fatal: could not reset submodule index

["Antoine W. Campagna" <awe@frontmatec.com>]

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