Three-way merges and conflicts

Comparing only the two branch tips can't tell you who changed what. If a line differs, did you change it, or did they? A three-way merge adds a third version, the merge base: the most recent commit both branches share.

         ┌─ ours   (main)
base ────┤
         └─ theirs (feature)

For each region of each file, Git compares both sides with the base:

So most merges resolve automatically. A conflict means one specific thing: both sides changed the same region relative to the base, in different ways. Git can't know which one you want, or whether you want both.

#Reading conflict markers

Turn on the base section. It shows what the code looked like before either change, which is often the key to resolving correctly:

git config --global merge.conflictStyle zdiff3

<<<<<<< HEAD
const timeout = 5_000
||||||| base
const timeout = 3_000
=======
const timeoutMs = 3_000
>>>>>>> feature

Here one side changed the value and the other renamed the variable. Neither side's version is right on its own. The correct resolution keeps both changes: const timeoutMs = 5_000.

#Resolving

1. git status lists the conflicted files.
2. Edit each one to the correct combined result, and remove the markers.
3. Build and run the tests. A merge without conflicts can still be wrong. If one side renamed a function and the other added a call to the old name, both changes apply cleanly and the result doesn't compile.
4. git add the files, then git commit (or git rebase --continue).

git merge --abort gets you back to the state before the merge. git rerere remembers how you resolved a conflict and reapplies it if the same conflict comes back, which helps with long-running branches.

#Fewer conflicts

Conflicts grow with how long branches diverge and how many people edit the same lines. Small, short-lived branches merged often (trunk-based development) avoid most of them. Moving code and changing it in the same commit is a reliable way to create them.

See fast-forward vs merge commits for what's recorded after the merge.