Summary of a problem or a feature request

I think there was a mistake in the php documentation (http://php.net/manual/en/ds-set.first.php)
Return typehint void for this method is incorrect. It should be like mixed
https://github.com/phpstan/phpstan/blob/master/src/Reflection/SignatureMap/functionMap.php#L2177

I also report a bug on php.net (https://bugs.php.net/bug.php?id=77702)

Code snippet that reproduces the problem

use Ds\Set;

$set = new Ds\Set();
$set->add('foo', 'bar');
var_dump($set->first());

Expected output

No errors