  trait CapSet extends Any
+
+  @compileTimeOnly("Should be be used only internally by the Scala compiler")
+  def capsOf[CS]: Any = ???
+
   /** Reach capabilities x* which appear as terms in @retains annotations are encoded
    *  as `caps.reachCapability(x)`. When converted to CaptureRef types in capture sets
    *  they are  represented as `x.type @annotation.internal.reachCapability`.

tests/pos/cc-poly-1.scala

@@ -0,0 +1,23 @@
+import language.experimental.captureChecking
+import annotation.experimental
+import caps.{CapSet, Capability}
+
+@experimental object Test:
+
+  class C extends Capability
+  class D
+
+  def f[X^](x: D^{X^}): D^{X^} = x
+  def g[X^](x: D^{X^}, y: D^{X^}): D^{X^} = x
+
+  def test(c1: C, c2: C) =
+    val d: D^{c1, c2} = D()
+    val x = f[CapSet^{c1, c2}](d)
+    val _: D^{c1, c2} = x
+    val d1: D^{c1} = D()
+    val d2: D^{c2} = D()
+    val y = g(d1, d2)
+    val _: D^{d1, d2} = y
+    val _: D^{c1, c2} = y
+
+

tests/pos/cc-poly-source.scala

@@ -0,0 +1,36 @@
+import language.experimental.captureChecking
+import annotation.experimental
+import caps.{CapSet, Capability}
+
+@experimental object Test:
+
+  class Label //extends Capability
+
+  class Listener
+
+  class Source[X^]:
+    private var listeners: Set[Listener^{X^}] = Set.empty
+    def register(x: Listener^{X^}): Unit =
+      listeners += x
+
+    def allListeners: Set[Listener^{X^}] = listeners
+
+  def test1(lbl1: Label^, lbl2: Label^) =
+    val src = Source[CapSet^{lbl1, lbl2}]
+    def l1: Listener^{lbl1} = ???
+    val l2: Listener^{lbl2} = ???
+    src.register{l1}
+    src.register{l2}
+    val ls = src.allListeners
+    val _: Set[Listener^{lbl1, lbl2}] = ls
+
+  def test2(lbls: List[Label^]) =
+    def makeListener(lbl: Label^): Listener^{lbl} = ???
+    val listeners = lbls.map(makeListener)
+    val src = Source[CapSet^{lbls*}]
+    for l <- listeners do
+      src.register(l)
+    val ls = src.allListeners
+    val _: Set[Listener^{lbls*}] = ls
+
+