Improve printing of capture sets before cc

Use the syntactic sugar instead of expanding with capsOf

Author: odersky

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -165,6 +165,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
   private def toTextRetainedElem[T <: Untyped](ref: Tree[T]): Text = ref match
     case ref: RefTree[?] if ref.typeOpt.exists =>
       toTextCaptureRef(ref.typeOpt)
+    case TypeApply(fn, arg :: Nil) if fn.symbol == defn.Caps_capsOf =>
+      toTextRetainedElem(arg)
     case _ =>
       toText(ref)
 
@@ -416,6 +418,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: SingletonType => toTextRef(tp)
       case ReachCapability(tp1) => toTextRef(tp1) ~ "*"
       case MaybeCapability(tp1) => toTextRef(tp1) ~ "?"
+      case tp: (TypeRef | TypeParamRef) => toText(tp) ~ "^"
       case _ => toText(tp)
 
   protected def isOmittablePrefix(sym: Symbol): Boolean =

tests/pos/cc-poly-1.scala

@@ -9,6 +9,7 @@ import caps.{CapSet, Capability}
 
   def f[X^](x: D^{X^}): D^{X^} = x
   def g[X^](x: D^{X^}, y: D^{X^}): D^{X^} = x
+  def h[X^](): D^{X^} = ???
 
   def test(c1: C, c2: C) =
     val d: D^{c1, c2} = D()
@@ -19,5 +20,7 @@ import caps.{CapSet, Capability}
     val y = g(d1, d2)
     val _: D^{d1, d2} = y
     val _: D^{c1, c2} = y
+    val z = h()
+