An alternative Capless-like CC scheme

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

@@ -482,27 +482,30 @@ object Capabilities:
         case info: OrType => viaInfo(info.tp1)(test) && viaInfo(info.tp2)(test)
         case _ => false
 
+      def trySubpath(y: TermRef, trySamePrefix: Boolean = true): Boolean =
+        y.prefix.match
+          case ypre: Capability =>
+            this.subsumes(ypre)
+            || trySamePrefix
+            && this.match
+                case x @ TermRef(xpre: Capability, _) if x.symbol == y.symbol =>
+                  // To show `{x.f} <:< {y.f}`, it is important to prove `x` and `y`
+                  // are equvalent, which means `x =:= y` in terms of subtyping,
+                  // not just `{x} =:= {y}` in terms of subcapturing.
+                  // It is possible to construct two singleton types `x` and `y`,
+                  // which subsume each other, but are not equal references.
+                  // See `tests/neg-custom-args/captures/path-prefix.scala` for example.
+                  withMode(Mode.IgnoreCaptures):
+                    TypeComparer.isSameRef(xpre, ypre)
+                case _ =>
+                  false
+          case _ => false
+
       try (this eq y)
       || maxSubsumes(y, canAddHidden = !vs.isOpen)
       || y.match
         case y: TermRef =>
-            y.prefix.match
-              case ypre: Capability =>
-                this.subsumes(ypre)
-                || this.match
-                    case x @ TermRef(xpre: Capability, _) if x.symbol == y.symbol =>
-                      // To show `{x.f} <:< {y.f}`, it is important to prove `x` and `y`
-                      // are equvalent, which means `x =:= y` in terms of subtyping,
-                      // not just `{x} =:= {y}` in terms of subcapturing.
-                      // It is possible to construct two singleton types `x` and `y`,
-                      // which subsume each other, but are not equal references.
-                      // See `tests/neg-custom-args/captures/path-prefix.scala` for example.
-                      withMode(Mode.IgnoreCaptures):
-                        TypeComparer.isSameRef(xpre, ypre)
-                    case _ =>
-                      false
-              case _ => false
-          || viaInfo(y.info)(subsumingRefs(this, _))
+          trySubpath(y) || viaInfo(y.info)(subsumingRefs(this, _))
         case Maybe(y1) => this.stripMaybe.subsumes(y1)
         case ReadOnly(y1) => this.stripReadOnly.subsumes(y1)
         case y: TypeRef if y.derivesFrom(defn.Caps_CapSet) =>
@@ -516,6 +519,12 @@ object Capabilities:
               this.subsumes(hi)
             case _ =>
               y.captureSetOfInfo.elems.forall(this.subsumes)
+        case Reach(y1: TermRef) =>
+          def isClassFunctionParam: Boolean =
+            def isClassParam = y1.symbol.is(ParamAccessor)
+            def isFunctionType = defn.isFunctionType(y1.widenDealias)
+            isClassParam && isFunctionType
+          isClassFunctionParam && trySubpath(y1, trySamePrefix = false)
         case _ => false
       || this.match
           case Reach(x1) => x1.subsumes(y.stripReach)

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

@@ -766,21 +766,7 @@ class CheckCaptures extends Recheck, SymTransformer:
       val appType = resultToFresh(
         super.recheckApplication(tree, qualType, funType, argTypes),
         Origin.ResultInstance(funType, tree.symbol))
-      val qualCaptures = qualType.captureSet
-      val argCaptures =
-        for (argType, formal) <- argTypes.lazyZip(funType.paramInfos) yield
-          if formal.hasAnnotation(defn.UseAnnot) then argType.deepCaptureSet else argType.captureSet
-      appType match
-        case appType @ CapturingType(appType1, refs)
-        if qualType.exists
-            && !tree.fun.symbol.isConstructor
-            && qualCaptures.mightSubcapture(refs)
-            && argCaptures.forall(_.mightSubcapture(refs)) =>
-          val callCaptures = argCaptures.foldLeft(qualCaptures)(_ ++ _)
-          appType.derivedCapturingType(appType1, callCaptures)
-            .showing(i"narrow $tree: $appType, refs = $refs, qual-cs = ${qualType.captureSet} = $result", capt)
-        case appType =>
-          appType
+      appType
 
     private def isDistinct(xs: List[Type]): Boolean = xs match
       case x :: xs1 => xs1.isEmpty || !xs1.contains(x) && isDistinct(xs1)
@@ -832,8 +818,12 @@ class CheckCaptures extends Recheck, SymTransformer:
         for (getterName, argType) <- mt.paramNames.lazyZip(argTypes) do
           val getter = cls.info.member(getterName).suchThat(_.isRefiningParamAccessor).symbol
           if !getter.is(Private) && getter.hasTrackedParts then
-            refined = refined.refinedOverride(getterName, argType.unboxed) // Yichen you might want to check this
-            allCaptures ++= argType.captureSet
+            refined = refined.refinedOverride(getterName, argType.unboxed) // TODO: This looks unsound.
+                                                                           // Try to find an counter-example.
+            if defn.isFunctionType(argType.widenDealias) then
+              allCaptures ++= argType.deepCaptureSet
+            else
+              allCaptures ++= argType.captureSet
         (refined, allCaptures)
 
