<?php
$test = new _ZendTestFiber(function (): void {
$fiber->start();
});
var_dump($test->start());

/php-src/Zend/zend_builtin_functions.c:1893: void zend_fetch_debug_backtrace(zval *, int, int, int): Assertion `call->func' failed.
Aborted (core dumped)