push.default=upstream doesn't play nicely with remote.pushdefault/branch.*.pushremote

(Please CC me as I am not subscribed.)

$ git config -l | grep '^branch.master\|^push.' push.default=upstream branch.master.remote=upstream branch.master.merge=refs/heads/master branch.master.pushremote=origin

$ git branch
* master

$ git push fatal: You are pushing to remote 'origin', which is not the upstream of your current branch 'master', without telling me what to push to update which remote branch.

push.default=upstream means "push back where it came from (*)". However, if I specifically define remote.pushdefault or branch.*.pushremote, this clearly means I don't want to do (*) in this case.

The question here then is, what remote branch should git push to - since pushremote only specifies a repo? I can suggest two options:

- A: push to branch.*.merge if it's defined; otherwise push to <current branch> (like push.default=current)
- B: push to <current branch> regardless

B is probably simpler and means you can amend the push.default manual entry more easily:

upstream: [..] This mode only makes sense if you are pushing to the same repository you would normally pull from (i.e. central workflow). <INS>If you are not pushing to the same repository, acts as "current".</INS>

(Note again; that this is a special case when push.default=upstream because of the "this mode only makes sense" semantics. The other values for push.default are simpler and do not result in a conflict.)

Alternatively, an even more simple behaviour is to adjust the error message to communicate the root cause:

$ git push fatal: branch.*.pushremote is not compatible with push.default = upstream

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