Flatten nested capture sets in retainedElementsRaw

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

@@ -94,6 +94,8 @@ extension (tp: Type)
   def retainedElementsRaw(using Context): List[Type] = tp match
     case OrType(tp1, tp2) =>
       tp1.retainedElementsRaw ++ tp2.retainedElementsRaw
+    case AnnotatedType(tp1, ann) if tp1.derivesFrom(defn.Caps_CapSet) && ann.symbol.isRetains =>
+      ann.tree.retainedSet.retainedElementsRaw
     case tp =>
       // Nothing is a special type to represent the empty set
       if tp.isNothingType then Nil

tests/pos-custom-args/captures/i23570.scala

@@ -0,0 +1,7 @@
+def f[C^](xs: List[() ->{C} Unit]): List[() ->{C} Unit] =
+  xs.reverse
+
+def test(io: Object^, async: Object^): Unit =
+  val ok = f[{io}](Nil)
+  val x = f[{io}]        // was error
+  val y = f[{io, async}] // was error

tests/pos/cc-use-alternatives.scala

@@ -0,0 +1,24 @@
+import language.experimental.captureChecking
+// no separation checking
+import caps.{cap, use}
+
+def foo1(@use xs: List[() => Unit]): Unit =
+  var x: () ->{xs*} Unit = xs.head
+  var ys = xs
+  while ys.nonEmpty do
+    ys = ys.tail
+    x = ys.head
+
+def foo2(@use xs: List[() => Unit]): Unit =
+  var x: () => Unit = xs.head // note: this would fail separation checking
+  var ys = xs
+  while ys.nonEmpty do
+    ys = ys.tail
+    x = ys.head
+
+def foo3[@use C^](xs: List[() ->{C} Unit]): Unit =
+  var x: () ->{C} Unit = xs.head
+  var ys = xs
+  while ys.nonEmpty do
+    ys = ys.tail
+    x = ys.head