in src/Assert.hack [667:678]
public function assertIsSortedByKey<Tv>(
Traversable<Tv> $collection,
(function(Tv): mixed) $key_extractor,
string $message = '',
): void {
$this->assertIsSorted(
$collection,
/* HH_FIXME[4240] unsafe comparison (PHPism) */
($a, $b) ==> $key_extractor($a) <= $key_extractor($b),
$message,
);
}