Also note that this introduces another shift/reduce conflict, could be related, but I don't understand it well enough. Feedback would be appreciated, I'm still learning bison.
That's because of how you use ArgumentsDecl_opt and ArgumentsDecl. I think that we could separate rules by arguments presence, something like https://gitlab.winehq.org/jacek/wine/-/commit/ab1870f29f941492b1965316859740... should avoid the problem.