Re: Why doesn't `git log -m` imply `-p`?

Alex Henrie [off-list ref] writes:

I read the following in `man git-log` today:

--diff-merges=separate, --diff-merges=m, -m
    This makes merge commits show the full diff with respect to each of
    the parents. Separate log entry and diff is generated for each
    parent.  -m doesn't produce any output without -p.

--diff-merges=combined, --diff-merges=c, -c
    With this option, diff output for a merge commit shows the
    differences from each of the parents to the merge result
    simultaneously instead of showing pairwise diff between a parent and
    the result one at a time. Furthermore, it lists only files which
    were modified from all parents.  -c implies -p.

--diff-merges=dense-combined, --diff-merges=cc, --cc
    With this option the output produced by --diff-merges=combined is
    further compressed by omitting uninteresting hunks whose contents
    in the parents have only two variants and the merge result picks one
    of them without modification.  --cc implies -p.

Why do -c and -cc imply -p, but -m does not? I tried to use both `git
log -c` and `git log -m` today and was confused when the latter didn't
produce any output. Could we change this behavior in a future version
of Git?

Patches to fix this are almost ready, but I'd like to make a warning
that you'd likely be even more confused by the current output of "-m",
unless you set "log.diffMerges" configuration option to "first-parent".
These diffs with respect to /second/ parent at the end of the output
made me really mad once upon a time.

-- Sergey Organov

On Mon, May 10, 2021 at 6:11 AM Sergey Organov [off-list ref] wrote:

Alex Henrie [off-list ref] writes:

quoted

I read the following in `man git-log` today:

--diff-merges=separate, --diff-merges=m, -m
    This makes merge commits show the full diff with respect to each of
    the parents. Separate log entry and diff is generated for each
    parent.  -m doesn't produce any output without -p.

--diff-merges=combined, --diff-merges=c, -c
    With this option, diff output for a merge commit shows the
    differences from each of the parents to the merge result
    simultaneously instead of showing pairwise diff between a parent and
    the result one at a time. Furthermore, it lists only files which
    were modified from all parents.  -c implies -p.

--diff-merges=dense-combined, --diff-merges=cc, --cc
    With this option the output produced by --diff-merges=combined is
    further compressed by omitting uninteresting hunks whose contents
    in the parents have only two variants and the merge result picks one
    of them without modification.  --cc implies -p.

Why do -c and -cc imply -p, but -m does not? I tried to use both `git
log -c` and `git log -m` today and was confused when the latter didn't
produce any output. Could we change this behavior in a future version
of Git?

Patches to fix this are almost ready, but I'd like to make a warning
that you'd likely be even more confused by the current output of "-m",
unless you set "log.diffMerges" configuration option to "first-parent".
These diffs with respect to /second/ parent at the end of the output
made me really mad once upon a time.

Good observation. I'm not sure what would be the ideal default here,
but certainly having -m turn on /some/ kind of diff is better than
nothing. Thanks!

-Alex