Bug report

The signature in functionMap.php is

'simplexml_import_dom' => ['SimpleXMLElement|false', 'node'=>'DOMNode', 'class_name='=>'string'],

And still, it looks like phpstan thinks the return type can be null (and not false), as can be seen in the linked playground. The error is only there for PHP 8, and not for 7.1-7.4.

Code snippet that reproduces the problem

https://phpstan.org/r/74fe87de-ea5a-4642-9577-0ecb1978da9c

Expected output

According to the documentation at php.net, the signature of simplexml_import_dom has not changed in PHP 8.0, so the code in the attached playground should not display an error.