Fix retainedElements rule

Author: noti0na1

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

@@ -83,15 +83,13 @@ extension (tp: Type)
       tp1.reach :: Nil
     case ReadOnlyCapability(tp1) =>
       tp1.readOnly :: Nil
-    case tp: CaptureRef if tp.isTrackableRef =>
-      tp :: Nil
-    case tp: TypeRef if tp.symbol.isType && tp.derivesFrom(defn.Caps_CapSet) =>
-      tp :: Nil
+    case tp: CaptureRef =>
+      if tp.isNothingType then Nil
+      else tp :: Nil // should be checked by wellformedness
     case OrType(tp1, tp2) =>
       tp1.retainedElements ++ tp2.retainedElements
     case _ =>
-      if tp.isNothingType then Nil
-      else throw IllegalCaptureRef(tp)
+      throw IllegalCaptureRef(tp)
 
   /** Is this type a CaptureRef that can be tracked?
    *  This is true for

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -788,7 +788,7 @@ object SymDenotations {
       * So the first call to a stable member might fail and/or produce side effects.
       */
     final def isStableMember(using Context): Boolean = {
-      def isUnstableValue = isOneOf(UnstableValueFlags) || info.isInstanceOf[ExprType] || isAllOf(InlineParam)
+      def isUnstableValue = isOneOf(UnstableValueFlags) || isAllOf(InlineParam)
       isType || is(StableRealizable) || exists && !isUnstableValue
     }
 

tests/neg-custom-args/captures/capt-wf-typer.scala

@@ -6,6 +6,6 @@ object foo
 
 def test(c: Cap, other: String): Unit =
   val x7: String^{c} = ??? // OK
-  val x8: String @retains[x7 + x7] = ??? // error
+  val x8: String @retains[(x7 + x7)] = ??? // error
   val x9: String @retains[foo] = ??? // error
   ()