Fix pathRoot and pathOwner

There was some accidental type confusion which made every capture root type end up
with cap as pathRoot and caps as pathOwner.

Author: odersky

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

@@ -192,25 +192,34 @@ extension (tp: Type)
     case _ =>
       tp
 
-  /** The first element of this path type. Note that class parameter references
-   *  are of the form this.C but their pathroot is still this.C, not this.
+  /** The first element of this path type, skipping selections
+   *  and qualifiers. Note that class parameter references are of
+   *  the form this.C but their pathroot is still this.C, not this.
    */
-  final def pathRoot(using Context): Type = tp.dealias match
-    case tp1: NamedType =>
-      if tp1.symbol.maybeOwner.isClass && tp1.symbol != defn.captureRoot && !tp1.symbol.is(TypeParam) then
-        tp1.prefix match
-          case pre: CaptureRef => pre.pathRoot
-          case _ => tp1
-      else tp1
-    case tp1 => tp1
-
-  /** If this part starts with `C.this`, the class `C`.
-   *  Otherwise, if it starts with a reference `r`, `r`'s owner.
-   *  Otherwise NoSymbol.
+  final def pathRoot(using Context): Type = tp match
+    case root(_) => tp
+    case QualifiedCapability(tp1) => tp1.pathRoot
+    case _ => tp.dealias match
+      case tp1: NamedType =>
+        if tp1.symbol.maybeOwner.isClass && !tp1.symbol.is(TypeParam) then
+          tp1.prefix match
+            case pre: CaptureRef => pre.pathRoot
+            case _ => tp1
+        else tp1
+      case tp1 => tp1
+
+  /** The logical owner of the root of this class:
+   *   - If this path starts with `C.this`, the class `C`.
+   *   - If it starts with a reference `r`, `r`'s owner.
+   *   - If it starts with cap, the `scala.caps` package class.
+   *   - If it starts with a fresh instance, its owner.
+   *   - If it starts with a ParamRef or a result root, NoSymbol.
    */
   final def pathOwner(using Context): Symbol = pathRoot match
+    case tp1: TermRef if tp1.isCap => defn.CapsModule.moduleClass
     case tp1: NamedType => tp1.symbol.owner
     case tp1: ThisType => tp1.cls
+    case tp1 @ root.Fresh(_) => tp1.ccOwner
     case _ => NoSymbol
 
   final def isParamPath(using Context): Boolean = tp.dealias match

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

@@ -313,7 +313,7 @@ sealed abstract class CaptureSet extends Showable:
    *      will also map any elements added in the future to themselves. This assumption
    *      can be tested to hold by setting the ccConfig.checkSkippedMaps setting to true.
    *    - If the map is some other map that does not map all elements to themselves,
-   *      freeze the current set (i.e. make it porvisionally solved) and return
+   *      freeze the current set (i.e. make it provisionally solved) and return
    *      the mapped elements as a constant set.
    */
   def map(tm: TypeMap)(using Context): CaptureSet =

tests/neg-custom-args/captures/i15772.check

@@ -18,14 +18,18 @@
    |          ^   refers to the universal root capability
    |          ^²  refers to a fresh root capability in the type of value arg
    |          cap is the universal root capability
--- Error: tests/neg-custom-args/captures/i15772.scala:35:33 ------------------------------------------------------------
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:35:33 ---------------------------------------
 35 |  val boxed2 : Observe[C]^ = box2(c) // error
    |                             ^^^^^^^
-   |                             reference cap is not included in the allowed capture set ?
-   |                             of the enclosing method main3
+   |Found:    (C{val arg: C^}^² => Unit) -> Unit
+   |Required: (C => Unit) =>² Unit
    |
-   |                             Note that the universal capability cap
-   |                             cannot be included in capture set ?
+   |where:    =>  refers to the universal root capability
+   |          =>² refers to a fresh root capability in the type of value boxed2
+   |          ^   refers to a fresh root capability in the type of value arg
+   |          ^²  refers to a fresh root capability created in value boxed2 when instantiating method c's type -> C^{cap}
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i15772.scala:46:2 ----------------------------------------
 46 |  x: (() -> Unit) // error
    |  ^

tests/neg-custom-args/captures/reaches.check

@@ -21,14 +21,6 @@
 34 |      (() => f.write()) :: Nil
    |
    | longer explanation available when compiling with `-explain`
--- Error: tests/neg-custom-args/captures/reaches.scala:39:31 -----------------------------------------------------------
-39 |    val next: () => Unit = cur.head   // error
-   |                           ^^^^^^^^
-   |                           reference cap is not included in the allowed capture set ?
-   |                           of the enclosing method runAll2
-   |
-   |                           Note that the universal capability cap
-   |                           cannot be included in capture set ?
 -- Error: tests/neg-custom-args/captures/reaches.scala:44:16 -----------------------------------------------------------
 44 |  val cur = Ref[List[Proc]](xs) // error
    |                ^^^^^^^^^^

tests/neg-custom-args/captures/reaches.scala

@@ -36,7 +36,7 @@ def runAll1(@use xs: List[Proc]): Unit =
 def runAll2(@consume xs: List[Proc]): Unit =
   var cur: List[Proc] = xs
   while cur.nonEmpty do
-    val next: () => Unit = cur.head   // error
+    val next: () => Unit = cur.head   // was error, now OK
     next()
     cur = cur.tail