Always charge deep capture set of function arguments to cv

Also: In Recheck-apply, use deep capture sets of arguments in computing the
result alternative.

Drop restrictions on leaking reach capabilities in markFree.

Revise visibility criterion for paths.

Together these changes now implement a classical capability system with reach capabilities.
References that are used later after passing some arguments are already recorded in capture sets
of earlier stages (exception: closure results).

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

@@ -274,6 +274,11 @@ extension (tp: Type)
     case _ =>
       tp
 
+  /** The first element of this path type */
+  final def pathRoot(using Context): Type = tp.dealias match
+    case tp1: NamedType if tp1.symbol.owner.isClass => tp1.prefix.pathRoot
+    case _ => tp
+
   /** If this is a unboxed capturing type with nonempty capture set, its boxed version.
    *  Or, if type is a TypeBounds of capturing types, the version where the bounds are boxed.
    *  The identity for all other types.

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

@@ -385,34 +385,8 @@ class CheckCaptures extends Recheck, SymTransformer:
         // Only captured references that are visible from the environment
         // should be included.
         val included = cs.filter: c =>
-          c.stripReach match
-            case ref: NamedType =>
-              val refSym = ref.symbol
-              val refOwner = refSym.owner
-              val isVisible = isVisibleFromEnv(refOwner, env)
-              if isVisible && !ref.isRootCapability then
-                ref match
-                  case ref: TermRef if ref.prefix `ne` NoPrefix =>
-                    // If c is a path of a class defined outside the environment,
-                    // we check the capture set of its info.
-                    checkSubsetEnv(ref.captureSetOfInfo, env)
-                  case _ =>
-              if !isVisible
-                  && (c.isReach || ref.isType)
-                  && (!ccConfig.useSealed || refSym.is(Param))
-                  && refOwner == env.owner
-              then
-                if refSym.hasAnnotation(defn.UnboxAnnot) then
-                  capt.println(i"exempt: $ref in $refOwner")
-                else
-                  // Reach capabilities that go out of scope have to be approximated
-                  // by their underlying capture set, which cannot be universal.
-                  // Reach capabilities of @unboxed parameters are exempted.
-                  val cs = CaptureSet.ofInfo(c)
-                  cs.disallowRootCapability: () =>
-                    report.error(em"Local reach capability $c leaks into capture scope of ${env.ownerString}", pos)
-                  checkSubset(cs, env.captured, pos, provenance(env))
-              isVisible
+          c.stripReach.pathRoot match
+            case ref: NamedType => isVisibleFromEnv(ref.symbol.owner, env)
             case ref: ThisType => isVisibleFromEnv(ref.cls, env)
             case _ => false
         checkSubset(included, env.captured, pos, provenance(env))
@@ -424,48 +398,14 @@ class CheckCaptures extends Recheck, SymTransformer:
     end markFree
 
     /** Include references captured by the called method in the current environment stack */
-    def includeCallCaptures(sym: Symbol, pos: SrcPos)(using Context): Unit =
-      if sym.exists && curEnv.isOpen then markFree(capturedVars(sym), pos)
-
-    private val prefixCalls = util.EqHashSet[GenericApply]()
-    private val unboxedArgs = util.EqHashSet[Tree]()
-
-    def handleCall(meth: Symbol, call: GenericApply, eval: () => Type)(using Context): Type =
-      if prefixCalls.remove(call) then return eval()
-
-      val unboxedParamNames =
-        meth.rawParamss.flatMap: params =>
-          params.collect:
-            case param if param.hasAnnotation(defn.UnboxAnnot) =>
-              param.name
-        .toSet
-
-      def markUnboxedArgs(call: GenericApply): Unit = call.fun.tpe.widen match
-        case MethodType(pnames) =>
-          for (pname, arg) <- pnames.lazyZip(call.args) do
-            if unboxedParamNames.contains(pname) then
-              unboxedArgs.add(arg)
-        case _ =>
-
-      def markPrefixCalls(tree: Tree): Unit = tree match
-        case tree: GenericApply =>
-          prefixCalls.add(tree)
-          markUnboxedArgs(tree)
-          markPrefixCalls(tree.fun)
-        case _ =>
-
-      markUnboxedArgs(call)
-      markPrefixCalls(call.fun)
-      val res = eval()
-      includeCallCaptures(meth, call.srcPos)
-      res
-    end handleCall
+    def includeCallCaptures(sym: Symbol, resType: Type, pos: SrcPos)(using Context): Unit = resType match
+      case _: MethodOrPoly => // wait until method is fully applied
+      case _ =>
+        if sym.exists && curEnv.isOpen then markFree(capturedVars(sym), pos)
 
