merge branch with updated submodule leaves repo in modified state

Hi all!

`git merge`: if a submodule is updated in the merged branch, the new version is not checked out after the merge. Is this correct behaviour?

`git switch` correctly updates the submodules when switching between the branches. I would expect `git merge` to leave the things in the most up-to-date state.

I tested this with the following settings.

  $ git version
  git version 2.24.0

  $ git config -l
  push.default=matching
  diff.submodule=log
  diff.colorwords=true
  diff.algorithm=minimal
  diff.exit-code=true
  commit.gpgsign=true
  submodule.fetchjobs=0
  submodule.recurse=true
  log.follow=true
   
MWE:

``` mkdir repoA repoB

# init repos with READMEs cd repoA git init echo "A" > README git add README git commit -m "Adds repoA README" README

cd ../repoB git init echo "B" > README git add README git commit -m "Adds repoB README" README

# construct nested structure: repoA/repoB cd ../repoA git submodule add ../repoB/.git git commit -m "Adds repoB submodule"

# update repoB in branch git switch -c update-repoB-submodule cd repoB echo "B updated" > README git commit -m "Updates repoB README" README cd .. git commit -m "Updates repoB submodule" repoB ```

Now, doing `git switch master` will have the correct version of repoB, where `cat repoB/README` outputs "B". A `git switch update-repoB-submodule` from there will yield `cat repoB/README` output "B updated".

However, switching to master and then merging will leave repoB at the old commit: ``` git switch master git merge update-repoB-submodule ``` A `git status` says there are new commits in repoB, it's actually a rewind which excludes the new commit (changing `repoB/README`), so `cat repoB/README` outputs "B" instead of "B updated". It feels unintended to get a repo which is in a modified state after a clean merge. Bug?

Cheers, Daniel