"This branch cannot be rebased safely" stops our "rebase and merge" workflow


In the GitHub - void-linux/void-packages: The Void source packages collection repository, we make the “rebase and merge” option mandatory, and it helps us a lot in keeping a linear history. Unfortunately, some time in the past week, many old PRs have suddenly greyed out the “Rebase and merge” button, with the following error:

This branch cannot be rebased safely
Rebasing the commits of this branch on top of the base branch cannot be performed automatically as this would create a different result than a regular merge. 

Some examples (not links because I’m a new user):

  • 29502
  • 29383

Even some new PRs are suffering from the same issue, for example:

  • 29975

This makes merging PRs a way more manual process (checking out the branch locally and pushing to master) and makes our experience with GH suboptimal.

A similar issue seems to have been reported in This branch cannot be rebased safely , but it wasn’t followed up and we hadn’t been affected until very recently. I assume something changed recently in how the “legality” of a rebase is determined, but it’s definitely too conservative.