       /** Augment result type of constructor with refinements and captures.

scala2-library-cc/src/scala/collection/Iterable.scala

@@ -684,9 +684,9 @@ trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] with Iterable
 
   def map[B](f: A => B): CC[B]^{this, f} = iterableFactory.from(new View.Map(this, f))
 
-  def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f} = iterableFactory.from(new View.FlatMap(this, f))
+  def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f*} = iterableFactory.from(new View.FlatMap(this, f))
 
-  def flatten[B](implicit asIterable: A -> IterableOnce[B]): CC[B]^{this} = flatMap(asIterable)
+  def flatten[B](implicit asIterable: A -> IterableOnce[B]): CC[B]^{this, asIterable*} = flatMap(asIterable)
 
   def collect[B](pf: PartialFunction[A, B]^): CC[B]^{this, pf} =
     iterableFactory.from(new View.Collect(this, pf))
@@ -911,7 +911,7 @@ object IterableOps {
     def map[B](f: A => B): CC[B]^{this, f} =
       self.iterableFactory.from(new View.Map(filtered, f))
 
-    def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f} =
+    def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f*} =
       self.iterableFactory.from(new View.FlatMap(filtered, f))
 
     def foreach[U](f: A => U): Unit = filtered.foreach(f)

scala2-library-cc/src/scala/collection/IterableOnce.scala

@@ -246,7 +246,7 @@ final class IterableOnceExtensionMethods[A](private val it: IterableOnce[A]) ext
   }
 
   @deprecated("Use .iterator.flatMap instead or consider requiring an Iterable", "2.13.0")
-  def flatMap[B](f: A => IterableOnce[B]^): IterableOnce[B]^{f} = it match {
+  def flatMap[B](f: A => IterableOnce[B]^): IterableOnce[B]^{f*} = it match {
     case it: Iterable[A] => it.flatMap(f)
     case _ => it.iterator.flatMap(f)
   }
@@ -439,7 +439,7 @@ trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A]^ =>
    *  @return       a new $coll resulting from applying the given collection-valued function
    *                `f` to each element of this $coll and concatenating the results.
    */
-  def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f}
+  def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f*}
 
   /** Converts this $coll of iterable collections into
    *  a $coll formed by the elements of these iterable

scala2-library-cc/src/scala/collection/Iterator.scala

@@ -588,8 +588,8 @@ trait Iterator[+A] extends IterableOnce[A] with IterableOnceOps[A, Iterator, Ite
     def next() = f(self.next())
   }
 
-  def flatMap[B](f: A => IterableOnce[B]^): Iterator[B]^{this, f} = new AbstractIterator[B] {
-    private[this] var cur: Iterator[B]^{f} = Iterator.empty
+  def flatMap[B](f: A => IterableOnce[B]^): Iterator[B]^{this, f*} = new AbstractIterator[B] {
+    private[this] var cur: Iterator[B]^{f*} = Iterator.empty
     /** Trillium logic boolean: -1 = unknown, 0 = false, 1 = true */
     private[this] var _hasNext: Int = -1
 
@@ -623,7 +623,7 @@ trait Iterator[+A] extends IterableOnce[A] with IterableOnceOps[A, Iterator, Ite
     }
   }
 
-  def flatten[B](implicit ev: A -> IterableOnce[B]): Iterator[B]^{this} =
+  def flatten[B](implicit ev: A -> IterableOnce[B]): Iterator[B]^{this, ev*} =
     flatMap[B](ev)
 
   def concat[B >: A](xs: => IterableOnce[B]^): Iterator[B]^{this, xs} = new Iterator.ConcatIterator[B](self).concat(xs)

scala2-library-cc/src/scala/collection/View.scala

@@ -57,8 +57,9 @@ object View extends IterableFactory[View] {
     *
     * @tparam A View element type
     */
-  def fromIteratorProvider[A](it: () => Iterator[A]^): View[A]^{it} = new AbstractView[A] {
-    def iterator: Iterator[A]^{it} = it()
+  def fromIteratorProvider[A](it: () => Iterator[A]^): View[A]^{it*} = new AbstractView[A] {  // TODO: this seems clearly unsound: not only `it*` but also `it` is used
+                                                                                              // why it capture-checks?
+    def iterator: Iterator[A]^{it*} = it()
   }
 
