What does caret bang (^!) after the commit hash do when calling git diff?

Question

Looking through the source code for brackets-git (a git extension for Brackets), I see that ^! (caret bang) is being appended to the commit hash when calling git diff. See GitCli.js, line 754:

function getDiffOfFileFromCommit(hash, file) {
    return git(["diff", "--no-ext-diff", "--no-color", hash + "^!", "--", file]);
}

This translates to the following on the command line, using the file in question as an example:

$ git diff --no-ext-diff --no-color 1f9ea6e^! -- src/git/GitCli.js

I know that ^ would refer to the parent of the commit. What does ^! do?

Answer 1

commit^! is a range specifier which means: this commit, but none of its parents. It's equivalent to specifying: commit ^parent1 ^parent2 ^parentN.

For diff this does not make sense (you can only compare two trees) From testing the command seems to show the differences between the merge base of the parents and the last parent. I think git (mis)interprets the parameters similar to the range A...B which will show the differences between the merge-base A B and B (git diff parent1...parent2 will produce the same diff). Not sure what will happen in the case of an octopus-merge.

I might be wrong though, these are just assumptions I drew from testing with a repository and looking into the git code (builtin/diff.c).