Comment by Anamon

> Not just squash merges, rebase-merges also don't work.

Are you sure? I almost exclusively rebase-merge, and I use ‘git branch --merged‘ all the time. It works perfectly fine for me.

Also conceptually it seems to make sense to me: you rebase your commits onto the tip of the target branch, so you can trivially follow the link from the tip of your source branch to the tip of your target branch, which as I understand it is what the command checks for.