Expand Capability types also in arguments of Capability classes (#23427)

Fixes #23422

Author: odersky

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

@@ -403,10 +403,14 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
        */
       def defaultApply(t: Type) =
         if t.derivesFromCapability
+          && t.typeParams.isEmpty
           && !t.isSingleton
           && (!sym.isConstructor || (t ne tp.finalResultType))
             // Don't add ^ to result types of class constructors deriving from Capability
-        then CapturingType(t, CaptureSet.CSImpliedByCapability(), boxed = false)
+        then
+          normalizeCaptures(mapOver(t)) match
+            case t1 @ CapturingType(_, _) => t1
+            case t1 => CapturingType(t1, CaptureSet.CSImpliedByCapability(), boxed = false)
         else normalizeCaptures(mapFollowingAliases(t))
 
       def innerApply(t: Type) =

tests/pos-custom-args/captures/i23422.scala

@@ -0,0 +1,10 @@
+import language.experimental.captureChecking
+import caps.*
+trait Cap
+class Inv[T] extends Capability
+class Inv2[T]
+class Inv3[T] extends Mutable
+def test(c: Cap^): Unit =
+  val t1: Inv[() ->{c} Unit] = Inv()  // error
+  val t2: Inv2[() ->{c} Unit] = Inv2()  // ok
+  val t3: Inv3[() ->{c} Unit] = Inv3()   // error, too