     override def recheckIdent(tree: Ident, pt: Type)(using Context): Type =
       if tree.symbol.is(Method) then
-        if tree.symbol.info.isParameterless then
-          // there won't be an apply; need to include call captures now
-          includeCallCaptures(tree.symbol, tree.srcPos)
+        includeCallCaptures(tree.symbol, tree.symbol.info, tree.srcPos)
       else if !tree.symbol.isStatic then
         //debugShowEnvs()
         def addSelects(ref: TermRef, pt: Type): TermRef = pt match
@@ -570,15 +510,16 @@ class CheckCaptures extends Recheck, SymTransformer:
             tp.derivedCapturingType(forceBox(parent), refs)
         mapArgUsing(forceBox)
       else
-        handleCall(meth, tree, () => super.recheckApply(tree, pt))
+        val res = super.recheckApply(tree, pt)
+        includeCallCaptures(meth, res, tree.srcPos)
+        res
     end recheckApply
 
     protected override
     def recheckArg(arg: Tree, formal: Type)(using Context): Type =
       val argType = recheck(arg, formal)
-      if unboxedArgs.contains(arg) then
-        capt.println(i"charging deep capture set of $arg: ${argType} = ${argType.deepCaptureSet}")
-        markFree(argType.deepCaptureSet, arg.srcPos)
+      capt.println(i"charging deep capture set of $arg: ${argType} = ${argType.deepCaptureSet}")
+      markFree(argType.deepCaptureSet, arg.srcPos)
       argType
 
     /** A specialized implementation of the apply rule.
@@ -589,27 +530,18 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  ---------------------
      *  E |- f(a): Tr^C
      *
-     *  If the function `f` does not have an `@unboxed` parameter, then
-     *  any unboxing it does would be charged to the environment of the function
-     *  so they have to appear in Cq. Since any capabilities of the result of the
-     *  application must already be present in the application, an upper
-     *  approximation of the result capture set is Cq \union Ca, where `Ca`
-     *  is the capture set of the argument.
-     *  If the function `f` does have an `@unboxed` parameter, then it could in addition
-     *  unbox reach capabilities over its formal parameter. Therefore, the approximation
-     *  would be `Cq \union dcs(Ca)` instead.
+     *  If the type of the function `f` does not mention any formal parameters
+     *  any capabilities of the result of the application must already be present in
+     *  the application. So an upper approximation of the result capture set is Cq \union Ca,
+     *  where `Ca` is the deep capture set of the argument.
      *  If the approximation is known to subcapture the declared result Cr, we pick it for C
-     *  otherwise we pick Cr.
+     *  otherwise we pick Cr. ???
      */
     protected override
     def recheckApplication(tree: Apply, qualType: Type, funType: MethodType, argTypes: List[Type])(using Context): Type =
       val appType = Existential.toCap(super.recheckApplication(tree, qualType, funType, argTypes))
       val qualCaptures = qualType.captureSet
-      val argCaptures =
-        for (arg, argType) <- tree.args.lazyZip(argTypes) yield
-          if unboxedArgs.remove(arg) // need to ensure the remove happens, that's why argCaptures is computed even if not needed.
-          then argType.deepCaptureSet
-          else argType.captureSet
+      val argCaptures = argTypes.map(_.deepCaptureSet)
       appType match
         case appType @ CapturingType(appType1, refs)
         if qualType.exists
@@ -704,8 +636,10 @@ class CheckCaptures extends Recheck, SymTransformer:
               i"Sealed type variable $pname", "be instantiated to",
               i"This is often caused by a local capability$where\nleaking as part of its result.",
               tree.srcPos)
-      try handleCall(meth, tree, () => Existential.toCap(super.recheckTypeApply(tree, pt)))
-      finally checkContains(tree)
+      val res = Existential.toCap(super.recheckTypeApply(tree, pt))
+      includeCallCaptures(meth, res, tree.srcPos)
+      checkContains(tree)
+      res
     end recheckTypeApply
 
