PHPUnit 9.3 breaks backward compatibility for assertXmlStringEqualsXmlFile()

[morozov]

Q	A
PHPUnit version	9.3.2
PHP version	7.4.9
Installation Method	Composer

Summary

The usage of a DOMDocument value for the $actualXml argument of assertXmlStringEqualsXmlFile() is broken (c331d28).

How to reproduce

<?php

declare(strict_types=1);

use DOMDocument;
use PHPUnit\Framework\TestCase;

class XmlTest extends TestCase
{
    public function test(): void
    {
        $this->assertXmlStringEqualsXmlFile(
            __DIR__ . '/phpunit.xml',
            new DOMDocument()
        );
    }
}

$ phpunit XmlTest.php
PHPUnit 9.3.2 by Sebastian Bergmann and contributors.

E                                                                                   1 / 1 (100%)

Time: 00:00.003, Memory: 8.00 MB

There was 1 error:

1) XmlTest::test
TypeError: Argument 1 passed to PHPUnit\Util\Xml\Loader::load() must be of the type string, object given, called in vendor/phpunit/phpunit/src/Framework/Assert.php on line 2126

Expected behavior

Only a deprecation warning is triggered. The usage of DOMDocument is supported by all PHPUnit 9.x releases.