Why are you allowing commits to land that break interfaces?
If this is a public function it should go through a deprecation cycle.
If this is a private function then it shouldn't be accessed from outside its module/class/whatever in the first place. If it is, then you've got the "messy codebase" I referred to in my original comment.
I'm not sure if you're being intentionally obtuse here, it's a very simple scenario that has nothing to do with public interfaces or deprecation cycles.
It does, but you're now looking at an edge case that rarely arises in practice in a well-maintained codebase, and certainly not one to design your entire branching/merging/CI strategy around.
If your codebase is a spaghettified nonsense then the problem arises much more often, and so I can understand the use of merge trains/whatnot.
In this case, we often have conflicts in private modules. (And everything is private since we don’t provide any libraries to anyone.)