On Mon, Dec 23, 2019 at 10:02:31AM -0800, Junio C Hamano wrote:

Please unlearn dot-dot and three-dots when using "git diff", which
is not about ranges but about comparing two endpoints.  If we were
reinventing Git today from scratch, we would make "git diff A..B" an
error.  You can consider it a bug that the command accepts a range
notation, but this will not change any time soon without a large
fight to find and fix uses of the syntax in scripts by longtime Git
users have written over the years.

Allowing dot-dot on the command line of "git diff", instead of
diagnosing them as errors and dying, was a stupid mistake we (well,
mostly Linus, but I am willing to take the blame too) made due to
laziness when we reused the machinery, which we invented to parse
the command line of "log" family of commands to specify ranges, to
parse the command line of "diff", which accidentally ended up
allowing the syntax for ranges where it shouldn't be allowed.

And worse yet, since there was only dot-dot and three-dots came much
later, "git diff A..B" ended up comparing the endpoints A and B,
because there didn't even A...B notation exist.

This is not limited to you but any user of modern Git is better off
to pretend "git diff A..B" does not exist; please unlearn dot-dot
and three-dots when using "git diff" and you'd be happier.

I agreen that you should not use `A..B`, but what is wrong with
`A...B`? The alternative is a lot more verbose.

git diff $(git merge-base A B) B

Kevin