On Fri, Sep 12, 2025 at 6:41 AM Guo Tingsheng [off-list ref] wrote:

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?