Our automated updates updated to 1.3.16 today and started failing phpstan. Based on the changes from 1.3.15 it's obvious what the issue is.
Our tests do something like this:
$mails = $this->getMails();
$this->assertCount(1, $mails);
$mail = \reset($mails);
$this->assertStringContainsString('foo', $mail['subject']);
Where getMails returns an array.
And this is throwing errors like Cannot access offset 'subject' on false.