From 305cb9caae2285c1d1bd626f13cdaa42e432223e Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Wed, 18 Sep 2024 15:33:16 +0200
Subject: [PATCH 01/12] Add path support for cc

---
 compiler/src/dotty/tools/dotc/ast/untpd.scala |   4 +-
 .../src/dotty/tools/dotc/cc/CaptureOps.scala  |   3 +-
 .../src/dotty/tools/dotc/cc/CaptureRef.scala  |  84 +++++++++-----
 .../src/dotty/tools/dotc/cc/CaptureSet.scala  |   8 +-
 .../dotty/tools/dotc/cc/CheckCaptures.scala   | 106 ++++++++++--------
 .../dotty/tools/dotc/parsing/Parsers.scala    |  28 ++---
 .../captures/class-contra.check               |  13 +--
 .../captures/class-contra.scala               |   3 +-
 .../captures/explain-under-approx.check       |  14 +--
 .../captures/filevar-multi-ios.scala          |  41 +++++++
 tests/neg-custom-args/captures/i15116.check   |  16 ++-
 tests/neg-custom-args/captures/path-box.scala |  20 ++++
 .../captures/path-connection.scala            |  46 ++++++++
 .../captures/path-illigal.scala               |   7 ++
 .../captures/path-simple.scala                |  27 +++++
 .../neg-custom-args/captures/singletons.scala |   8 +-
 .../captures/filevar-expanded.scala           |   3 +-
 tests/pos-custom-args/captures/filevar.scala  |   3 +-
 18 files changed, 314 insertions(+), 120 deletions(-)
 create mode 100644 tests/neg-custom-args/captures/filevar-multi-ios.scala
 create mode 100644 tests/neg-custom-args/captures/path-box.scala
 create mode 100644 tests/neg-custom-args/captures/path-connection.scala
 create mode 100644 tests/neg-custom-args/captures/path-illigal.scala
 create mode 100644 tests/neg-custom-args/captures/path-simple.scala

diff --git a/compiler/src/dotty/tools/dotc/ast/untpd.scala b/compiler/src/dotty/tools/dotc/ast/untpd.scala
index 935e42d5e05c..4684464d477f 100644
--- a/compiler/src/dotty/tools/dotc/ast/untpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/untpd.scala
@@ -524,8 +524,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   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 makeCapsOf(tp: Tree)(using Context): Tree =
+    TypeApply(Select(scalaDot(nme.caps), nme.capsOf), tp :: Nil)
 
   def makeCapsBound()(using Context): Tree =
     makeRetaining(
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala b/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
index 29c6528e36de..79cc7d136e45 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
@@ -194,7 +194,8 @@ extension (tp: Type)
       true
     case tp: TermRef =>
       ((tp.prefix eq NoPrefix)
-      || tp.symbol.is(ParamAccessor) && tp.prefix.isThisTypeOf(tp.symbol.owner)
+      || tp.symbol.isField && !tp.symbol.isStatic && (
+        tp.prefix.isThisTypeOf(tp.symbol.owner) || tp.prefix.isTrackableRef)
       || tp.isRootCapability
       ) && !tp.symbol.isOneOf(UnstableValueFlags)
     case tp: TypeRef =>
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index f00c6869cd80..05162907b608 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
@@ -61,18 +61,19 @@ trait CaptureRef extends TypeProxy, ValueType:
     case tp: TermParamRef => tp.underlying.derivesFrom(defn.Caps_Exists)
     case _ => false
 
-  /** Normalize reference so that it can be compared with `eq` for equality */
-  final def normalizedRef(using Context): CaptureRef = this match
-    case tp @ AnnotatedType(parent: CaptureRef, annot) if tp.isTrackableRef =>
-      tp.derivedAnnotatedType(parent.normalizedRef, annot)
-    case tp: TermRef if tp.isTrackableRef =>
-      tp.symbol.termRef
-    case _ => this
+  // With the support of pathes, we don't need to normalize the `TermRef`s anymore.
+  // /** Normalize reference so that it can be compared with `eq` for equality */
+  // final def normalizedRef(using Context): CaptureRef = this match
+  //   case tp @ AnnotatedType(parent: CaptureRef, annot) if tp.isTrackableRef =>
+  //     tp.derivedAnnotatedType(parent.normalizedRef, annot)
+  //   case tp: TermRef if tp.isTrackableRef =>
+  //     tp.symbol.termRef
+  //   case _ => this
 
   /** The capture set consisting of exactly this reference */
   final def singletonCaptureSet(using Context): CaptureSet.Const =
     if mySingletonCaptureSet == null then
-      mySingletonCaptureSet = CaptureSet(this.normalizedRef)
+      mySingletonCaptureSet = CaptureSet(this)
     mySingletonCaptureSet.uncheckedNN
 
   /** The capture set of the type underlying this reference */
@@ -99,25 +100,56 @@ trait CaptureRef extends TypeProxy, ValueType:
    *   x: x1.type /\ x1 subsumes y  ==>  x subsumes y
    */
   final def subsumes(y: CaptureRef)(using Context): Boolean =
-    (this eq y)
-    || this.isRootCapability
-    || y.match
-        case y: TermRef =>
-          (y.prefix eq this)
-          || y.info.match
-              case y1: SingletonCaptureRef => this.subsumes(y1)
-              case _ => false
-        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)
+    def compareCaptureRefs(x: Type, y: Type): Boolean =
+      (x eq y)
+      || y.match
+          case y: CaptureRef => x.match
+            case x: CaptureRef => x.subsumes(y)
             case _ => false
-        case x: TermParamRef => subsumesExistentially(x, y)
-        case x: TypeRef => assumedContainsOf(x).contains(y)
-        case _ => false
+          case _ => false
+
+    def compareUndelying(x: Type): Boolean = x match
+      case x: SingletonCaptureRef => x.subsumes(y)
+      case x: AndType => compareUndelying(x.tp1) || compareUndelying(x.tp2)
+      case x: OrType => compareUndelying(x.tp1) && compareUndelying(x.tp2)
+      case _ => false
+
+    if (this eq y) || this.isRootCapability then return true
+
+    // similar to compareNamed in TypeComparer
+    y match
+      case y: TermRef =>
+        this match
+          case x: TermRef =>
+            val xSym = x.symbol
+            val ySym = y.symbol
+
+            // check x.f and y.f
+            if (xSym ne NoSymbol)
+                && (xSym eq ySym)
+                && compareCaptureRefs(x.prefix, y.prefix)
+              || (x.name eq y.name)
+                && x.isPrefixDependentMemberRef
+                && compareCaptureRefs(x.prefix, y.prefix)
+                && x.signature == y.signature
+                && !(xSym.isClass && ySym.isClass)
+            then return true
+          case _ =>
+
+        // shorten
+        if compareCaptureRefs(this, y.prefix) then return true
+        // underlying
+        if compareCaptureRefs(this, y.info) then return true
+      case MaybeCapability(y1) => return this.stripMaybe.subsumes(y1)
+      case _ =>
+
+    return this.match
+      case ReachCapability(x1) => x1.subsumes(y.stripReach)
+      case x: TermRef => compareUndelying(x.info)
+      case CapturingType(x1, _) => compareUndelying(x1)
+      case x: TermParamRef => subsumesExistentially(x, y)
+      case x: TypeRef => assumedContainsOf(x).contains(y)
+      case _ => false
 
   def assumedContainsOf(x: TypeRef)(using Context): SimpleIdentitySet[CaptureRef] =
     CaptureSet.assumedContains.getOrElse(x, SimpleIdentitySet.empty)
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
index 44d5e2cf4b88..81b4287961ba 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
@@ -374,7 +374,7 @@ object CaptureSet:
 
