PHPMailer with Gmail/XOAUTH2 doesn't work when league/oauth2-client = 2.8.0 (latest) #3125

After a recent composer update, I noticed that PHPMailer was no longer able to send any email with Gmail, using XOAUTH2, eg. with a script like this:

https://github.com/PHPMailer/PHPMailer/blob/master/examples/gmail_xoauth.phps

Using PHP 8.3 on Ubuntu.

So for example this configuration works fine:

"phpmailer/phpmailer": "6.9.3",
"league/oauth2-client": "2.7.0",
"league/oauth2-google": "4.0.1",

whereas with this 2.8.0 version (latest):

"phpmailer/phpmailer": "6.9.3",
"league/oauth2-client": "2.8.0",
"league/oauth2-google": "4.0.1",

It will error in a generic authentication way:

SMTP ERROR: AUTH command failed: 334
SMTP Error: Could not authenticate.
CLIENT -> SERVER: QUIT
SERVER -> CLIENT: 535-5.7.8 Username and Password not accepted.

Obviously the solution for now is to just use the version that works! :)

But I was wondering if anyone could double-check this and confirm the issue. I assume people will start hitting this problem more and more as they set up new projects or upgrade to these latest versions.

The other thing is, while I suspect it's a problem with oauth2-client (though I suppose it's not impossible it's an issue with the PHPMailer implementation of it, now, either?) I'm not sure how to really file a bug report for this, because I don't know how to test it outside of the context of the PHPMailer example! I guess I can try to report it with this example if I can't figure anything else out, but maybe someone more knowledgeable here can suggest something more specific?