in src/ExpectObj.hack [359:368]
public function toBeSortedByKey<Tv>(
(function(Tv): arraykey) $key_extractor,
string $msg = '',
mixed ...$args
): void where T as Traversable<Tv> {
$msg = \vsprintf($msg, $args);
$actual = $this->var;
$this->assertIsSortedByKey($actual, $key_extractor, $msg);
}