   def apply(elems: CaptureRef*)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty
-    else Const(SimpleIdentitySet(elems.map(_.normalizedRef.ensuring(_.isTrackableRef))*))
+    else Const(SimpleIdentitySet(elems.map(_.ensuring(_.isTrackableRef))*))
 
   def apply(elems: Refs)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty else Const(elems)
@@ -508,7 +508,11 @@ object CaptureSet:
         !noUniversal
       else elem match
         case elem: TermRef if level.isDefined =>
-          elem.symbol.ccLevel <= level
+          elem.prefix match
+            case prefix: CaptureRef =>
+              levelOK(prefix)
+            case _ =>
+              elem.symbol.ccLevel <= level
         case elem: ThisType if level.isDefined =>
           elem.cls.ccLevel.nextInner <= level
         case ReachCapability(elem1) =>
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index b05ab8542137..ec1e63137311 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -122,10 +122,6 @@ object CheckCaptures:
    *  This check is performed at Typer.
    */
   def checkWellformed(parent: Tree, ann: Tree)(using Context): Unit =
-    parent.tpe match
-      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
@@ -373,45 +369,54 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  the environment's owner
      */
     def markFree(cs: CaptureSet, pos: SrcPos)(using Context): Unit =
+      // A captured reference with the symbol `sym` is visible from the environment
+      // if `sym` is not defined inside the owner of the environment.
+      inline def isVisibleFromEnv(sym: Symbol, env: Env) =
+        if env.kind == EnvKind.NestedInOwner then
+          !sym.isProperlyContainedIn(env.owner)
+        else
+          !sym.isContainedIn(env.owner)
+
+      def checkSubsetEnv(cs: CaptureSet, env: Env)(using Context): Unit =
+        // 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
+            case ref: ThisType => isVisibleFromEnv(ref.cls, env)
+            case _ => false
+        checkSubset(included, env.captured, pos, provenance(env))
+        capt.println(i"Include call or box capture $included from $cs in ${env.owner} --> ${env.captured}")
+
       if !cs.isAlwaysEmpty then
         forallOuterEnvsUpTo(ctx.owner.topLevelClass): env =>
-          // Whether a symbol is defined inside the owner of the environment?
-          inline def isContainedInEnv(sym: Symbol) =
-            if env.kind == EnvKind.NestedInOwner then
-              sym.isProperlyContainedIn(env.owner)
-            else
-              sym.isContainedIn(env.owner)
-          // A captured reference with the symbol `sym` is visible from the environment
-          // if `sym` is not defined inside the owner of the environment
-          inline def isVisibleFromEnv(sym: Symbol) = !isContainedInEnv(sym)
-          // 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)
-                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
-              case ref: ThisType => isVisibleFromEnv(ref.cls)
-              case _ => false
-          checkSubset(included, env.captured, pos, provenance(env))
-          capt.println(i"Include call or box capture $included from $cs in ${env.owner} --> ${env.captured}")
+          checkSubsetEnv(cs, env)
     end markFree
 
     /** Include references captured by the called method in the current environment stack */
@@ -488,21 +493,28 @@ class CheckCaptures extends Recheck, SymTransformer:
         case _ => denot
 
       val selType = recheckSelection(tree, qualType, name, disambiguate)
-      val selCs = selType.widen.captureSet
-      if selCs.isAlwaysEmpty
-          || selType.widen.isBoxedCapturing
+      val selWiden = selType.widen
+      def isStableSel = selType match
+        case selType: NamedType => selType.symbol.isStableMember
+        case _ => false
+        
+      if pt == LhsProto
           || qualType.isBoxedCapturing
-          || pt == LhsProto
+          || selType.isTrackableRef
+          || selWiden.isBoxedCapturing
+          || selWiden.captureSet.isAlwaysEmpty
       then
         selType
       else
         val qualCs = qualType.captureSet
-        capt.println(i"pick one of $qualType, ${selType.widen}, $qualCs, $selCs in $tree")
+        val selCs = selType.captureSet
+        capt.println(i"pick one of $qualType, ${selType.widen}, $qualCs, $selCs ${selWiden.captureSet} in $tree")
+
         if qualCs.mightSubcapture(selCs)
             && !selCs.mightSubcapture(qualCs)
             && !pt.stripCapturing.isInstanceOf[SingletonType]
         then
-          selType.widen.stripCapturing.capturing(qualCs)
+          selWiden.stripCapturing.capturing(qualCs)
             .showing(i"alternate type for select $tree: $selType --> $result, $qualCs / $selCs", capt)
         else
           selType
diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 47391a4114cf..dc3ae4cf7639 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -1559,21 +1559,23 @@ object Parsers {
       case _ => None
     }
 
-    /** CaptureRef  ::=  ident [`*` | `^`] | `this`
+    /** CaptureRef  ::=  (ident | `this`) [`*` | `^`]
      */
     def captureRef(): Tree =
-      if in.token == THIS then simpleRef()
-      else
-        val id = termIdent()
-        if isIdent(nme.raw.STAR) then
-          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
+      val ref = singleton()
+      if isIdent(nme.raw.STAR) then
+        in.nextToken()
+        atSpan(startOffset(ref)):
+          PostfixOp(ref, Ident(nme.CC_REACH))
+      else if isIdent(nme.UPARROW) then
+        in.nextToken()
+        def toTypeSel(r: Tree): Tree = r match
+          case id: Ident => cpy.Ident(id)(id.name.toTypeName)
+          case Select(qual, id) => Select(qual, id.toTypeName)
+          case _ => r
+        atSpan(startOffset(ref)):
+          makeCapsOf(toTypeSel(ref))
+      else ref
 
     /**  CaptureSet ::=  `{` CaptureRef {`,` CaptureRef} `}`    -- under captureChecking
      */
diff --git a/tests/neg-custom-args/captures/class-contra.check b/tests/neg-custom-args/captures/class-contra.check
index 9fc009ac3d48..808118bd1795 100644
--- a/tests/neg-custom-args/captures/class-contra.check
+++ b/tests/neg-custom-args/captures/class-contra.check
@@ -1,10 +1,7 @@
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/class-contra.scala:12:39 ---------------------------------
-12 |  def fun(x: K{val f: T^{a}}) = x.setf(a) // error
-   |                                       ^
-   |   Found:    (a : T^{x, y})
-   |   Required: T^{}
-   |
-   |   Note that a capability (K.this.f : T^) in a capture set appearing in contravariant position
-   |   was mapped to (x.f : T^{a}) which is not a capability. Therefore, it was under-approximated to the empty set.
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/class-contra.scala:12:40 ---------------------------------
+12 |  def fun1(k: K{val f: T^{a}}) = k.setf(a) // error
+   |                                        ^
+   |                                        Found:    (a : T^{x, y})
+   |                                        Required: T^{k.f}
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/class-contra.scala b/tests/neg-custom-args/captures/class-contra.scala
index 210fd4e331f1..8ef8e7485a18 100644
--- a/tests/neg-custom-args/captures/class-contra.scala
+++ b/tests/neg-custom-args/captures/class-contra.scala
@@ -9,5 +9,6 @@ class T
 
