Squash merge issue example

This repository is there to expose a dangerous behavior on the squash and merge action.

Let's say you started a branch B based on the work of someone else's branch A.

In your branch B, you remove some code that was introduced in branch A.

Now branch A is squash merged in master before the branch B is, without modifications.

You have no conflict with master, the diff in github shows only the work of the branch B, but it shows 2 commits.

Now if you squash merge the branch B, here is what happens:

all commits from the branch A (that are still in branch B) and all commits of the branch B are squashed together
due to the fact they were related to master before the merge of the branch A, the fusion of the code from A and its removal from B will result in an empty diff
the empty diff is now part of the squashed commit
the rebase applies the diff of the squash commit on master, so it applies the empty diff

So once your branch is squash merged on master, the code you remove from A will remain on master.

⚠️ This is not a bug from GitHub, this is a bad usage of git, you can do this mistake with or without GitHub.

Commits

You can see in the repository an example of the issue.

The foo.txt file contains the 4 lines, the removal from the line 2 has been lost during the squash and merge action.