branch{u} not working even though upstream configured

What could be a reason why branch@{u} doesn't work even though the branch does have a remote tracking branch configured?

We have a user who reports the following:

$ git push --set-upstream origin test-456
branch 'test-456' set up to track 'origin/test-456'.
Everything up-to-date
$ git merge-base test-456 test-456@{u}
fatal: upstream branch 'refs/heads/test-456' not stored as a
       remote-tracking branch

Git remote shows that the remote tracking branch is configured as expected:

$ git remote -v show origin
* remote origin
  [...]
  Local branches configured for 'git pull':
    test-456          merges with remote test-456
  Local refs configured for 'git push':
    test-456          pushes to test-456           (up to date)

Any ideas?

On Wed, Jul 03, 2024 at 01:52:05PM +0200, Stefan Haller wrote:

Hmm, in a simple test this seems to work for me. Can you show us what's in your local .git/config, especially the relevant remote and branch sections? Likewise, can you show us "git for-each-ref" output to see what refs you have (though from the message I think we are not even getting as far as looking at the remote tracking branch itself)?

I was able to produce a similar error if "refs/heads/test-456" is not covered by the remote.origin.fetch refspec. That's usually:

  [remote "origin"]
  fetch = +refs/heads/*:refs/remotes/origin/*

but if I delete that second line, or replace it with something like:

  fetch = +refs/heads/some-specific-branch:refs/remotes/origin/some-specific-branch

then I see the same error you saw.

-Peff