 def test(x: Cap, y: Cap) =
   val a: T^{x, y} = ???
-  def fun(x: K{val f: T^{a}}) = x.setf(a) // error
+  def fun1(k: K{val f: T^{a}}) = k.setf(a) // error
+  def fun2(k: K{val f: a.type}) = k.setf(a)
   ()
\ No newline at end of file
diff --git a/tests/neg-custom-args/captures/explain-under-approx.check b/tests/neg-custom-args/captures/explain-under-approx.check
index 2d2b05b4b95a..c186fc6adb11 100644
--- a/tests/neg-custom-args/captures/explain-under-approx.check
+++ b/tests/neg-custom-args/captures/explain-under-approx.check
@@ -1,20 +1,14 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/explain-under-approx.scala:12:10 -------------------------
 12 |  col.add(Future(() => 25)) // error
    |          ^^^^^^^^^^^^^^^^
-   |       Found:    Future[Int]{val a: (async : Async^)}^{async}
-   |       Required: Future[Int]^{}
-   |
-   |       Note that a capability Collector.this.futs* in a capture set appearing in contravariant position
-   |       was mapped to col.futs* which is not a capability. Therefore, it was under-approximated to the empty set.
+   |          Found:    Future[Int]{val a: (async : Async^)}^{async}
+   |          Required: Future[Int]^{col.futs*}
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/explain-under-approx.scala:15:11 -------------------------
 15 |  col1.add(Future(() => 25)) // error
    |           ^^^^^^^^^^^^^^^^
-   |      Found:    Future[Int]{val a: (async : Async^)}^{async}
-   |      Required: Future[Int]^{}
-   |
-   |      Note that a capability Collector.this.futs* in a capture set appearing in contravariant position
-   |      was mapped to col1.futs* which is not a capability. Therefore, it was under-approximated to the empty set.
+   |           Found:    Future[Int]{val a: (async : Async^)}^{async}
+   |           Required: Future[Int]^{col1.futs*}
    |
    | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/filevar-multi-ios.scala b/tests/neg-custom-args/captures/filevar-multi-ios.scala
new file mode 100644
index 000000000000..8ffc8d8e299c
--- /dev/null
+++ b/tests/neg-custom-args/captures/filevar-multi-ios.scala
@@ -0,0 +1,41 @@
+import language.experimental.modularity
+import compiletime.uninitialized
+
+class IO extends caps.Capability
+
+class File:
+  def write(x: String): Unit = ???
+
+object test1:
+
+  class Service(val io: IO, val io2: IO):
+    var file: File^{io} = uninitialized
+    var file2: File^{io2} = uninitialized
+    def log = file.write("log")
+
+  def withFile[T](io: IO)(op: File^{io} => T): T =
+    op(new File)
+
+  def test(io3: IO, io4: IO) =
+    withFile(io3): f =>
+      val o = Service(io3, io4)
+      o.file = f // error
+      o.file2 = f // error
+      o.log
+
+object test2:
+
+  class Service(tracked val io: IO, tracked val io2: IO):
+    var file: File^{io} = uninitialized
+    var file2: File^{io2} = uninitialized
+    def log = file.write("log")
+
+  def withFile[T](io: IO)(op: File^{io} => T): T =
+    op(new File)
+
+  def test(io3: IO, io4: IO) =
+    withFile(io3): f =>
+      val o = Service(io3, io4)
+      o.file = f
+      o.file2 = f // error
+      o.log
diff --git a/tests/neg-custom-args/captures/i15116.check b/tests/neg-custom-args/captures/i15116.check
index df05324866e1..0a16af9f6704 100644
--- a/tests/neg-custom-args/captures/i15116.check
+++ b/tests/neg-custom-args/captures/i15116.check
@@ -18,13 +18,17 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15116.scala:5:13 ----------------------------------------
 5 |  val x = Foo(m)   // error
   |          ^^^^^^
-  |          Found:    Foo{val m: String^{Baz.this}}^{Baz.this}
+  |          Found:    Foo{val m²: (Baz.this.m : String^)}^{Baz.this.m}
   |          Required: Foo
   |
+  |          where:    m  is a value in trait Baz
+  |                    m² is a value in class Foo
+  |
+  |
   |          Note that the expected type Foo
   |          is the previously inferred type of value x
   |          which is also the type seen in separately compiled sources.
-  |          The new inferred type Foo{val m: String^{Baz.this}}^{Baz.this}
+  |          The new inferred type Foo{val m: (Baz.this.m : String^)}^{Baz.this.m}
   |          must conform to this type.
   |
   | longer explanation available when compiling with `-explain`
@@ -48,13 +52,17 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15116.scala:9:13 ----------------------------------------
 9 |  val x = Foo(m)   // error
   |          ^^^^^^
-  |          Found:    Foo{val m: String^{Baz2.this}}^{Baz2.this}
+  |          Found:    Foo{val m²: (Baz2.this.m : String^)}^{Baz2.this.m}
   |          Required: Foo
   |
+  |          where:    m  is a value in trait Baz2
+  |                    m² is a value in class Foo
+  |
+  |
   |          Note that the expected type Foo
   |          is the previously inferred type of value x
   |          which is also the type seen in separately compiled sources.
-  |          The new inferred type Foo{val m: String^{Baz2.this}}^{Baz2.this}
+  |          The new inferred type Foo{val m: (Baz2.this.m : String^)}^{Baz2.this.m}
   |          must conform to this type.
   |
   | longer explanation available when compiling with `-explain`
diff --git a/tests/neg-custom-args/captures/path-box.scala b/tests/neg-custom-args/captures/path-box.scala
new file mode 100644
index 000000000000..3213c236aaf5
--- /dev/null
+++ b/tests/neg-custom-args/captures/path-box.scala
@@ -0,0 +1,20 @@
+class A:
+  val m: A^ = ???
+  val self: this.type = this
+
+case class Box[+T](value: T)
+
+def testBox1(a: A^): Box[A^{a}] =
+  Box(a.m)
+
+def testBox2(a: A^): Box[A^{a.m}] =
+  Box(a.m)
+
+def testBox3(a: A^): Box[A^{a.m}] =
+  Box(a) // error
+
+def testBox4(a: A^): Box[A^{a.m}] =
+  Box(a.m.m.m)
+
+def testBox5(a: A^): Box[A^{a.m}] =
+  Box(a.m.m.self)
\ No newline at end of file
diff --git a/tests/neg-custom-args/captures/path-connection.scala b/tests/neg-custom-args/captures/path-connection.scala
new file mode 100644
index 000000000000..3b3820488c8d
--- /dev/null
+++ b/tests/neg-custom-args/captures/path-connection.scala
@@ -0,0 +1,46 @@
+import language.experimental.modularity
+
+trait Reader:
+  def read(): String
+
+trait Sender:
+  def send(msg: String): Unit
+
+class Connection extends Reader, Sender:
+  def read() = "hello"
+  def send(msg: String) = ()
+
+  val readOnly: Reader^ = new Reader:
+    def read() = Connection.this.read()
+
+class ReaderProxy(tracked val r: Reader^) extends Reader:
+  def read() = "(Proxy)" + r.read()
+
+class SenderProxy(tracked val s: Sender^) extends Sender:
+  def send(msg: String) = s.send("(Proxy) " + msg)
+
+def testConnection(c: Connection^)(
+    handle1: Reader^{c.readOnly} => String,
+    handle2: Sender^{c} => Unit,
+    handle3: Reader^{c} => String,
+    ) =
+  val m1 = c.read()
+  c.send("hello")
+
+  val m2 = c.readOnly.read()
+
+  val m3a = handle1(c.readOnly)
+  val m3b = handle3(c.readOnly)
+
+  val m4a = handle1(c) // error
+  val m4b = handle3(c)
+
+  val m5a = handle1(new ReaderProxy(c.readOnly))
+  val m5b = handle3(new ReaderProxy(c.readOnly))
+
+  val m6a = handle1(new ReaderProxy(c)) // error
+  val m6b = handle3(new ReaderProxy(c))
+
+  handle2(c)
+
+  handle2(new SenderProxy(c))
\ No newline at end of file
diff --git a/tests/neg-custom-args/captures/path-illigal.scala b/tests/neg-custom-args/captures/path-illigal.scala
new file mode 100644
index 000000000000..f09db0087ef7
--- /dev/null
+++ b/tests/neg-custom-args/captures/path-illigal.scala
@@ -0,0 +1,7 @@
+class A:
+  val m: A^ = ???
+  var n: A^ = ???
+
+def test1(a: A^) =
+  val c1: A^{a.m} = a.m
+  val f1: A^{a.n} = a.n // error
\ No newline at end of file
diff --git a/tests/neg-custom-args/captures/path-simple.scala b/tests/neg-custom-args/captures/path-simple.scala
new file mode 100644
index 000000000000..93b6dacebe74
--- /dev/null
+++ b/tests/neg-custom-args/captures/path-simple.scala
@@ -0,0 +1,27 @@
+
+class A:
+  val m: A^ = ???
+  val self: this.type = this
+
+case class C(ca: A^)
+
+def test1(a: A^, b: A^) =
+  val c1: A^{a} = a.m
+  val c2: A^{a.m} = a.m
+  val c3: A^{b} = a.m // error
+
+  val d1: A^{a} = a.self
+  val d2: A^{a.self} = a.self
+  val d3: A^{a.self} = a
+
+  val e1: A^{a.m} = a.self.m
+  val e2: A^{a.self.m} = a.self.m
+  val e3: A^{a.self.m} = a.m
+
+def test2(a: A^) =
+  val b: a.type = a
+  val c1: C^{a} = new C(a)
+  val c2: C^{a} = new C(a.m)
+  val c3: C^{a.m} = new C(a.m)
+  val c4: C^{b} = new C(a)
+  val c5: C^{a} = new C(b)
\ No newline at end of file
diff --git a/tests/neg-custom-args/captures/singletons.scala b/tests/neg-custom-args/captures/singletons.scala
index 194e6e850dcd..be0ee67ab1bc 100644
--- a/tests/neg-custom-args/captures/singletons.scala
+++ b/tests/neg-custom-args/captures/singletons.scala
@@ -1,6 +1,6 @@
 val x = () => ()
 
