Don't do git merge

Steps to describe the issue

1. Create a base branch.

• lh_mouse@lhmouse-xps ~ $ mkdir test_repo/
• lh_mouse@lhmouse-xps ~ $ cd test_repo/
• lh_mouse@lhmouse-xps ~/test_repo $ git init -b base
  Initialized empty Git repository in /home/lh_mouse/test_repo/.git/
• lh_mouse@lhmouse-xps ~/test_repo $ echo one >> f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git add f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git commit -m "one"
  [base (root-commit) 05df618] one
  1 file changed, 1 insertion(+)
  create mode 100644 f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git log --oneline
  05df618 (HEAD -> base) one

2. Fork a branch and commit something.

• lh_mouse@lhmouse-xps ~/test_repo $ git checkout -B dev
  Switched to a new branch 'dev'
• lh_mouse@lhmouse-xps ~/test_repo $ echo "dev two" >> f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git commit -m"dev two" f.txt
  [dev 42a89ec] dev two
  1 file changed, 1 insertion(+)
• lh_mouse@lhmouse-xps ~/test_repo $ git log --oneline
  42a89ec (HEAD -> dev) dev two
  05df618 (base) one

3. Return to the base branch and commit something else.

• lh_mouse@lhmouse-xps ~/test_repo $ git checkout base
  Switched to branch 'base'
• lh_mouse@lhmouse-xps ~/test_repo $ echo "base three" >> f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git commit -m"base three" f.txt
  [base b3d829b] base three
  1 file changed, 1 insertion(+)
• lh_mouse@lhmouse-xps ~/test_repo $ git log --oneline
  b3d829b (HEAD -> base) base three
  05df618 one

4. Try merging the two branches with a conflict.

• lh_mouse@lhmouse-xps ~/test_repo $ git merge --no-ff dev
  Auto-merging f.txt
  CONFLICT (content): Merge conflict in f.txt
  Automatic merge failed; fix conflicts and then commit the result.
• lh_mouse@lhmouse-xps ~/test_repo $ git diff

  diff --cc f.txt
  index 33f639d,e9837ff..0000000
  --- a/f.txt
  +++ b/f.txt
  @@@ -1,2 -1,2 +1,6 @@@
    one
  ++<<<<<<< HEAD
   +base three
  ++=======
  + dev two
  ++>>>>>>> dev

5. Resolve the conflict by discarding base changes.

• lh_mouse@lhmouse-xps ~/test_repo $ nano f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git diff

  diff --cc f.txt
  index 33f639d,e9837ff..0000000
  --- a/f.txt
  +++ b/f.txt

• lh_mouse@lhmouse-xps ~/test_repo $ git add f.txt
• lh_mouse@lhmouse-xps ~/test_repo $ git commit
  [base 3d2c56d] Merge branch 'dev' into base
• lh_mouse@lhmouse-xps ~/test_repo $ git log --oneline
  3d2c56d (HEAD -> base) Merge branch 'dev' into base
  b3d829b base three
  42a89ec (dev) dev two
  05df618 one

6. Examine the file, which contains no change from the base branch.

• lh_mouse@lhmouse-xps ~/test_repo $ cat f.txt

  one
  dev two
  

7. Examine the history, which contains no deletion; the change has vanished into thin air.

• lh_mouse@lhmouse-xps ~/test_repo $ git log -p

  commit 3d2c56d4bf6edcf327103b6a5e19bfc3a7e1de66 (HEAD -> base)
  Merge: b3d829b 42a89ec
  Author: LIU Hao <[email protected]>
  Date:   2024-01-24 15:18:24 +0800
  
      Merge branch 'dev' into base
  
  commit b3d829b43f577bda4498456cff31d980c9934410
  Author: LIU Hao <[email protected]>
  Date:   2024-01-24 15:01:45 +0800
  
      base three
  
  diff --git a/f.txt b/f.txt
  index 5626abf..33f639d 100644
  --- a/f.txt
  +++ b/f.txt
  @@ -1 +1,2 @@
   one
  +base three
  
  commit 42a89ec457bf1391a6e674fec759e2f318c04a00 (dev)
  Author: LIU Hao <[email protected]>
  Date:   2024-01-24 14:57:32 +0800
  
      dev two
  
  diff --git a/f.txt b/f.txt
  index 5626abf..e9837ff 100644
  --- a/f.txt
  +++ b/f.txt
  @@ -1 +1,2 @@
   one
  +dev two
  
  commit 05df618feb13c0e7e0a157da2112c2bd43d24881
  Author: LIU Hao <[email protected]>
  Date:   2024-01-24 14:51:57 +0800
  
      one
  
  diff --git a/f.txt b/f.txt
  new file mode 100644
  index 0000000..5626abf
  --- /dev/null
  +++ b/f.txt
  @@ -0,0 +1 @@
  +one

Conclusion

Incorrect use of git merge may result in invisible data loss in history. In addition, it also complicates bisecting.

So, don't do git merge.