Re: [BUG] git merge sometimes loses changes from one branch

From: Elijah Newren <newren@gmail.com>
Date: Sep 12, 2025, 22:56 UTC
Message-ID: <CABPp-BGeZ6no5LdBGKjykQLqUHRjFis7ZtGdEwnzg0gy_CkLqA@mail.gmail.com>
In-Reply-To: <SN6PR03MB4285CA9885AE7952EEF762E1AE08A@SN6PR03MB4285.namprd03.prod.outlook.com>

On Fri, Sep 12, 2025 at 6:41 AM Guo Tingsheng <CoriCraft16@outlook.com> wrote:

Show 61 quoted lines

>
> Hello Git developers,
>
> I would like to report a potential issue in Git's merge behavior, where changes from one branch may be lost even when the other branch does not modify the same location.
>
> Environment:
> - git version: 2.43.0
> - OS: Ubuntu 24.04 LTS
>
> Steps to reproduce:
> 1. On branch A, modify a file at a certain location:
>
>    @@ -1026,7 +1028,9 @@
>    public final class AutoBuilderCompilationTest {
>                 "  }",
>                 "}");
>         Compilation compilation =
>    -        javac().withProcessors(new AutoBuilderProcessor()).compile(javaFileObject);
>    +        javac()
>    +            .withProcessors(new AutoBuilderProcessor())
>    +            .compile(javaFileObject);
>         assertThat(compilation).failed();
>         assertThat(compilation)
>             .hadErrorContaining(
>
> 2. On branch B, make no changes to this location.
>
> 3. Merge branch A and branch B.
>
> Expected result:
> - The merge should retain the changes made in branch A:
>
>    @@ -1026,7 +1028,9 @@
>    public final class AutoBuilderCompilationTest {
>                 "  }",
>                 "}");
>         Compilation compilation =
>    -        javac().withProcessors(new AutoBuilderProcessor()).compile(javaFileObject);
>    +        javac()
>    +            .withProcessors(new AutoBuilderProcessor())
>    +            .compile(javaFileObject);
>         assertThat(compilation).failed();
>         assertThat(compilation)
>             .hadErrorContaining(
>
> Actual result:
> - Git sometimes produces a merged result where the modification from branch A is lost, yielding:
>
>    @@ -1026,7 +1028,9 @@
>    public final class AutoBuilderCompilationTest {
>                 "  }",
>                 "}");
>         Compilation compilation =
>            javac().withProcessors(new AutoBuilderProcessor()).compile(javaFileObject);
>         assertThat(compilation).failed();
>         assertThat(compilation)
>             .hadErrorContaining(
>
> Additional information:
> - This issue occurs even when the other branch (B) does not modify the same location.
> - It appears to happen non-deterministically and may lead to subtle bugs in real-world merges.

My only guess here would be having a non-unique merge-base, where one of the merge bases also introduced this change, so that it appears that A made no change relative to the consolidated merge base while also making it look like branch B undid the change relative to that consolidated merge base. But, that's a wild guess and I have no idea if that's related to what you're seeing or if there's some unrelated issue at play.

Can you create a testcase to reproduce this, either a repository that others can clone together with the commands to trigger it, or a bunch of commands to both create a simple repository and demonstrate the issue?