-val y1: x.type = x          // ok
-val y2: x.type^{} = x       // error: singleton type cannot have capture set
-val y3: x.type^{x} = x      // error: singleton type cannot have capture set // error
-val y4: x.type^ = x         // error: singleton type cannot have capture set
+val y1: x.type = x
+val y2: x.type^{} = x
+val y3: x.type^{x} = x // error
+val y4: x.type^ = x
diff --git a/tests/pos-custom-args/captures/filevar-expanded.scala b/tests/pos-custom-args/captures/filevar-expanded.scala
index a883471e8d2e..58e7a0e67e0a 100644
--- a/tests/pos-custom-args/captures/filevar-expanded.scala
+++ b/tests/pos-custom-args/captures/filevar-expanded.scala
@@ -1,4 +1,5 @@
 import language.experimental.captureChecking
+import language.experimental.modularity
 import compiletime.uninitialized
 
 object test1:
@@ -22,7 +23,7 @@ object test2:
   class File:
     def write(x: String): Unit = ???
 
-  class Service(io: IO^):
+  class Service(tracked val io: IO^):
     var file: File^{io} = uninitialized
     def log = file.write("log")
 
diff --git a/tests/pos-custom-args/captures/filevar.scala b/tests/pos-custom-args/captures/filevar.scala
index 9ab34fe617b5..dc8d0b18908b 100644
--- a/tests/pos-custom-args/captures/filevar.scala
+++ b/tests/pos-custom-args/captures/filevar.scala
@@ -1,4 +1,5 @@
 import language.experimental.captureChecking
+import language.experimental.modularity
 import compiletime.uninitialized
 
 object test1:
@@ -22,7 +23,7 @@ object test2:
   class File:
     def write(x: String): Unit = ???
 
-  class Service(io: IO):
+  class Service(tracked val io: IO):
     var file: File^{io} = uninitialized
     def log = file.write("log")
 

From 57dd0304822385d987fc780b5f86874161bcc722 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Thu, 19 Sep 2024 15:30:57 +0200
Subject: [PATCH 02/12] Add comment for path-dependent limitation

---
 compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala | 5 +----
 compiler/src/dotty/tools/dotc/cc/Setup.scala         | 5 +++++
 tests/neg-custom-args/captures/path-connection.scala | 2 ++
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index ec1e63137311..b3a1ab44d6cd 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -494,10 +494,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
       val selType = recheckSelection(tree, qualType, name, disambiguate)
       val selWiden = selType.widen
-      def isStableSel = selType match
-        case selType: NamedType => selType.symbol.isStableMember
-        case _ => false
-        
+
       if pt == LhsProto
           || qualType.isBoxedCapturing
           || selType.isTrackableRef
diff --git a/compiler/src/dotty/tools/dotc/cc/Setup.scala b/compiler/src/dotty/tools/dotc/cc/Setup.scala
index 22e7899eeea1..76ae41649517 100644
--- a/compiler/src/dotty/tools/dotc/cc/Setup.scala
+++ b/compiler/src/dotty/tools/dotc/cc/Setup.scala
@@ -518,6 +518,11 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           info match
             case mt: MethodOrPoly =>
               val psyms = psymss.head
+              // TODO: the substitution does not work for param-dependent method types.
+              // For example, `(x: T, y: x.f.type) => Unit`. In this case, when we
+              // substitute `x.f.type`, `x` becomes a `TermParamRef`. But the new method
+              // type is still under initialization and `paramInfos` is still `null`,
+              // so the new `NamedType` will not have a denoation.
               mt.companion(mt.paramNames)(
                 mt1 =>
                   if !paramSignatureChanges && !mt.isParamDependent && prevLambdas.isEmpty then
diff --git a/tests/neg-custom-args/captures/path-connection.scala b/tests/neg-custom-args/captures/path-connection.scala
index 3b3820488c8d..c65aa75b1ed2 100644
--- a/tests/neg-custom-args/captures/path-connection.scala
+++ b/tests/neg-custom-args/captures/path-connection.scala
@@ -19,6 +19,8 @@ class ReaderProxy(tracked val r: Reader^) extends Reader:
 class SenderProxy(tracked val s: Sender^) extends Sender:
   def send(msg: String) = s.send("(Proxy) " + msg)
 
+// TODO: We have to put `c` in the different argument list to make it work.
+// See the comments in `integrateRT`.
 def testConnection(c: Connection^)(
     handle1: Reader^{c.readOnly} => String,
     handle2: Sender^{c} => Unit,

From 2f9f4c4be97d64683a4154c3d4f6fb306f8d373c Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Tue, 24 Sep 2024 18:05:57 +0200
Subject: [PATCH 03/12] Add alternative subsumes implementations

This is done for comparing old with new
---
 .../src/dotty/tools/dotc/cc/CaptureRef.scala  | 46 +++++++++++++++++--
 1 file changed, 41 insertions(+), 5 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index 05162907b608..107b1a178069 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
@@ -93,23 +93,59 @@ trait CaptureRef extends TypeProxy, ValueType:
   final def invalidateCaches() =
     myCaptureSetRunId = NoRunId
 
