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function send() in includes/UserMailer.php has a first parameter, $to, that can be a singleton or an array. If a singleton, it must be a MailAddress object. If you pass an array of MailAddress objects, however, you get the warning:

Object of class MailAddress to string conversion in .../includes/UserMailer.php on line 115

The line is:

wfDebug( __METHOD__.': sending mail to '  . implode( ',', $to ) . "\n" ); 

This is strange. send() seems to be expecting an array of strings, or a single MailAddress, but not an array of MailAddress objects.

This is either a bug to be fixed, or extra documentation is needed about valid values for $to.