Tweaks to path checking and massage tests

Needed to make stdlib2-cc go through.

There were two errors. One in LayListIterable required a type annotation
and a tweak to markFree. The other in Vieew.scala required a cast, but this could be fixed
with better handling of pattern matching. path-patmat-should-be-pos.scala is a minimization.

Author: odersky

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

@@ -101,6 +101,19 @@ trait CaptureRef extends TypeProxy, ValueType:
    *   TODO: Document path cases
    */
   final def subsumes(y: CaptureRef)(using Context): Boolean =
+
+    def subsumingRefs(x: Type, y: Type): Boolean = x match
+      case x: CaptureRef => y match
+        case y: CaptureRef => x.subsumes(y)
+        case _ => false
+      case _ => false
+
+    def viaInfo(info: Type)(test: Type => Boolean): Boolean = info.match
+      case info: SingletonCaptureRef => test(info)
+      case info: AndType => test(info.tp1) || test(info.tp2)
+      case info: OrType => test(info.tp1) && test(info.tp2)
+      case _ => false
+
     (this eq y)
     || this.isRootCapability
     || y.match
@@ -109,25 +122,21 @@ trait CaptureRef extends TypeProxy, ValueType:
               case ypre: CaptureRef =>
                 this.subsumes(ypre)
                 || this.match
-                    case x @ TermRef(xpre: CaptureRef, _) =>
-                      x.symbol == y.symbol && xpre =:= ypre
+                    case x @ TermRef(xpre: CaptureRef, _) if x.symbol == y.symbol =>
+                      subsumingRefs(xpre, ypre) && subsumingRefs(ypre, xpre)
                     case _ =>
                       false
               case _ => false
-          || y.info.match
-              case y1: SingletonCaptureRef => this.subsumes(y1)
-              case _ => false
+          || viaInfo(y.info)(subsumingRefs(this, _))
         case MaybeCapability(y1) => this.stripMaybe.subsumes(y1)
         case _ => false
     || this.match
         case ReachCapability(x1) => x1.subsumes(y.stripReach)
-        case x: TermRef =>
-          x.info match
-            case x1: SingletonCaptureRef => x1.subsumes(y)
-            case _ => false
+        case x: TermRef => viaInfo(x.info)(subsumingRefs(_, y))
         case x: TermParamRef => subsumesExistentially(x, y)
         case x: TypeRef => assumedContainsOf(x).contains(y)
         case _ => false
+  end subsumes
 
   def assumedContainsOf(x: TypeRef)(using Context): SimpleIdentitySet[CaptureRef] =
     CaptureSet.assumedContains.getOrElse(x, SimpleIdentitySet.empty)

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

@@ -466,16 +466,24 @@ class CheckCaptures extends Recheck, SymTransformer:
         if tree.symbol.info.isParameterless then
           // there won't be an apply; need to include call captures now
           includeCallCaptures(tree.symbol, tree.srcPos)
-      else
+      else if !tree.symbol.isStatic then
         //debugShowEnvs()
         def addSelects(ref: TermRef, pt: Type): TermRef = pt match
-          case pt: PathSelectionProto => addSelects(ref.select(pt.sym).asInstanceOf[TermRef], pt.pt)
+          case pt: PathSelectionProto if ref.isTracked =>
+            // if `ref` is not tracked then the selection could not give anything new
+            // class SerializationProxy in stdlib-cc/../LazyListIterable.scala has an example where this matters.
+            addSelects(ref.select(pt.sym).asInstanceOf[TermRef], pt.pt)
           case _ => ref
-        markFree(tree.symbol, addSelects(tree.symbol.termRef, pt), tree.srcPos)
+        val ref = tree.symbol.termRef
+        val pathRef = addSelects(ref, pt)
+        //if pathRef ne ref then
+        //  println(i"add selects $ref --> $pathRef")
+        markFree(tree.symbol, if false then ref else pathRef, tree.srcPos)
       super.recheckIdent(tree, pt)
 
     override def selectionProto(tree: Select, pt: Type)(using Context): Type =
-      if !tree.symbol.isOneOf(UnstableValueFlags) then PathSelectionProto(tree.symbol, pt)
+      val sym = tree.symbol
+      if !sym.isOneOf(UnstableValueFlags) && !sym.isStatic then PathSelectionProto(sym, pt)
       else super.selectionProto(tree, pt)
 