+  final def subsumes(y: CaptureRef)(using Context): Boolean =
+    val was = subsumesOld(y)
+    val now = subsumesNew(y)
+    if was != now then
+      println(i"diff for  $this subsumes $y, now: $now, ${this.getClass}, ${y.getClass}")
+    was
+
+  final def subsumesOld(y: CaptureRef)(using Context): Boolean =
+    (this eq y)
+    || this.isRootCapability
+    || y.match
+        case y: TermRef =>
+            y.prefix.match
+              case ypre: CaptureRef =>
+                this.subsumesOld(ypre)
+                || this.match
+                    case x @ TermRef(xpre: CaptureRef, _) =>
+                      x.symbol == y.symbol && xpre =:= ypre
+                    case _ =>
+                      false
+              case _ => false
+          || y.info.match
+              case y1: SingletonCaptureRef => this.subsumesOld(y1)
+              case _ => false
+        case MaybeCapability(y1) => this.stripMaybe.subsumesOld(y1)
+        case _ => false
+    || this.match
+        case ReachCapability(x1) => x1.subsumesOld(y.stripReach)
+        case x: TermRef =>
+          x.info match
+            case x1: SingletonCaptureRef => x1.subsumesOld(y)
+            case _ => false
+        case x: TermParamRef => subsumesExistentially(x, y)
+        case x: TypeRef => assumedContainsOf(x).contains(y)
+        case _ => false
+
   /** x subsumes x
    *   this subsumes this.f
    *   x subsumes y  ==>  x* subsumes y, x subsumes y?
    *   x subsumes y  ==>  x* subsumes y*, x? subsumes y?
    *   x: x1.type /\ x1 subsumes y  ==>  x subsumes y
    */
-  final def subsumes(y: CaptureRef)(using Context): Boolean =
+  final def subsumesNew(y: CaptureRef)(using Context): Boolean =
     def compareCaptureRefs(x: Type, y: Type): Boolean =
       (x eq y)
       || y.match
           case y: CaptureRef => x.match
-            case x: CaptureRef => x.subsumes(y)
+            case x: CaptureRef => x.subsumesNew(y)
             case _ => false
           case _ => false
 
     def compareUndelying(x: Type): Boolean = x match
-      case x: SingletonCaptureRef => x.subsumes(y)
+      case x: SingletonCaptureRef => x.subsumesNew(y)
       case x: AndType => compareUndelying(x.tp1) || compareUndelying(x.tp2)
       case x: OrType => compareUndelying(x.tp1) && compareUndelying(x.tp2)
       case _ => false
@@ -140,11 +176,11 @@ trait CaptureRef extends TypeProxy, ValueType:
         if compareCaptureRefs(this, y.prefix) then return true
         // underlying
         if compareCaptureRefs(this, y.info) then return true
-      case MaybeCapability(y1) => return this.stripMaybe.subsumes(y1)
+      case MaybeCapability(y1) => return this.stripMaybe.subsumesNew(y1)
       case _ =>
 
     return this.match
-      case ReachCapability(x1) => x1.subsumes(y.stripReach)
+      case ReachCapability(x1) => x1.subsumesNew(y.stripReach)
       case x: TermRef => compareUndelying(x.info)
       case CapturingType(x1, _) => compareUndelying(x1)
       case x: TermParamRef => subsumesExistentially(x, y)

From 0c50a31003a9090d4e131ee6654be5a5e033433e Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Tue, 24 Sep 2024 18:07:50 +0200
Subject: [PATCH 04/12] Revert to previous subsumes scheme

Add the path cases without changing the whole logic
---
 .../src/dotty/tools/dotc/cc/CaptureRef.scala  | 82 +++----------------
 1 file changed, 12 insertions(+), 70 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index 107b1a178069..195f07f778eb 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
@@ -93,21 +93,21 @@ trait CaptureRef extends TypeProxy, ValueType:
   final def invalidateCaches() =
     myCaptureSetRunId = NoRunId
 
+  /** x subsumes x
+   *   this subsumes this.f
+   *   x subsumes y  ==>  x* subsumes y, x subsumes y?
+   *   x subsumes y  ==>  x* subsumes y*, x? subsumes y?
+   *   x: x1.type /\ x1 subsumes y  ==>  x subsumes y
+   *   TODO: Document path cases
+   */
   final def subsumes(y: CaptureRef)(using Context): Boolean =
-    val was = subsumesOld(y)
-    val now = subsumesNew(y)
-    if was != now then
-      println(i"diff for  $this subsumes $y, now: $now, ${this.getClass}, ${y.getClass}")
-    was
-
-  final def subsumesOld(y: CaptureRef)(using Context): Boolean =
     (this eq y)
     || this.isRootCapability
     || y.match
         case y: TermRef =>
             y.prefix.match
               case ypre: CaptureRef =>
-                this.subsumesOld(ypre)
+                this.subsumes(ypre)
                 || this.match
                     case x @ TermRef(xpre: CaptureRef, _) =>
                       x.symbol == y.symbol && xpre =:= ypre
@@ -115,78 +115,20 @@ trait CaptureRef extends TypeProxy, ValueType:
                       false
               case _ => false
           || y.info.match
-              case y1: SingletonCaptureRef => this.subsumesOld(y1)
+              case y1: SingletonCaptureRef => this.subsumes(y1)
               case _ => false
-        case MaybeCapability(y1) => this.stripMaybe.subsumesOld(y1)
+        case MaybeCapability(y1) => this.stripMaybe.subsumes(y1)
         case _ => false
     || this.match
-        case ReachCapability(x1) => x1.subsumesOld(y.stripReach)
+        case ReachCapability(x1) => x1.subsumes(y.stripReach)
         case x: TermRef =>
           x.info match
-            case x1: SingletonCaptureRef => x1.subsumesOld(y)
+            case x1: SingletonCaptureRef => x1.subsumes(y)
             case _ => false
         case x: TermParamRef => subsumesExistentially(x, y)
         case x: TypeRef => assumedContainsOf(x).contains(y)
         case _ => false
 
-  /** x subsumes x
-   *   this subsumes this.f
-   *   x subsumes y  ==>  x* subsumes y, x subsumes y?
-   *   x subsumes y  ==>  x* subsumes y*, x? subsumes y?
-   *   x: x1.type /\ x1 subsumes y  ==>  x subsumes y
-   */
-  final def subsumesNew(y: CaptureRef)(using Context): Boolean =
-    def compareCaptureRefs(x: Type, y: Type): Boolean =
-      (x eq y)
-      || y.match
-          case y: CaptureRef => x.match
-            case x: CaptureRef => x.subsumesNew(y)
-            case _ => false
-          case _ => false
-
-    def compareUndelying(x: Type): Boolean = x match
-      case x: SingletonCaptureRef => x.subsumesNew(y)
-      case x: AndType => compareUndelying(x.tp1) || compareUndelying(x.tp2)
-      case x: OrType => compareUndelying(x.tp1) && compareUndelying(x.tp2)
-      case _ => false
-
-    if (this eq y) || this.isRootCapability then return true
-
-    // similar to compareNamed in TypeComparer
-    y match
-      case y: TermRef =>
-        this match
-          case x: TermRef =>
-            val xSym = x.symbol
-            val ySym = y.symbol
-
-            // check x.f and y.f
-            if (xSym ne NoSymbol)
-                && (xSym eq ySym)
-                && compareCaptureRefs(x.prefix, y.prefix)
-              || (x.name eq y.name)
-                && x.isPrefixDependentMemberRef
-                && compareCaptureRefs(x.prefix, y.prefix)
-                && x.signature == y.signature
-                && !(xSym.isClass && ySym.isClass)
-            then return true
-          case _ =>
-
-        // shorten
-        if compareCaptureRefs(this, y.prefix) then return true
-        // underlying
-        if compareCaptureRefs(this, y.info) then return true
-      case MaybeCapability(y1) => return this.stripMaybe.subsumesNew(y1)
-      case _ =>
-
-    return this.match
-      case ReachCapability(x1) => x1.subsumesNew(y.stripReach)
-      case x: TermRef => compareUndelying(x.info)
-      case CapturingType(x1, _) => compareUndelying(x1)
-      case x: TermParamRef => subsumesExistentially(x, y)
-      case x: TypeRef => assumedContainsOf(x).contains(y)
-      case _ => false
-
   def assumedContainsOf(x: TypeRef)(using Context): SimpleIdentitySet[CaptureRef] =
     CaptureSet.assumedContains.getOrElse(x, SimpleIdentitySet.empty)
 

