Fix setup of CapSet arguments.

These arguments tell the whole truth; they cannot possibly be decorated with another capture set. So
we should not add a capture set variable.

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

@@ -605,8 +605,10 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
         !refs.isEmpty
       case tp: (TypeRef | AppliedType) =>
         val sym = tp.typeSymbol
-        if sym.isClass then !sym.isPureClass
-        else instanceCanBeImpure(tp.superType)
+        if sym.isClass
+        then !sym.isPureClass
+        else !tp.derivesFrom(defn.Caps_CapSet) // CapSet arguments don't get other capture set variables added
+          && instanceCanBeImpure(tp.superType)
       case tp: (RefinedOrRecType | MatchType) =>
         instanceCanBeImpure(tp.underlying)
       case tp: AndType =>

tests/pos/polycap.scala

@@ -0,0 +1,14 @@
+import language.experimental.captureChecking
+
+class Source[+T, Cap^]
+
+def completed[T, Cap^](result: T): Source[T, Cap] =
+  //val fut = new Source[T, Cap]()
+  val fut2 = new Source[T, Cap]()
+  fut2: Source[T, Cap]
+
+
+
+
+
+