What exactly is a merge conflict?

Question

You will have a conflict if you merge:

That is because:

The common ancestor would be master (with a second line empty)
the source content is branch3 (with a second line including “3”)
the destination content is on latest of master (with a second line including “2”, from the merge of branch2 to master)

Git will ask you to choose which content to keep (“3”, “2”, or both).

First, do the merges after:

git config merge.conflictstyle diff3

See “Fix merge conflicts in Git?”.

Notes:

With Git 2.30 (Q1 2021), a new default merge strategy is in place: ORT (“Ostensibly Recursive’s Twin“), with clearer conflict messages (Gti 2.36, Q2 2022)
you can preview those conflicts with (Git 2.38, Q3 2022):
```
git merge-tree --write-tree --no-messages branch1 branch2
```
(That would not touch the index or working tree!)