From 54c02426699b8095165f3f3df0bfa6dfa0375b1f Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Wed, 25 Sep 2024 11:57:39 +0200
Subject: [PATCH 05/12] Add logic to mark paths as used

If we refer to a path `a.b`, we should mark `a.b` as used,
which is better than marking `a`.
---
 .../dotty/tools/dotc/cc/CheckCaptures.scala   | 27 +++++++++++++------
 .../dotty/tools/dotc/transform/Recheck.scala  | 15 +++++------
 .../dotty/tools/dotc/typer/ProtoTypes.scala   |  6 +++--
 tests/pos-custom-args/captures/path-use.scala | 19 +++++++++++++
 4 files changed, 49 insertions(+), 18 deletions(-)
 create mode 100644 tests/pos-custom-args/captures/path-use.scala

diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index b3a1ab44d6cd..05bcecf86067 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -13,7 +13,7 @@ import Trees.*
 import typer.RefChecks.{checkAllOverrides, checkSelfAgainstParents, OverridingPairsChecker}
 import typer.Checking.{checkBounds, checkAppliedTypesIn}
 import typer.ErrorReporting.{Addenda, NothingToAdd, err}
-import typer.ProtoTypes.{AnySelectionProto, LhsProto}
+import typer.ProtoTypes.{LhsProto, WildcardSelectionProto}
 import util.{SimpleIdentitySet, EqHashMap, EqHashSet, SrcPos, Property}
 import transform.{Recheck, PreRecheck, CapturedVars}
 import Recheck.*
@@ -183,6 +183,9 @@ object CheckCaptures:
   /** Attachment key for bodies of closures, provided they are values */
   val ClosureBodyValue = Property.Key[Unit]
 
+  /** A prototype that indicates selection with an immutable value */
+  class PathSelectionProto(val sym: Symbol, val pt: Type)(using Context) extends WildcardSelectionProto
+
 class CheckCaptures extends Recheck, SymTransformer:
   thisPhase =>
 
@@ -357,12 +360,13 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  the environment in which `sym` is defined.
      */
     def markFree(sym: Symbol, pos: SrcPos)(using Context): Unit =
-      if sym.exists then
-        val ref = sym.termRef
-        if ref.isTracked then
-          forallOuterEnvsUpTo(sym.enclosure): env =>
-            capt.println(i"Mark $sym with cs ${ref.captureSet} free in ${env.owner}")
-            checkElem(ref, env.captured, pos, provenance(env))
+      markFree(sym, sym.termRef, pos)
+
+    def markFree(sym: Symbol, ref: TermRef, pos: SrcPos)(using Context): Unit =
+      if sym.exists && ref.isTracked then
+        forallOuterEnvsUpTo(sym.enclosure): env =>
+          capt.println(i"Mark $sym with cs ${ref.captureSet} free in ${env.owner}")
+          checkElem(ref, env.captured, pos, provenance(env))
 
     /** Make sure (projected) `cs` is a subset of the capture sets of all enclosing
      *  environments. At each stage, only include references from `cs` that are outside
@@ -464,9 +468,16 @@ class CheckCaptures extends Recheck, SymTransformer:
           includeCallCaptures(tree.symbol, tree.srcPos)
       else
         //debugShowEnvs()
-        markFree(tree.symbol, tree.srcPos)
+        def addSelects(ref: TermRef, pt: Type): TermRef = pt match
+          case pt: PathSelectionProto => addSelects(ref.select(pt.sym).asInstanceOf[TermRef], pt.pt)
+          case _ => ref
+        markFree(tree.symbol, addSelects(tree.symbol.termRef, pt), 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)
+      else super.selectionProto(tree, pt)
+
     /** A specialized implementation of the selection rule.
      *
      *  E |- f: T{ m: R^Cr }^{f}
diff --git a/compiler/src/dotty/tools/dotc/transform/Recheck.scala b/compiler/src/dotty/tools/dotc/transform/Recheck.scala
index 7520767c918c..8df9e5966920 100644
--- a/compiler/src/dotty/tools/dotc/transform/Recheck.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Recheck.scala
@@ -12,7 +12,7 @@ import DenotTransformers.{DenotTransformer, IdentityDenotTransformer, SymTransfo
 import NamerOps.linkConstructorParams
 import NullOpsDecorator.stripNull
 import typer.ErrorReporting.err
-import typer.ProtoTypes.*
+import typer.ProtoTypes.{AnySelectionProto, LhsProto}
 import typer.TypeAssigner.seqLitType
 import typer.ConstFold
 import typer.ErrorReporting.{Addenda, NothingToAdd}
@@ -206,13 +206,12 @@ abstract class Recheck extends Phase, SymTransformer:
       tree.tpe
 
     def recheckSelect(tree: Select, pt: Type)(using Context): Type =
-      recheckSelection(tree, recheckSelectQualifier(tree), tree.name, pt)
+      recheckSelection(tree,
+        recheck(tree.qualifier, selectionProto(tree, pt)).widenIfUnstable,
+        tree.name, pt)
 
-    def recheckSelectQualifier(tree: Select)(using Context): Type =
-      val proto =
-        if tree.symbol == defn.Any_asInstanceOf then WildcardType
-        else AnySelectionProto
-      recheck(tree.qualifier, proto).widenIfUnstable
+    def selectionProto(tree: Select, pt: Type)(using Context): Type =
+      if tree.symbol == defn.Any_asInstanceOf then WildcardType else AnySelectionProto
 
     def recheckSelection(tree: Select, qualType: Type, name: Name,
         sharpen: Denotation => Denotation)(using Context): Type =
@@ -311,7 +310,7 @@ abstract class Recheck extends Phase, SymTransformer:
     def recheckApply(tree: Apply, pt: Type)(using Context): Type =
       val (funtpe0, qualType) = tree.fun match
         case fun: Select =>
-          val qualType = recheckSelectQualifier(fun)
+          val qualType = recheck(fun.qualifier, selectionProto(fun, WildcardType)).widenIfUnstable
           (recheckSelection(fun, qualType, fun.name, WildcardType), qualType)
         case _ =>
           (recheck(tree.fun), NoType)
diff --git a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
index a69a63d1ceef..53e0b456ed9a 100644
--- a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
@@ -324,6 +324,8 @@ object ProtoTypes {
       case tp: UnapplyFunProto => new UnapplySelectionProto(name, nameSpan)
       case tp => SelectionProto(name, IgnoredProto(tp), typer, privateOK = true, nameSpan)
 
+  class WildcardSelectionProto extends SelectionProto(nme.WILDCARD, WildcardType, NoViewsAllowed, true, NoSpan)
+
   /** A prototype for expressions [] that are in some unspecified selection operation
    *
    *    [].?: ?
@@ -332,9 +334,9 @@ object ProtoTypes {
    *  operation is further selection. In this case, the expression need not be a value.
    *  @see checkValue
    */
-  @sharable object AnySelectionProto extends SelectionProto(nme.WILDCARD, WildcardType, NoViewsAllowed, true, NoSpan)
+  @sharable object AnySelectionProto extends WildcardSelectionProto
 
-  @sharable object SingletonTypeProto extends SelectionProto(nme.WILDCARD, WildcardType, NoViewsAllowed, true, NoSpan)
+  @sharable object SingletonTypeProto extends WildcardSelectionProto
 
   /** A prototype for selections in pattern constructors */
   class UnapplySelectionProto(name: Name, nameSpan: Span) extends SelectionProto(name, WildcardType, NoViewsAllowed, true, nameSpan)
diff --git a/tests/pos-custom-args/captures/path-use.scala b/tests/pos-custom-args/captures/path-use.scala
new file mode 100644
index 000000000000..5eb2b60fd218
--- /dev/null
+++ b/tests/pos-custom-args/captures/path-use.scala
@@ -0,0 +1,19 @@
+import language.experimental.namedTuples
+
+class IO
+
+class C(val f: IO^):
+  val procs: List[Proc] = ???
+
+type Proc = () => Unit
+
+def test(io: IO^) =
+  val c = C(io)
+  val f = () => println(c.f)
+  val _: () ->{c.f} Unit = f
+
+  val x = c.procs
+  val _: List[() ->{c.procs*} Unit] = x
+
+  val g = () => println(c.procs.head)
+  val _: () ->{c.procs*} Unit = g

