And how do you know that wasn't the commit you were looking for? The whole point of bisect is that you don't have to do a manual bug hunt; you can create a script that tests for the thing you are looking for. If it's something static sure, that's easy. But if it's something more complicated and you don't know what's causing it (a prime candidate for bisect), you will need to build and analyze the behavior of the built executable. Non-building commit = more manual bughunting.
But aren't we talking about squashing those commits? You're not going to lose anything by skipping commits between PRs instead of squashing everything between PRs into one commit, you can only narrow it down more.
> you can create a script that tests for the thing you are looking for.
Yes, which you can tell to skip anything that doesn't compile/pass tests. Return 125 from your script and it'll be skipped.
> Non-building commit = more manual bughunting.
But you can just do a loop of "make || git skip" and skip everything that doesn't commit, surely. Or if you're using a script, just return 125 for things you want to skip.