Why is the diff reversed for a merge commit with ^! parent notation

Why does the the ^! reverse the diff for merge commit?

The r1^! notation includes commit r1 but excludes all of its parents. By itself, this notation denotes the single commit r1.

This correctly shows me the diff for a non-merge commit.

$ git diff <commit>^!

But for a merge commit, the diff is reversed.

$ git diff <merge-commit>^!

I get similar results for difftool.

Why so?

Solution

The short answer is that it is a bug.

You can see what git diff effectively sees by using git rev-parse. Here's the ^! suffix applied to an ordinary commit:

$ git rev-parse 699d47e1d^!
699d47e1d2777ad1c2a867671e35daa821769f29
^4aaf5b0b21ac1fc294066593ac5243b3eaff897b

and here it is applied to a merge commit:

$ git rev-parse 117ddefdb^!
117ddefdb4dfd9b40ae60967a7327754d8ce7a87
^5e5a7cd9327cdbe6b50b5a0ead9b2ee5fb30789c
^699d47e1d2777ad1c2a867671e35daa821769f29

When you supply the compressed notation to git diff, Git passes it through the same expansion code that git rev-parse uses. Instead of printing it out, though, git diff then tries to back-interpret the result. If there are two commit hashes (with various flags) it runs a diff between the two named commits. If there are three or more, it does something different.