An alternative way to refer to * with no -Ycc set

Author: odersky

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

@@ -15,8 +15,11 @@ private val BoxedType: Key[BoxedTypeCache] = Key()
 
 /** The arguments of a @retains or @retainsByName annotation */
 private[cc] def retainedElems(tree: Tree)(using Context): List[Tree] = tree match
-  case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) => elems
-  case _ => Nil
+  case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) =>
+    elems.map(elem =>
+      if elem.symbol == defn.Caps_universal then ref(defn.captureRoot) else elem)
+  case _ =>
+    Nil
 
 /** An exception thrown if a @retains argument is not syntactically a CaptureRef */
 class IllegalCaptureRef(tpe: Type) extends Exception

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

@@ -961,6 +961,7 @@ class Definitions {
   def RuntimeTupleFunctionsModule(using Context): Symbol = requiredModule("scala.runtime.TupledFunctions")
 
   @tu lazy val CapsModule: Symbol = requiredModule("scala.caps")
+    @tu lazy val Caps_universal: Symbol = CapsModule.requiredValue("universal")
     @tu lazy val Caps_unsafeUnbox: Symbol = CapsModule.requiredMethod("unsafeUnbox")
 
   // Annotation base classes

library/src/scala/caps.scala

@@ -9,3 +9,6 @@ object caps:
    *  avoids the error that would be raised when unboxing `*`.
    */
   extension [T](x: T) def unsafeUnbox: T = x
+
+  /** An alternative for `*`, which can be used without -Ycc */
+  val universal: Any = ()

tests/pos-custom-args/captures/caps-universal.scala

@@ -0,0 +1,8 @@
+import caps.*
+import annotation.retains
+
+val foo: Int => Int = x => x
+val bar: (Int -> Int) @retains(universal) = foo
+val baz: {*} Int -> Int = bar
+
+