Don't drop empty capture sets of capability classes

We need a way to distinguish C^{} and C^{cap} when printing. We
print C^{cap} as C is C is a capability class.

Author: odersky

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

@@ -34,7 +34,8 @@ object CapturingType:
    */
   def apply(parent: Type, refs: CaptureSet, boxed: Boolean = false)(using Context): Type =
     assert(!boxed || !parent.derivesFrom(defn.Caps_CapSet))
-    if refs.isAlwaysEmpty && !refs.keepAlways then parent
+    if refs.isAlwaysEmpty && !refs.keepAlways && !parent.derivesFromCapability then
+      parent
     else parent match
       case parent @ CapturingType(parent1, refs1) if boxed || !parent.isBoxed =>
         apply(parent1, refs ++ refs1, boxed)