/hphp/hack/test/typecheck/recursive_optional.php

 1<?hh // partial
 2
 3/* HH_FIXME[4336] */
 4function my_compact<T>(Vector<?T> $vector): Vector<T> {
 5}
 6
 7function test(bool $b): ?string {
 8
 9  $nullable = Vector { null }; // Vector([?T]])
10  $non_nullable = my_compact($nullable); // Vector([T])
11
12  if ($b) {
13    $res = $nullable;
14  } else {
15    $res = $non_nullable;
16  }
17  // integrating if branches unifies ?T with T, resulting in recursive type
18  hh_show($res[0]); // Toption(Toption(Toption(Toption(Toption(...)))))
19  return $res[0];
20}