Why is git submodule update not automatic on git checkout?

Question

When switching branches with git checkout I would assume that most of the time you would want to update your submodules.

• In what situation do you not want to update submodules after switching?
• What would break if this was done automatically by git checkout?

Updated with example:

• Branch A has submodule S at 3852f1
• Branch B has submodule S at fd72d7

On branch A, git checkout B will result in a working copy of branch B with submodule S at 3852f1 (with a modified S). git submodule update will checkout S at fd72d7.

Answer 1

git checkout --recurse-submodules was added to git 2.13

This is mentioned on the release notes at: https://github.com/git/git/commit/e1104a5ee539408b81566066aaa6963cb87d5cd6#diff-c24776ff22455a30fbb78e378b7df0b0R139

submodule.recurse option was added to git 2.14

Set as:

git config --global submodule.recurse true

man git-config says:

Specifies if commands recurse into submodules by default. This applies to all commands that have a --recurse-submodules option. Defaults to false.

I feel that not updating modules by default is a bad Git default behavior that goes against most user's expectations and limits the adoption of submodules, I really wish the devs would change it.