Allow SharableCapablity anywhere on a path

Author: odersky

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

@@ -285,6 +285,16 @@ extension (tp: Type)
     case tp1: TypeRef if !tp1.symbol.is(Param) => tp1.prefix.pathRoot
     case tp1 => tp1
 
+  /** The first element of a path type, but stop at references extending
+   *  SharableCapability
+   */
+  final def pathRootOrShared(using Context): Type =
+    if tp.derivesFrom(defn.Caps_SharedCapability) 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.

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

@@ -410,7 +410,7 @@ class SepChecker(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.pathRoot
+      val proot = hiddenRef.pathRootOrShared
       if !proot.widen.derivesFrom(defn.Caps_SharedCapability) then
         proot match
           case ref: TermRef =>
@@ -448,7 +448,7 @@ class SepChecker(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
     role match
       case _: TypeRole.Argument | _: TypeRole.Qualifier =>
         for ref <- refsToCheck do
-          if !ref.derivesFrom(defn.Caps_SharedCapability) then
+          if !ref.pathRootOrShared.derivesFrom(defn.Caps_SharedCapability) then
             consumed.put(ref, pos)
       case _ =>
   end checkConsumedRefs