version control - git merge squash and recurring conflicts

Question

I have a git repository with master and alt branches. alt branch contains modified version of master code, and i am trying to merge changes from master to alt like this:

git merge --squash master

Merge results in conflict:

Auto-merging myproject/foo/bar
CONFLICT (content): Merge conflict in myproject/foo/bar
Squash commit -- not updating HEAD
Automatic merge failed; fix conflicts and then commit the result.

After I resolve conflicts and commit changes everything seems fine, but when i run git merge --squash master again (without doing any changes on any branches) i will get same conflict error.

Why is that? What did i miss?

Answer 1

By squashing the merge, you've created a commit which has the effect of, but is not really, a merge.

That is, the working tree has the modifications you'd expect, but the metadata doesn't: crucially, the commit doesn't have two parents (one on master and one on alt) and therefore subsequent merges can't figure out the last common ancestor.

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Useful uses of squash

1. merging a completely finished feature branch onto master. I'll accumulate any useful information into the squashed commit, but specifically don't want this feature's incremental development history polluting the master commit timeline.
2. merging several independent features (or contributions from different developers) onto the same integration branch, again without preserving their incremental histories. I could rebase them all together, and then use rebase -i to squash their commits, but this is easier

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Useless uses of squash

Any merge where you want to keep the history and ancestry metadata intact, such as any time you want repeated recursive merges to work correctly, specifically what the OP is trying to do.

squashjust isn't really a good default, which is why it isn't the default.