     /** Faced with a tree of form `caps.contansImpl[CS, r.type]`, check that `R` is a tracked
@@ -1156,12 +1090,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         (erefs /: erefs.elems): (erefs, eref) =>
           eref match
             case eref: ThisType if isPureContext(ctx.owner, eref.cls) =>
-
-              def pathRoot(aref: Type): Type = aref match
-                case aref: NamedType if aref.symbol.owner.isClass => pathRoot(aref.prefix)
-                case _ => aref
-
-              def isOuterRef(aref: Type): Boolean = pathRoot(aref) match
+              def isOuterRef(aref: Type): Boolean = aref.pathRoot match
                 case aref: NamedType => eref.cls.isProperlyContainedIn(aref.symbol.owner)
                 case aref: ThisType => eref.cls.isProperlyContainedIn(aref.cls)
                 case _ => false
@@ -1171,7 +1100,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               // Include implicitly added outer references in the capture set of the class of `eref`.
               for outerRef <- outerRefs.elems do
                 if !erefs.elems.contains(outerRef)
-                    && !pathRoot(outerRef).isInstanceOf[ThisType]
+                    && !outerRef.pathRoot.isInstanceOf[ThisType]
                     // we don't need to add outer ThisTypes as these are anyway added as path
                     // prefixes at the use site. And this exemption is required since capture sets
                     // of non-local classes are always empty, so we can't add an outer this to them.
@@ -1328,6 +1257,12 @@ class CheckCaptures extends Recheck, SymTransformer:
 
     /** If actual is a tracked CaptureRef `a` and widened is a capturing type T^C,
      *  improve `T^C` to `T^{a}`, following the VAR rule of CC.
+     *  TODO: We probably should do this also for other top-level occurrences of captures
+     *  E.g.
+     *    class Foo { def a: C^{io}; val def: C^{async} }
+     *    val foo: Foo^{io, async}
+     *  Then
+     *    foo: Foo { def a: C^{foo}; def b: C^{foo} }^{foo}
      */
     private def improveCaptures(widened: Type, actual: Type)(using Context): Type = actual match
       case ref: CaptureRef if ref.isTracked =>
@@ -1388,21 +1323,6 @@ class CheckCaptures extends Recheck, SymTransformer:
           !setup.isPreCC(overriding) && !setup.isPreCC(overridden)
 
         override def checkInheritedTraitParameters: Boolean = false
-
-        /** Check that overrides don't change the @unbox status of their parameters */
-        override def additionalChecks(member: Symbol, other: Symbol)(using Context): Unit =
-          for
-            (params1, params2) <- member.rawParamss.lazyZip(other.rawParamss)
-            (param1, param2) <- params1.lazyZip(params2)
-          do
-            if param1.hasAnnotation(defn.UnboxAnnot) != param2.hasAnnotation(defn.UnboxAnnot) then
-              report.error(
-                OverrideError(
-                    i"has a parameter ${param1.name} with different @unbox status than the corresponding parameter in the overridden definition",
-                    self, member, other, self.memberInfo(member), self.memberInfo(other)
-                  ),
-                if member.owner == clazz then member.srcPos else clazz.srcPos
-              )
       end OverridingPairsCheckerCC
 
