|Someone wrote in mjg59,|
Contributor A signs X's CLA.
Contributor B signs X's CLA and wants to patch the code contributor A as submitted.
X uses its relicensing powers to give B a license which states you are allowed to modify the code for the express purpose of contributing back.
It's a tight specific purpose re-licensing scheme where X uses the extra permissions you have granted them so they can take in derived works.
Now whether or not they actually have to do the licensing paperwork to create that tight purpose specific permissive licensing is something the lawyers would need to debate. But I think even if you were able to force some sort of GPL violation, they could instantly remedy it by granting the necessary permissive license to contributor B for the express purpose of commiting the derived work.