That's something that I think we've mentioned before - technically I think even historically we've agreed it would need agreement from copyright holders.
At the same time, before we accept a patch to mantis, we should be checking that:
a) the author of the patch is willing for the code to be used under the GPL
b) transferring copyright to 'the mantis team' [which I'd probably be inclined to go as far to say is probably an undefined entity]. It is the Mantis Team that is listed as the copyright owner in each file.
Some of the larger projects then have agreements people need to accept before they can add code etc etc etc etc.
"And despite the broad right of distribution conveyed by the GPL, enforcement of copyright is generally not possible for distributors: only the copyright holder or someone having assignment of the copyright can enforce the license. If there are multiple authors of a copyrighted work, successful enforcement depends on having the cooperation of all authors."
"In order to make sure that all of our copyrights can meet the recordkeeping and other requirements of registration, and in order to be able to enforce the GPL most effectively, FSF requires that each author of code incorporated in FSF projects provide a copyright assignment, and, where appropriate, a disclaimer of any work-for-hire ownership claims by the programmer's employer. That way we can be sure that all the code in FSF projects is free code, whose freedom we can most effectively protect, and therefore on which other developers can completely rely."
That to me suggests that it would be impossible for us to enforce a gpl/copyright issue against mantis:
a) All authors? - "If there are multiple authors of a copyrighted work, successful enforcement depends on having the cooperation of all authors.""
b) who/what is "Mantis Team" that currently owns the copyright
c) "provide a copyright assignment, and, where appropriate, a disclaimer of any work-for-hire ownership" -> we have those copyright assignments right? :)