Re: Merge commit diff results are confusing and inconsistent

[Robert Dailey <rcdailey.lists@gmail.com>]

On Tue, May 7, 2019 at 9:10 AM Robert Dailey <rcdailey.lists@gmail.com> wrote:
> Your example is very helpful. I understand what you're saying for
> conflicted lines. But the "whatever the default merge resolution would
> have been" doesn't exist, because there's no reality where line 1 in
> color.txt can be something "automatic" (i.e. deduced by git). The only
> reality for the merge commit is some hand-edited replacement to line
> 1. So there is no "diff what I see with some alternate reality".
>
> The majority use case I'm interested in is seeing net-positive changes
> that happen in merge commits. Normally I take for granted that merge
> commits have nothing meaningful in them (meaningful here defined as
> something unexpected for a merge commit). But what if someone makes a
> poor decision and does some crazy refactoring in 1 file and amends it
> into a merge commit? Let's also say that these changes are done to a
> file that wasn't modified in any parent (say a unrelated.txt next to
> your color.txt). Since neither parent cares about that file for the
> purposes of the merge, I am trying to make sense of a revision
> specification that can be used to see what they did to that file.
>
> Even ignoring that issue, the more concerning observation of mine is
> that `diff @^!` produces any output at all. If you exclude both
> parents, why do I see a diff for parent 2 (I see the complete diff of
> the branch that was merged in)?
>
> Again, thank you for your example, you definitely made things very
> clear for me. I see where the confusion is. And I think --cc is a good
> way to get more context. At this point I'm just concerned about the
> @^! behavior with merge commits & diff.

Also I'm really confused how you got diff-tree to work. If I pick any
arbitrary SHA1 of a merge commit in my existing repo's history,
diff-tree produces only a SHA1 as the result:

$ git diff-tree --cc bdd47a73d
bdd47a73d18948aa46a8a7aa964543f0d989ffd4

I tried with just `-c` as well; same result.