     /** A specialized implementation of the selection rule.
@@ -1141,11 +1149,14 @@ class CheckCaptures extends Recheck, SymTransformer:
         (erefs /: erefs.elems): (erefs, eref) =>
           eref match
             case eref: ThisType if isPureContext(ctx.owner, eref.cls) =>
-              erefs ++ arefs.filter {
-                case aref: TermRef => eref.cls.isProperlyContainedIn(aref.symbol.owner)
+              def isOuterRef(aref: Type): Boolean = aref match
+                case aref: TermRef =>
+                  val owner = aref.symbol.owner
+                  if owner.isClass then isOuterRef(aref.prefix)
+                  else eref.cls.isProperlyContainedIn(owner)
                 case aref: ThisType => eref.cls.isProperlyContainedIn(aref.cls)
                 case _ => false
-              }
+              erefs ++ arefs.filter(isOuterRef)
             case _ =>
               erefs
 

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

@@ -150,7 +150,10 @@ object View extends IterableFactory[View] {
   object Filter {
     def apply[A](underlying: Iterable[A]^, p: A => Boolean, isFlipped: Boolean): Filter[A]^{underlying, p} =
       underlying match {
-        case filter: Filter[A] if filter.isFlipped == isFlipped => new Filter(filter.underlying, a => filter.p(a) && p(a), isFlipped)
+        case filter: Filter[A] if filter.isFlipped == isFlipped =>
+          new Filter(filter.underlying, a => filter.p(a) && p(a), isFlipped)
+            .asInstanceOf[Filter[A]^{underlying, p}]
+              // !!! asInstanceOf needed once paths were added, see path-patmat-should-be-pos.scala for minimization
         case _ => new Filter(underlying, p, isFlipped)
       }
   }

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

@@ -1366,7 +1366,9 @@ object LazyListIterable extends IterableFactory[LazyListIterable] {
         case SerializeEnd => initRead = true
         case a => init += a.asInstanceOf[A]
       }
-      val tail = in.readObject().asInstanceOf[LazyListIterable[A]]
+      val tail: LazyListIterable[A] = in.readObject().asInstanceOf[LazyListIterable[A]]
+        // Explicit type annotation needed so that tail.state below is dropped from capture set.
+        // Before paths were added, it was tail that was added, and the `asSeenFrom` to a pure type made it work.
       // scala/scala#10118: caution that no code path can evaluate `tail.state`
       // before the resulting LazyListIterable is returned
       val it = init.toList.iterator

tests/neg-custom-args/captures/path-patmat-should-be-pos.scala

@@ -0,0 +1,26 @@
+class It[A]
+
+class Filter[A](val underlying: It[A]^, val p: A => Boolean) extends It[A]
+object Filter:
+  def apply[A](underlying: It[A]^, p: A => Boolean): Filter[A]^{underlying, p} =
+    underlying match
+      case filter: Filter[A]^ =>
+        val x = new Filter(filter.underlying, a => filter.p(a) && p(a))
+        x: Filter[A]^{underlying, p} // error
+          // !!! should work, it seems to be the case that the system does not recognize that
+          // underlying and filter are aliases.
+
+    // On the other hand, the following works:
+    locally:
+      val filter: underlying.type & Filter[A] = ???
+      val a: It[A]^{filter.underlying} = ???
+      val b: It[A]^{underlying} = a
+      val x = new Filter(filter.underlying, a => filter.p(a) && p(a))
+      x: Filter[A]^{underlying, p}
+
+    locally:
+      val filter: underlying.type & Filter[A]^ = ???
+      val a: It[A]^{filter.underlying} = ???
+      val b: It[A]^{underlying} = a
+      val x = new Filter(filter.underlying, a => filter.p(a) && p(a))
+      x: Filter[A]^{underlying, p}