From 3224b2108832f25487e51a51a79c83645b02a8ec Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Wed, 25 Sep 2024 21:01:05 +0200
Subject: [PATCH 06/12] 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.
---
 .../src/dotty/tools/dotc/cc/CaptureRef.scala  | 27 ++++++++++++-------
 .../dotty/tools/dotc/cc/CheckCaptures.scala   | 25 ++++++++++++-----
 .../src/scala/collection/View.scala           |  5 +++-
 .../immutable/LazyListIterable.scala          |  4 ++-
 .../captures/path-patmat-should-be-pos.scala  | 26 ++++++++++++++++++
 5 files changed, 69 insertions(+), 18 deletions(-)
 create mode 100644 tests/neg-custom-args/captures/path-patmat-should-be-pos.scala

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index 195f07f778eb..bbaf0c7d2fa0 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/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)
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index 05bcecf86067..19acebde8651 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/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
 
diff --git a/scala2-library-cc/src/scala/collection/View.scala b/scala2-library-cc/src/scala/collection/View.scala
index 31c544a46beb..132934dbe3bd 100644
--- a/scala2-library-cc/src/scala/collection/View.scala
+++ b/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)
       }
   }
diff --git a/scala2-library-cc/src/scala/collection/immutable/LazyListIterable.scala b/scala2-library-cc/src/scala/collection/immutable/LazyListIterable.scala
index 2f7b017a6729..28ce8da104aa 100644
--- a/scala2-library-cc/src/scala/collection/immutable/LazyListIterable.scala
+++ b/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
diff --git a/tests/neg-custom-args/captures/path-patmat-should-be-pos.scala b/tests/neg-custom-args/captures/path-patmat-should-be-pos.scala
new file mode 100644
index 000000000000..aca6102204a3
--- /dev/null
+++ b/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}

From a0c7361669cc60228b097cbbed2bfbdd483f9c05 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Wed, 9 Oct 2024 15:33:11 +0200
Subject: [PATCH 07/12] Update comments for parser; remove unnecessary checks

---
 compiler/src/dotty/tools/dotc/ast/untpd.scala       |  2 +-
 compiler/src/dotty/tools/dotc/cc/CaptureOps.scala   |  3 +--
 compiler/src/dotty/tools/dotc/parsing/Parsers.scala | 12 +++++-------
 3 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/ast/untpd.scala b/compiler/src/dotty/tools/dotc/ast/untpd.scala
index 4684464d477f..e8e3646bd087 100644
--- a/compiler/src/dotty/tools/dotc/ast/untpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/untpd.scala
@@ -524,7 +524,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def makeRetaining(parent: Tree, refs: List[Tree], annotName: TypeName)(using Context): Annotated =
     Annotated(parent, New(scalaAnnotationDot(annotName), List(refs)))
 
-  def makeCapsOf(tp: Tree)(using Context): Tree =
+  def makeCapsOf(tp: RefTree)(using Context): Tree =
     TypeApply(Select(scalaDot(nme.caps), nme.capsOf), tp :: Nil)
 
   def makeCapsBound()(using Context): Tree =
diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala b/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
index 79cc7d136e45..aad6ca8ddeac 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
@@ -194,8 +194,7 @@ extension (tp: Type)
       true
     case tp: TermRef =>
       ((tp.prefix eq NoPrefix)
-      || tp.symbol.isField && !tp.symbol.isStatic && (
-        tp.prefix.isThisTypeOf(tp.symbol.owner) || tp.prefix.isTrackableRef)
+      || tp.symbol.isField && !tp.symbol.isStatic && tp.prefix.isTrackableRef
       || tp.isRootCapability
       ) && !tp.symbol.isOneOf(UnstableValueFlags)
     case tp: TypeRef =>
diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index dc3ae4cf7639..8eb2a7a1e045 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -1559,22 +1559,20 @@ object Parsers {
       case _ => None
     }
 
-    /** CaptureRef  ::=  (ident | `this`) [`*` | `^`]
+    /** CaptureRef  ::=  SimpleRef { `.` id } [`*` | `^`]
      */
     def captureRef(): Tree =
-      val ref = singleton()
+      val ref = dotSelectors(simpleRef())
       if isIdent(nme.raw.STAR) then
         in.nextToken()
         atSpan(startOffset(ref)):
           PostfixOp(ref, Ident(nme.CC_REACH))
       else if isIdent(nme.UPARROW) then
         in.nextToken()
-        def toTypeSel(r: Tree): Tree = r match
-          case id: Ident => cpy.Ident(id)(id.name.toTypeName)
-          case Select(qual, id) => Select(qual, id.toTypeName)
-          case _ => r
         atSpan(startOffset(ref)):
-          makeCapsOf(toTypeSel(ref))
+          convertToTypeId(ref) match
+            case ref: RefTree => makeCapsOf(ref)
+            case ref => ref
       else ref
 
     /**  CaptureSet ::=  `{` CaptureRef {`,` CaptureRef} `}`    -- under captureChecking

From d930adf334e1f86ead4b31d212544155a8c2587f Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Wed, 9 Oct 2024 17:00:48 +0200
Subject: [PATCH 08/12] More robust check for subsumes

---
 compiler/src/dotty/tools/dotc/cc/CaptureRef.scala | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index bbaf0c7d2fa0..61f18008cbad 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
@@ -110,8 +110,8 @@ trait CaptureRef extends TypeProxy, ValueType:
 
     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 info: AndType => viaInfo(info.tp1)(test) || viaInfo(info.tp2)(test)
+      case info: OrType => viaInfo(info.tp1)(test) && viaInfo(info.tp2)(test)
       case _ => false
 
     (this eq y)
@@ -123,7 +123,7 @@ trait CaptureRef extends TypeProxy, ValueType:
                 this.subsumes(ypre)
                 || this.match
                     case x @ TermRef(xpre: CaptureRef, _) if x.symbol == y.symbol =>
-                      subsumingRefs(xpre, ypre) && subsumingRefs(ypre, xpre)
+                      withMode(Mode.IgnoreCaptures) {TypeComparer.isSameRef(xpre, ypre)}
                     case _ =>
                       false
               case _ => false

From 5bc20acfe93fb86d0d9557bcfa2166a01c1bb75e Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Wed, 9 Oct 2024 18:29:38 +0200
Subject: [PATCH 09/12] Update syntax comment

---
 compiler/src/dotty/tools/dotc/parsing/Parsers.scala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 8eb2a7a1e045..aa62b664ba7c 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -1559,7 +1559,8 @@ object Parsers {
       case _ => None
     }
 
-    /** CaptureRef  ::=  SimpleRef { `.` id } [`*` | `^`]
+    /** CaptureRef  ::=  { SimpleRef `.` } SimpleRef [`*`]
+     *                |  [ { SimpleRef `.` } SimpleRef `.` ] id `^`
      */
     def captureRef(): Tree =
       val ref = dotSelectors(simpleRef())

From 1b0634a2b8d56e2693ef378f326f250bcab71d1b Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Fri, 27 Sep 2024 19:04:45 +0200
Subject: [PATCH 10/12] Account for added outer refs in the capture sets of
 classes

