add test case for scala#21507

Author: Linyxus

tests/pos-custom-args/captures/i21507.scala

@@ -0,0 +1,10 @@
+import language.experimental.captureChecking
+
+trait Box[Cap^]:
+  def store(f: (() -> Unit)^{Cap^}): Unit
+
+def run[Cap^](f: Box[Cap]^{Cap^} => Unit): Box[Cap]^{Cap^} =
+  new Box[Cap]:
+    private var item: () ->{Cap^} Unit = () => ()
+    def store(f: () ->{Cap^} Unit): Unit =
+      item = f  // was error, now ok