From: Guo Tingsheng <coricraft16@outlook.com>
Date: Fri, 12 Sep 2025 13:33:20 GMT
Subject: [BUG] git apply misplaces patch when similar code fragments exist in the same file
Message-ID: <SN6PR03MB428591529FBE9413B427B030AE08A@SN6PR03MB4285.namprd03.prod.outlook.com>

Hello Git developers,

I would like to report a potential issue in Git's patch application mechanism, where a patch can be incorrectly applied to a similar but unintended code location if line numbers have shifted.

Environment:
- git version: 2.43.0
- OS: Ubuntu 24.04 LTS

Steps to reproduce:
1. Start from Commit_1, containing the following file:

   function HeaderComponent() {
       return `
           <div class="layout-section">
               <button>Click Me</button>
           </div>
       `;
   }

   function FooterComponent() {
       return `
           <div class="layout-section">
               <button>Click Me</button>
           </div>
       `;
   }

2. In another branch, Commit_2 introduces additional import statements before HeaderComponent, shifting its return statement further down (around line 10).  
   In Commit_2, the button text in HeaderComponent is modified as follows:

   @@ -10,6 +10,6 @@
        return `
            <div class="layout-section">
   -            <button>Click Me</button>
   +            <button>点击</button>
            </div>
        `;
    }

3. Generate a patch from Commit_2.

4. Apply this patch on top of Commit_1 using:
   git apply commit2.patch

Expected result:
- The patch should apply the change to HeaderComponent, modifying its button text.

Actual result:
- The patch is incorrectly applied to FooterComponent instead, producing:

   function HeaderComponent() {
       return `
           <div class="layout-section">
               <button>Click Me</button>
           </div>
       `;
   }

   function FooterComponent() {
       return `
           <div class="layout-section">
               <button>点击</button>
           </div>
       `;
   }

Additional information:
- This issue occurs when a file contains multiple similar code fragments, and the patch context fails to match due to line number shifts between commits.
- As a result, git apply may incorrectly match against the wrong occurrence of the repeated code, introducing unintended changes.
- This can potentially lead to subtle bugs in real-world projects (e.g., configuration files, HTML/JS components).

Thanks,  
Cori