Re: Different behaviour for --find-renames between git diff and git merge?

From: Elijah Newren <newren@gmail.com>
Date: Dec 13, 2025, 01:57 UTC
Message-ID: <CABPp-BH80R4LJDRKQnPmh5Am_HAcCgxWiA8vRoN8LgLRUMz+JQ@mail.gmail.com>
In-Reply-To: <3742e7de-7d88-4e77-b711-9fed867a8c23@gmail.com>

On Fri, Dec 12, 2025 at 10:06 AM Luca Balsanelli <lucabalsanelli@gmail.com> wrote:

Show 5 quoted lines

>
> Hi,
>
>    I'm scratching my head to understand why on the following case `git
> diff` and `git merge` give a different interpretation about a rename.

I don't see any difference...

Show 23 quoted lines

>     git switch master
>     touch aaa
>     git add aaa
>     git commit -m 'aaa'
>
>     git switch -c branch
>     echo -en 'A\nB\nC\n' > aaa
>     git add aaa
>     git commit -m 'A\nB\nC\n > aaa'
>
>     git switch master
>     echo -en 'A\nB\n' > aaa
>     mkdir dir
>     mv aaa dir/
>     git add aaa dir/
>     git commit -m 'A\nB\n > aaa -> dir/'
>
> The `|merge.renames` config variable is true. Changing `git diff
> --find-renames=50%` (the default) or `git merge -s ort -X
> find-renames=50%` ||to something lower does not change the following.
> |
>
> `git diff` prints

Actually, it doesn't; more on that below...

>
>     diff --git a/aaa b/dir/aaa
>     similarity index 71%

Did you not follow your own recipe? Maybe you inserted an extra space or left off the 'n' in 'echo -en' when you ran this? The number should have been 66%.

Show 11 quoted lines

>     rename from aaa
>     rename to dir/aaa
>     index bbd2b90..986ad36 100644
>     --- a/aaa
>     +++ b/dir/aaa
>     @@ -1,4 +1,3 @@
>       A
>       B
>     -C
>
>      that is the similarity index is 71% and it detects the rename.

At this point, if you actually run `git diff` you see the following:

$ git diff $

i.e. nothing. I suspect you gave `git diff` additional arguments but didn't tell us. Let's look at a few options:

$ git diff master~1 master

diff --git a/aaa b/aaa
deleted file mode 100644
index e69de29..0000000
diff --git a/dir/aaa b/dir/aaa
new file mode 100644
index 0000000..35d242b
--- /dev/null
+++ b/dir/aaa
@@ -0,0 +1,2 @@
+A
+B
$

So, on master, aaa was deleted, and dir/aaa was added.

$ git diff master~1 branch
diff --git a/aaa b/aaa
index e69de29..b1e6722 100644
--- a/aaa
+++ b/aaa
@@ -0,0 +1,3 @@
+A
+B
+C
$

On branch, aaa was modified.

$ git diff branch master
diff --git a/aaa b/dir/aaa
similarity index 66%
rename from aaa
rename to dir/aaa
index b1e6722..35d242b 100644
--- a/aaa
+++ b/dir/aaa
@@ -1,3 +1,2 @@
 A
 B
-C
$

So, only if you diff the endpoints of the two branches do you see a
rename; if you look from the merge base to either branch, there isn't
one.

> `git merge branch`, instead, gives
>
>     CONFLICT (modify/delete): aaa deleted in HEAD and modified in
>     branch.  Version branch of aaa left in tree.
>     Automatic merge failed; fix conflicts and then commit the result

Yes, this exactly matches what diff showed above -- on HEAD (master),
'aaa' was deleted, and on branch, 'aaa' was modified.

> Why it is that? I always supposed that the rename detection was the same
> for `git diff`, `git merge`. Reading the documentation I do not find any
> hint why `git diff` and `git merge` are behaving differently.

Hope that helps...