Fix retainedElements throwing exceptions on illegal refs too early

Author: noti0na1

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

@@ -78,18 +78,22 @@ extension (tree: Tree)
 
 extension (tp: Type)
 
-  def retainedElements(using Context): List[CaptureRef] = tp match
+  def retainedElementsRaw(using Context): List[Type] = tp match
     case ReachCapability(tp1) =>
       tp1.reach :: Nil
     case ReadOnlyCapability(tp1) =>
       tp1.readOnly :: Nil
-    case tp: CaptureRef =>
+    case OrType(tp1, tp2) =>
+      tp1.retainedElementsRaw ++ tp2.retainedElementsRaw
+    case tp =>
+      // Nothing is a special type to represent the empty set
       if tp.isNothingType then Nil
       else tp :: Nil // should be checked by wellformedness
-    case OrType(tp1, tp2) =>
-      tp1.retainedElements ++ tp2.retainedElements
-    case _ =>
-      throw IllegalCaptureRef(tp)
+
+  def retainedElements(using Context): List[CaptureRef] =
+    retainedElementsRaw.map:
+      case tp: CaptureRef => tp
+      case tp => throw IllegalCaptureRef(tp)
 
   /** Is this type a CaptureRef that can be tracked?
    *  This is true for

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

@@ -105,7 +105,7 @@ object CheckCaptures:
           report.error(em"$elem cannot be tracked since it is not a parameter or local value", pos)
       case tpe =>
         report.error(em"$elem: $tpe is not a legal element of a capture set", pos)
-    for elem <- ann.retainedSet.retainedElements do
+    for elem <- ann.retainedSet.retainedElementsRaw do
       elem match
         case ref: TypeRef =>
           val refSym = ref.symbol

tests/neg-custom-args/captures/capt1.check

@@ -55,7 +55,7 @@
    | longer explanation available when compiling with `-explain`
 -- Error: tests/neg-custom-args/captures/capt1.scala:38:13 -------------------------------------------------------------
 38 |  val z3 = h[(() -> Cap) @retains[x.type]](() => x)(() => C())  // error
-   |             ^^^^^^^^^^^^^^^^^^^^^^^
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |             Type variable X of method h cannot be instantiated to box () ->{x} C^ since
    |             the part C^ of that type captures the root capability `cap`.
    |

tests/neg-custom-args/captures/i21868.scala

@@ -1,13 +1,14 @@
+import language.experimental.captureChecking
 import caps.*
 
 trait AbstractWrong:
   type C <: CapSet
   def f(): Unit^{C} // error
 
 trait Abstract1:
-  type C >: CapSet <: CapSet^
+  type C >: CapSet <: CapSet^ // error
   def f(): Unit^{C}
 
-// class Abstract2:
-//   type C^
-//   def f(): Unit^{C^}
+trait Abstract2:
+  type C <: {cap}
+  def f(): Unit^{C}