为何修改文件不同行仍出现Git合并冲突？

Atlassian的说明提到：冲突通常发生在两人修改了文件的同一行，或一人删除文件而另一人修改该文件时。

但在以下Git操作序列中，两个分支（master-dev5和master-dev6）分别修改了readme文件的不同行（第3行和第4行），然而将master-dev6合并到master或反向合并时却出现了冲突。这与上述说明不符，请问该冲突为何会发生？

$ git checkout -b master-dev5 master
$ git checkout -b master-dev6 master
$ git checkout master-dev5
$ cat readme
begin
added by dev1, added by dev2
added by dev3, added by dev4
$ echo "added by dev5">>readme
$ cat readme
begin
added by dev1, added by dev2
added by dev3, added by dev4
added by dev5
$ git add readme
$ git commit -m "dev5 add something on line4"
$ git checkout master-dev6
$ cat readme
begin
added by dev1, added by dev2
added by dev3, added by dev4
$ vim readme
$ cat readme
begin
added by dev1, added by dev2
added by dev3, added by dev4, added by dev6
$ git add readme
$ git commit -m "dev6 add something on line3"
$ git checkout master
$ cat readme
begin
added by dev1, added by dev2
added by dev3, added by dev4
$ git merge master-dev5
Updating 3b87e7c..28e91e1
Fast-forward
 readme | 1 +
 1 file changed, 1 insertion(+)

$ git merge master-dev6
Auto-merging readme
CONFLICT (content): Merge conflict in readme
Automatic merge failed; fix conflicts and then commit the result.

$ cat readme
begin
added by dev1, added by dev2
<<<<<<< HEAD
added by dev3, added by dev4
added by dev5
=======
added by dev3, added by dev4, added by dev6
>>>>>>> master-dev6

$ git merge --abort
$ cat readme
begin
added by dev1, added by dev2
added by dev3, added by dev4
added by dev5
$ git checkout master-dev6
$ git merge master
Auto-merging readme
CONFLICT (content): Merge conflict in readme
Automatic merge failed; fix conflicts and then commit the result.

$ cat readme
begin
added by dev1, added by dev2
<<<<<<< HEAD
added by dev3, added by dev4, added by dev6
=======
added by dev3, added by dev4
added by dev5
>>>>>>> master

Git判断合并冲突，看的不是最终文件里的行号，而是相对于共同祖先版本的变更上下文。

具体到这个场景：

• 两个分支的共同祖先版本中，readme的最后一行是：added by dev3, added by dev4
• master-dev5分支的变更是：在这一行的后面新增了一行（added by dev5）
• master-dev6分支的变更是：修改了这一行的内容（末尾加上added by dev6）

当合并时，Git需要把这两个变更合并到一起。但master-dev5的新增操作，让原祖先版本的最后一行，在当前分支里变成了倒数第二行；而master-dev6的修改是针对祖先版本的最后一行。Git无法自动确定：是应该先修改原行再新增，还是先新增再修改哪一行？两个变更的上下文区域出现了重叠，所以触发了冲突。

Atlassian的说明只是常见冲突场景的简化总结，实际Git的冲突判断逻辑更关注变更的上下文关联，而非最终文件的行号。你的操作里，两个分支的变更在共同祖先的视角下，刚好影响了关联的上下文区域，所以出现了看似“不同行却冲突”的情况。

内容的提问来源于stack exchange，提问作者techie11