Don't add ^ to singleton capabilities during Setup

Author: odersky

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

@@ -317,7 +317,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
             this(expandThrowsAlias(res, exc, Nil))
           case t =>
             // Map references to capability classes C to C^
-            if t.derivesFromCapability && t.typeSymbol != defn.Caps_Exists
+            if t.derivesFromCapability && !t.isSingleton && t.typeSymbol != defn.Caps_Exists
             then CapturingType(t, CaptureSet.universal, boxed = false)
             else normalizeCaptures(mapOver(t))
     end toCapturing

tests/pos-custom-args/captures/cap-problem.scala

@@ -0,0 +1,13 @@
+import language.experimental.captureChecking
+
+trait Suspend:
+  type Suspension
+
+  def resume(s: Suspension): Unit
+
+import caps.Capability
+
+trait Async(val support: Suspend) extends Capability
+
+class CancelSuspension(ac: Async, suspension: ac.support.Suspension):
+  ac.support.resume(suspension)