Make PHP Storm Aware of inherited Singleton Classes

I have following Code in the Parent Class:

class parent {
    /**
     * Memory of the instances of the classes.
     * @since  1.0.0
     * @access protected
     * @static
     * @var array
     */
    protected static $instances = [];

    /**
     * Singleton
     * Thanks to: https://stackoverflow.com/a/45500679/8148987
     * @since  1.0.0
     * @access public
     * @static
     * @return object
     */
    public static function instance() {

        if ( empty( self::$instances[static::class] ) ) {
            $instance                       = new static();
            self::$instances[static::class] = $instance;
        } else {
            $instance = self::$instances[static::class];
        }

        return $instance;
    }
}

The Child Class:

class child extends parent {

    public function test() {

    }

}

I can do following with this code:

$class_child = child::instance();

But PHP is not aware of the method test().

If I write $class_child-> no proposals are listed. What canI do? The solution mentioned here https://stackoverflow.com/a/32014968/8148987 will not work in my case.