Why does this merge produce a conflict

Question

When I run this command:

 git log HEAD..other_branch -- some_file.txt

There's no output at all which I assume meants that there were no changes to some_file.txt in other_branch. Yet when I run git merge other_branch I get a whole slew of conflicts in some_file.txt.

When I run:

git log HEAD...other_branch -- some_file.txt 

I get two commits. One where the file was modified and one where the branch where it was modified was merged into HEAD.

I assumed that because the file was only changed in one branch that there wouldn't be any conflicts. Why are there conflicts? Is there a way to see what will conflict (and why) before I run git merge?

Answer 1

To add to manojlds's contribution:

Very nice. But then the following should accomplish the same?

git diff master:some_file.txt someBranch:some_file.txt

that way you don't need to checkout before diffing against cached

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Original answer:

The command

git log HEAD..other_branch -- some_file.txt

is identical to

git log ^HEAD other_branch -- some_file.txt

which means give me the log of all commits reachable from other_branch but NOT reachable from HEAD for some_file.txt. If this command gives you no output, it means that some_file.txt has not changed on other_branch at all.

On the other hand:

git log HEAD...other_branch -- some_file.txt 

is the symmetric difference between HEAD and other_branch, i.e. the commits that are in HEAD and in other_branch but not in both, and are the commits that will be merged when you merge the two branches. So something has probably happened to some_file.txt on HEAD that cause this conflict with the version on other_branch