Fix handling paths extending SharedCapability (#22564)

This partially reverts commit 7b3d3f4.
It looks like this fixes the problems we had with CI timeouts as well.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -232,9 +232,7 @@ extension (tp: Type)
       case tp @ ReachCapability(_) =>
         tp.singletonCaptureSet
       case ReadOnlyCapability(ref) =>
-        val refDcs = ref.deepCaptureSet(includeTypevars)
-        if refDcs.isConst then CaptureSet(refDcs.elems.map(_.readOnly))
-        else refDcs // this case should not happen for correct programs
+        ref.deepCaptureSet(includeTypevars).readOnly
       case tp: SingletonCaptureRef if tp.isTrackableRef =>
         tp.reach.singletonCaptureSet
       case _ =>
@@ -285,20 +283,10 @@ extension (tp: Type)
    *  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: TermRef if tp1.symbol.maybeOwner.isClass => tp1.prefix.pathRoot
-    case tp1: TypeRef if !tp1.symbol.is(Param) => tp1.prefix.pathRoot
+    case tp1: NamedType if tp1.symbol.maybeOwner.isClass && !tp1.symbol.is(TypeParam) =>
+      tp1.prefix.pathRoot
     case tp1 => tp1
 
-  /** The first element of a path type, but stop at references extending
-   *  SharedCapability.
-   */
-  final def pathRootOrShared(using Context): Type =
-    if tp.derivesFromSharedCapability then tp
-    else tp.dealias match
-      case tp1: TermRef if tp1.symbol.maybeOwner.isClass => tp1.prefix.pathRoot
-      case tp1: TypeRef if !tp1.symbol.is(Param) => tp1.prefix.pathRoot
-      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.

Diff for: compiler/src/dotty/tools/dotc/cc/SepCheck.scala

@@ -537,9 +537,8 @@ class SepCheck(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
     val badParams = mutable.ListBuffer[Symbol]()
     def currentOwner = role.dclSym.orElse(ctx.owner)
     for hiddenRef <- prune(refsToCheck, tpe, role) do
-      val proot = hiddenRef.pathRootOrShared
-      if !proot.widen.derivesFromSharedCapability then
-        proot match
+      if !hiddenRef.derivesFromSharedCapability then
+        hiddenRef.pathRoot match
           case ref: TermRef =>
             val refSym = ref.symbol
             if currentOwner.enclosingMethodOrClass.isProperlyContainedIn(refSym.maybeOwner.enclosingMethodOrClass) then
@@ -575,7 +574,7 @@ class SepCheck(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
     role match
       case _: TypeRole.Argument | _: TypeRole.Qualifier =>
         for ref <- refsToCheck do
-          if !ref.pathRootOrShared.derivesFromSharedCapability then
+          if !ref.derivesFromSharedCapability then
             consumed.put(ref, pos)
       case _ =>
   end checkConsumedRefs