---
 .../dotty/tools/dotc/cc/CheckCaptures.scala   | 45 ++++++++++++++-----
 .../captures/outerRefsUses.scala              | 10 +++++
 2 files changed, 45 insertions(+), 10 deletions(-)
 create mode 100644 tests/neg-custom-args/captures/outerRefsUses.scala

diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index 19acebde8651..4d905a5df4ab 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -1089,7 +1089,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
       if actualBoxed eq actual then
         // Only `addOuterRefs` when there is no box adaptation
-        expected1 = addOuterRefs(expected1, actual)
+        expected1 = addOuterRefs(expected1, actual, tree.srcPos)
       if isCompatible(actualBoxed, expected1) then
         if debugSuccesses then tree match
             case Ident(_) =>
@@ -1130,8 +1130,12 @@ class CheckCaptures extends Recheck, SymTransformer:
      *     that are outside `Cls`. These are all accessed through `Cls.this`,
      *     so we can assume they are already accounted for by `Ce` and adding
      *     them explicitly to `Ce` changes nothing.
+     *   - To make up for this, we also add these variables to the capture set of `Cls`,
+     *     so that all instances of `Cls` will capture these outer references.
+     *  So in a sense we use `{Cls.this}` as a placeholder for certain outer captures.
+     *  that we needed to be subsumed by `Cls.this`.
      */
-    private def addOuterRefs(expected: Type, actual: Type)(using Context): Type =
+    private def addOuterRefs(expected: Type, actual: Type, pos: SrcPos)(using Context): Type =
 
       def isPure(info: Type): Boolean = info match
         case info: PolyType => isPure(info.resType)
@@ -1144,19 +1148,40 @@ class CheckCaptures extends Recheck, SymTransformer:
         else isPure(owner.info) && isPureContext(owner.owner, limit)
 
       // Augment expeced capture set `erefs` by all references in actual capture
-      // set `arefs` that are outside some `this.type` reference in `erefs`
+      // set `arefs` that are outside some `C.this.type` reference in `erefs` for an enclosing
+      // class `C`. If an added reference is not a ThisType itself, add it to the capture set
+      // (i.e. use set) of the `C`. This makes sure that any outer reference implicitly subsumed
+      // by `C.this` becomes a capture reference of every instance of `C`.
       def augment(erefs: CaptureSet, arefs: CaptureSet): CaptureSet =
         (erefs /: erefs.elems): (erefs, eref) =>
           eref match
             case eref: ThisType if isPureContext(ctx.owner, eref.cls) =>
-              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)
+
+              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
+                case aref: NamedType => eref.cls.isProperlyContainedIn(aref.symbol.owner)
                 case aref: ThisType => eref.cls.isProperlyContainedIn(aref.cls)
                 case _ => false
-              erefs ++ arefs.filter(isOuterRef)
+
+              val outerRefs = arefs.filter(isOuterRef)
+
+              // 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]
+                    // 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.
+                then
+                  def provenance =
+                    i""" of the enclosing class ${eref.cls}.
+                       |The reference was included since we tried to establish that $arefs <: $erefs"""
+                  checkElem(outerRef, capturedVars(eref.cls), pos, provenance)
+
+              erefs ++ outerRefs
             case _ =>
               erefs
 
@@ -1341,7 +1366,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         *  @param sym  symbol of the field definition that is being checked
         */
         override def checkSubType(actual: Type, expected: Type)(using Context): Boolean =
-          val expected1 = alignDependentFunction(addOuterRefs(expected, actual), actual.stripCapturing)
+          val expected1 = alignDependentFunction(addOuterRefs(expected, actual, srcPos), actual.stripCapturing)
           val actual1 =
             val saved = curEnv
             try
diff --git a/tests/neg-custom-args/captures/outerRefsUses.scala b/tests/neg-custom-args/captures/outerRefsUses.scala
new file mode 100644
index 000000000000..cd03c8c41efd
--- /dev/null
+++ b/tests/neg-custom-args/captures/outerRefsUses.scala
@@ -0,0 +1,10 @@
+class IO
+def test(io: IO^) =
+  class C:
+    def foo() = () =>
+      val x: IO^{this} = io
+      ()
+  val c = new C
+  val _: C^{io} = c // ok
+  val _: C = c // error
+  ()

From f16d5652e10dca2832c6a4c246910a7e733bded9 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Tue, 29 Oct 2024 17:35:53 +0100
Subject: [PATCH 11/12] Add note and a test to show the prefix rule for path
 subsuming

---
 .../src/dotty/tools/dotc/cc/CaptureRef.scala  |  6 +++
 .../captures/path-prefix.scala                | 44 +++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 tests/neg-custom-args/captures/path-prefix.scala

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index 61f18008cbad..8676390eda04 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
@@ -123,6 +123,12 @@ trait CaptureRef extends TypeProxy, ValueType:
                 this.subsumes(ypre)
                 || this.match
                     case x @ TermRef(xpre: CaptureRef, _) 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 for subtyping,
+                      // not just `{x} =:= {y}`.
+                      // It is posible to construct two singleton types `x` and `y`,
+                      // which subumse 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
diff --git a/tests/neg-custom-args/captures/path-prefix.scala b/tests/neg-custom-args/captures/path-prefix.scala
new file mode 100644
index 000000000000..af5817636d0b
--- /dev/null
+++ b/tests/neg-custom-args/captures/path-prefix.scala
@@ -0,0 +1,44 @@
+import language.experimental.modularity
+import language.experimental.captureChecking
+import caps.Capability
+
+class F:
+  val f: AnyRef^ = ???
+
+case class B(tracked val a: A) extends F, Capability
+
+class A extends F, Capability:
+  val b: B { val a: A.this.type } = B(this)
+
+def test(a: A) =
+  val x: a.b.type = a.b
+  val y: x.a.type = x.a
+  // x and y are two distinct singleton types with following properties:
+  // x =:= a.b
+  // y =:= x.a =:= a.b.a =:= a
+
+  val cx: AnyRef^{x} = ???
+  val cy: AnyRef^{y} = ???
+  val caf: AnyRef^{a.f} = ???
+  val cabf: AnyRef^{a.b.f} = ???
+  val cxf: AnyRef^{x.f} = ???
+  val cyf: AnyRef^{y.f} = ???
+
+  // x and y subsume to each other:
+  // * {x} <:< {y}: the underlying singleton of y is x.a,
+  //   and the underlying singleton of x.a is a,
+  //   which is a prefix for the underlying type of x (a.b),
+  //   hence {x} <:< {y};
+  // * {y} <:< {x}: by underlying singleton of y is x.a, whose prefix is x.
+  // Hence, {x} =:= {y}.
+  val x2y: AnyRef^{y} = cx
+  val y2x: AnyRef^{x} = cy
+
+  val yf2af: AnyRef^{a.f} = cyf
+  val af2yf: AnyRef^{y.f} = caf
+  val xf2abf: AnyRef^{a.b.f} = cxf
+  val abf2xf: AnyRef^{x.f} = cabf
+
+  // Since `x !=:= y`, {x.f} !=:= {y.f}
+  val yf2xf: AnyRef^{x.f} = cyf // error
+  val xf2yf: AnyRef^{y.f} = cxf // error

From ad58323fb6e9a9c0bafa51db869168a084cee691 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Tue, 29 Oct 2024 17:40:05 +0100
Subject: [PATCH 12/12] Fix typo in the comment

---
 compiler/src/dotty/tools/dotc/cc/CaptureRef.scala | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
index 8676390eda04..590beda42903 100644
--- a/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
@@ -124,10 +124,10 @@ trait CaptureRef extends TypeProxy, ValueType:
                 || this.match
                     case x @ TermRef(xpre: CaptureRef, _) 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 for subtyping,
-                      // not just `{x} =:= {y}`.
-                      // It is posible to construct two singleton types `x` and `y`,
-                      // which subumse each other, but are not equal references.
+                      // 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 _ =>