   /**
@@ -310,7 +311,7 @@ object View extends IterableFactory[View] {
   /** A view that flatmaps elements of the underlying collection. */
   @SerialVersionUID(3L)
   class FlatMap[A, B](underlying: SomeIterableOps[A]^, f: A => IterableOnce[B]^) extends AbstractView[B] {
-    def iterator: Iterator[B]^{underlying, f} = underlying.iterator.flatMap(f)
+    def iterator: Iterator[B]^{underlying, f*} = underlying.iterator.flatMap(f)
     override def knownSize: Int = if (underlying.knownSize == 0) 0 else super.knownSize
     override def isEmpty: Boolean = iterator.isEmpty
   }

scala2-library-cc/src/scala/collection/WithFilter.scala

@@ -45,7 +45,7 @@ abstract class WithFilter[+A, +CC[_]] extends Serializable {
     *                of the filtered outer $coll and
     *                concatenating the results.
     */
-  def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f}
+  def flatMap[B](f: A => IterableOnce[B]^): CC[B]^{this, f*}
 
   /** Applies a function `f` to all elements of the `filtered` outer $coll.
     *

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

@@ -592,15 +592,15 @@ final class LazyListIterable[+A] private(@untrackedCaptures lazyState: () => Laz
     */
   // optimisations are not for speed, but for functionality
   // see tickets #153, #498, #2147, and corresponding tests in run/ (as well as run/stream_flatmap_odds.scala)
-  override def flatMap[B](f: A => IterableOnce[B]^): LazyListIterable[B]^{this, f} =
+  override def flatMap[B](f: A => IterableOnce[B]^): LazyListIterable[B]^{this, f*} =
     if (knownIsEmpty) LazyListIterable.empty
     else LazyListIterable.flatMapImpl(this, f)
 
   /** @inheritdoc
     *
     * $preservesLaziness
     */
-  override def flatten[B](implicit asIterable: A -> IterableOnce[B]): LazyListIterable[B]^{this} = flatMap(asIterable)
+  override def flatten[B](implicit asIterable: A -> IterableOnce[B]): LazyListIterable[B]^{this, asIterable*} = flatMap(asIterable)
 
   /** @inheritdoc
     *
@@ -1061,11 +1061,11 @@ object LazyListIterable extends IterableFactory[LazyListIterable] {
     }
   }
 
-  private def flatMapImpl[A, B](ll: LazyListIterable[A]^, f: A => IterableOnce[B]^): LazyListIterable[B]^{ll, f} = {
+  private def flatMapImpl[A, B](ll: LazyListIterable[A]^, f: A => IterableOnce[B]^): LazyListIterable[B]^{ll, f*} = {
     // DO NOT REFERENCE `ll` ANYWHERE ELSE, OR IT WILL LEAK THE HEAD
     var restRef: LazyListIterable[A]^{ll} = ll  // restRef is captured by closure arg to newLL, so A is not recognized as parametric
     newLL {
-      var it: Iterator[B]^{ll, f} = null
+      var it: Iterator[B]^{ll, f*} = null
       var itHasNext       = false
       var rest            = restRef           // var rest = restRef.elem
       while (!itHasNext && !rest.isEmpty) {
@@ -1185,7 +1185,7 @@ 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
 
@@ -1307,7 +1307,7 @@ object LazyListIterable extends IterableFactory[LazyListIterable] {
     extends collection.WithFilter[A, LazyListIterable] {
     private[this] val filtered = lazyList.filter(p)
     def map[B](f: A => B): LazyListIterable[B]^{this, f} = filtered.map(f)
-    def flatMap[B](f: A => IterableOnce[B]^): LazyListIterable[B]^{this, f} = filtered.flatMap(f)
+    def flatMap[B](f: A => IterableOnce[B]^): LazyListIterable[B]^{this, f*} = filtered.flatMap(f)
     def foreach[U](f: A => U): Unit = filtered.foreach(f)
     def withFilter(q: A => Boolean): collection.WithFilter[A, LazyListIterable]^{this, q} = new WithFilter(filtered, q)
   }
@@ -1353,7 +1353,7 @@ object LazyListIterable extends IterableFactory[LazyListIterable] {
     final class DeferredState[A] {
       private[this] var _state: (() => State[A]^) @uncheckedCaptures = _
 
-      def eval(): State[A]^ = {
+      def eval(): State[A]^{this} = {
         val state = _state
         if (state == null) throw new IllegalStateException("uninitialized")
         state()