fatal: could not reset submodule index

Hi,

I would like to add submodules to existing projects.
Some branches would have the submodules and some branches would not.
Since we often switch from one branch to another, I would like the submodules to update automatically so I activate the option submodule.recurse.
But I am experiencing a problem if I do the following:
  1. Clone repo, with master containing no submodules
  2. Checkout a branch that contains submodules. It results in this error:
	fatal: not a git repository: ../.git/modules/submodule
	fatal: could not reset submodule index
I think git is trying to update the submodule but the submodule has not yet been initialized (runs "git submodule update" without "--init").
Is there a way to ask git to initialize the submodule automatically ?

Here is the full reproduction instructions:

# Create a repository mkdir main cd main git init touch main.txt git add main.txt git commit -a -m "Initial commit" cd ..

# Create a second repository mkdir sub cd sub git init touch sub.txt git add sub.txt git commit -a -m "Initial commit of repo sub" cd ..

# Add the second repository as submodule, on a separate branch cd main git branch with-submodule git checkout with-submodule git submodule add ../sub sub git commit -a -m "Add submodule"

# Set main repo back to master branch (without the submodule) git checkout master cd ..

# Make a clone and checkout the branch git clone main clone1 cd clone1 git checkout with-submodule # Submodule is not automatically updated (sub folder is empty) git submodule update --init --recursive # Now the submodule content is there # But I want to automatically update submodules when checking out a branch cd ..

# Trying again, adding --recursive during clone git clone --recursive main clone2 cd clone2 git checkout with-submodule # Submodule is still not automatically updated (sub folder is empty) git submodule update --init --recursive cd ..

# Trying again, adding --recurse-submodules during checkout git clone --recursive main clone3 cd clone3 git checkout --recurse-submodules with-submodule # Fails with these error messages : # fatal: not a git repository: ../.git/modules/sub # fatal: could not reset submodule index # It seems like Git tries to update the submodule but without having initialized the submodule cd ..

# Trying again with submodule.recurse git config --global submodule.recurse true git clone main clone4 cd clone4 git checkout with-submodule # Submodule is still not automatically updated (sub folder is empty) # It seems like submodule.recurse does not affect git clone

# Trying again with both submodule.recurse and --recursive git config --global submodule.recurse true git clone --recursive main clone5 cd clone5 git checkout with-submodule # Fails with these error messages : # fatal: not a git repository: ../.git/modules/sub # fatal: could not reset submodule index # Same issue as with "git checkout --recurse-submodules"

# I tested this in git-bash on Windows 10
$ git --version
	git version 2.17.1.windows.2
# And in Ubuntu in WSL
$ git --version
	git version 2.17.1