Add reach capabilities on the fly in getBoxed

Author: odersky

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

@@ -320,18 +320,27 @@ extension (tp: Type)
   /** The capture set consisting of all top-level captures of `tp` that appear under a box.
    *  Unlike for `boxed` this also considers parents of capture types, unions and
    *  intersections, and type proxies other than abstract types.
+   *  Furthermore, if the original type is a capture ref `x`, it replaces boxed universal sets
+   *  on the fly with x*.
    */
   def boxedCaptureSet(using Context): CaptureSet =
-    def getBoxed(tp: Type): CaptureSet = tp match
+    def getBoxed(tp: Type, pre: Type): CaptureSet = tp match
       case tp @ CapturingType(parent, refs) =>
-        val pcs = getBoxed(parent)
-        if tp.isBoxed then refs ++ pcs else pcs
+        val pcs = getBoxed(parent, pre)
+        if !tp.isBoxed then
+          pcs
+        else if pre.exists && refs.containsRootCapability then
+          val reachRef = if refs.isReadOnly then pre.reach.readOnly else pre.reach
+          pcs ++ reachRef.singletonCaptureSet
+        else
+          pcs ++ refs
+      case ref: CaptureRef if ref.isTracked && !pre.exists => getBoxed(ref, ref)
       case tp: TypeRef if tp.symbol.isAbstractOrParamType => CaptureSet.empty
-      case tp: TypeProxy => getBoxed(tp.superType)
-      case tp: AndType => getBoxed(tp.tp1) ** getBoxed(tp.tp2)
-      case tp: OrType => getBoxed(tp.tp1) ++ getBoxed(tp.tp2)
+      case tp: TypeProxy => getBoxed(tp.superType, pre)
+      case tp: AndType => getBoxed(tp.tp1, pre) ** getBoxed(tp.tp2, pre)
+      case tp: OrType => getBoxed(tp.tp1, pre) ++ getBoxed(tp.tp2, pre)
       case _ => CaptureSet.empty
-    getBoxed(tp)
+    getBoxed(tp, NoType)
 
   /** Is the boxedCaptureSet of this type nonempty? */
   def isBoxedCapturing(using Context): Boolean =