       def traverse(t: Tree)(using Context) =

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

@@ -1057,7 +1057,6 @@ class Definitions {
   @tu lazy val ExperimentalAnnot: ClassSymbol = requiredClass("scala.annotation.experimental")
   @tu lazy val ThrowsAnnot: ClassSymbol = requiredClass("scala.throws")
   @tu lazy val TransientAnnot: ClassSymbol = requiredClass("scala.transient")
-  @tu lazy val UnboxAnnot:  ClassSymbol = requiredClass("scala.caps.unbox")
   @tu lazy val UncheckedAnnot: ClassSymbol = requiredClass("scala.unchecked")
   @tu lazy val UncheckedStableAnnot: ClassSymbol = requiredClass("scala.annotation.unchecked.uncheckedStable")
   @tu lazy val UncheckedVarianceAnnot: ClassSymbol = requiredClass("scala.annotation.unchecked.uncheckedVariance")

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

@@ -1796,7 +1796,7 @@ object Types extends TypeUtils {
 
     /** Is this either not a method at all, or a parameterless method? */
     final def isParameterless(using Context): Boolean = stripPoly match {
-      case mt: MethodType => false
+      case mt: MethodOrPoly => false
       case _ => true
     }
 

scala2-library-cc/src/scala/collection/immutable/LazyListIterable.scala

@@ -384,7 +384,7 @@ final class LazyListIterable[+A] private(@untrackedCaptures lazyState: () => Laz
     * @param suffix The collection that gets appended to this lazy list
     * @return The lazy list containing elements of this lazy list and the iterable object.
     */
-  def lazyAppendedAll[B >: A](suffix: => collection.IterableOnce[B]^): LazyListIterable[B]^{this, suffix} =
+  def lazyAppendedAll[B >: A](suffix: => collection.IterableOnce[B]^): LazyListIterable[B]^{this, suffix*} =
     newLL {
       if (isEmpty) suffix match {
         case lazyList: LazyListIterable[B]       => lazyList.state // don't recompute the LazyListIterable
@@ -497,7 +497,7 @@ final class LazyListIterable[+A] private(@untrackedCaptures lazyState: () => Laz
     *
     * $preservesLaziness
     */
-  def prepended[B >: A](elem: B): LazyListIterable[B] = newLL(sCons(elem, this))
+  def prepended[B >: A](elem: B): LazyListIterable[B]^{this} = newLL(sCons(elem, this))
 
   /** @inheritdoc
     *
@@ -1137,7 +1137,7 @@ object LazyListIterable extends IterableFactory[LazyListIterable] {
     /** Construct a LazyListIterable consisting of the concatenation of the given LazyListIterable and
       *  another LazyListIterable.
       */
-    def #:::[B >: A](prefix: LazyListIterable[B]^): LazyListIterable[B]^{prefix, l} = prefix lazyAppendedAll l
+    def #:::[B >: A](prefix: LazyListIterable[B]^): LazyListIterable[B]^{prefix, l*} = prefix lazyAppendedAll l
 
   object #:: {
     def unapply[A](s: LazyListIterable[A]^): Option[(A, LazyListIterable[A]^{s})] =
@@ -1155,7 +1155,7 @@ object LazyListIterable extends IterableFactory[LazyListIterable] {
   /** Creates a State from an Iterator, with another State appended after the Iterator
     * is empty.
     */
-  private def stateFromIteratorConcatSuffix[A](it: Iterator[A]^)(suffix: => State[A]^): State[A]^{it, suffix} =
+  private def stateFromIteratorConcatSuffix[A](it: Iterator[A]^)(suffix: => State[A]^): State[A]^{it, suffix*} =
     if (it.hasNext) sCons(it.next(), newLL(stateFromIteratorConcatSuffix(it)(suffix)))
     else suffix
 

tests/neg-custom-args/captures/cc-selftype-unsound.scala

@@ -11,5 +11,5 @@ def magic(l: Logger^): Logger =
       Boxed[Logger^{this}](l)  // error
   val x = new Foo
   val y = x.foo.unbox  // y: Logger^{x}
-  val z: Logger = y  // now the capability becomes pure
+  val z: Logger = y  // error
   z