Document bug NodeList::getChildrenOfItemsOfType() does not refine

tests/NodeListGetChildrenOfItemsOfTypeTest.hack

@@ -0,0 +1,36 @@
+/*
+ *  Copyright (c) 2017-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+namespace Facebook\HHAST;
+
+use namespace HH\Lib\C;
+use type Facebook\HackTest\HackTest;
+
+final class NodeListGetChildrenOfItemsOfTypeTest extends HackTest {
+  public function testRefinesType(): void {
+    $node_list = static::getNodeListOfItemsOfIExpression();
+    $literal_expression =
+      $node_list->getChildrenOfItemsOfType(LiteralExpression::class)
+      |> C\firstx($$);
+    static::takesT<LiteralExpression>($literal_expression);
+  }
+
+  private static function getNodeListOfItemsOfIExpression(
+  ): NodeList<ListItem<IExpression>> {
+    // NodeList(123,)
+    return new NodeList(
+      vec[new ListItem(
+        new LiteralExpression(new DecimalLiteralToken(null, null, '123')),
+        new CommaToken(null, null),
+      )],
+    );
+  }
+
+  private static function takesT<<<__Explicit>> T>(T $_): void {}
+}