Handle capture parameters in dcs

Author: Linyxus

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

@@ -1688,7 +1688,8 @@ object CaptureSet:
         else this(acc, parent)
 
       def abstractTypeCase(acc: CaptureSet, t: TypeRef, upperBound: Type) =
-        if includeTypevars && upperBound.isExactlyAny then fresh(Origin.DeepCS(t))
+        if t.derivesFrom(defn.Caps_CapSet) then t.singletonCaptureSet
+        else if includeTypevars && upperBound.isExactlyAny then fresh(Origin.DeepCS(t))
         else this(acc, upperBound)
 
     collect(CaptureSet.empty, tp)

tests/pos-custom-args/captures/i24309b.scala

@@ -0,0 +1,6 @@
+import language.experimental.captureChecking
+def runOps[C^](ops: List[() ->{C} Unit]): Unit = ops.foreach(_())  // ok
+trait Ops[C^] { def toList: List[() ->{C} Unit] }
+def runOpsAlt1[C1^](ops: Ops[C1]): Unit = runOps[{C1}](???)  // was error, now ok
+def runOpsAlt2[C2^](ops: Ops[{}]^{C2}): Unit = runOps[{C2}](???)  // ok
+def runOpsAlt3[C3^](ops: Ops[C3]^{C3}): Unit = runOps[{C3}](???)  // was error, no ok