This API is part of the Hack Standard Library, not HHVM itself.
Facebook Engineer?

This function is available as C\firstx() in Facebook's www repository.

Returns the first element of the given Traversable, or throws if the Traversable is empty


namespace HH\Lib\C;

function firstx<T>(
  Traversable<T> $traversable,
): T;

For possibly empty Traversables, see C\first. For possibly null Traversables, see C\nfirst. For single-element Traversables, see C\onlyx. For Awaitables that yield Traversables, see C\firstx_async.