Using the slash character in Git branch name

Are you sure branch labs does not already exist (as in this thread)?

You can't have both a file, and a directory with the same name.

You're trying to get git to do basically this:

% cd .git/refs/heads
% ls -l
total 0
-rw-rw-r-- 1 jhe jhe 41 2009-11-14 23:51 labs
-rw-rw-r-- 1 jhe jhe 41 2009-11-14 23:51 master
% mkdir labs
mkdir: cannot create directory 'labs': File exists

You're getting the equivalent of the "cannot create directory" error.
When you have a branch with slashes in it, it gets stored as a directory hierarchy under .git/refs/heads.

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Note that labs must not be an existing branch, as ddruganov points out in the comments:

 git switch -c 19023-commerce/19033-commerce-view 19023-commerce

 # Fails with:

 fatal: cannot lock ref 'refs/heads/19073-commerce-view/99999-test-branch': 
 'refs/heads/19073-commerce-view' exists; 
  cannot create 'refs/heads/19073-commerce-view/99999-test-branch'

As explained in "git push: refs/heads/my/subbranch exists, cannot create":

• If branch b exists, no branch named b/anything can be created.
• Likewise, if branch dev/b exists, dev/b/c cannot be created.

This is a git internal limitation.

It is possible to have hierarchical branch names (branch names with slash). For example in my repository I have such branch(es). One caveat is that you can't have both branch 'foo' and branch 'foo/bar' in repository.

Your problem is not with creating branch with slash in name.

$ git branch foo/bar
error: unable to resolve reference refs/heads/labs/feature: Not a directory
fatal: Failed to lock ref for update: Not a directory

The above error message talks about 'labs/feature' branch, not 'foo/bar' (unless it is a mistake in copy'n'paste, i.e you edited parts of session). What is the result of git branch or git rev-parse --symbolic-full-name HEAD?

I forgot that I had already an unused labs branch. Deleting it solved my problem:

git branch -d labs
git checkout -b labs/feature

Explanation:

Each name can only be a parent branch or a normal branch, not both. Thats why the branches labs and labs/feature can't exists both at the same time.

The reason: Branches are stored in the file system and there you also can't have a file labs and a directory labs at the same level.