Re: [Bug report] git diff stat shows unrelated diff

[Junio C Hamano <gitster@pobox.com>]

Elijah Newren <newren@gmail.com> writes:

> The only thing I seem to be able to retain is the following:  "git
> diff D..E is totally useless and should be an error because (1) it
> doesn't do what I expect and (2) for folks that want the behavior
> currently gotten with that syntax can instead just use a space instead
> of a double dot."

That sums up pretty nicely.  diff is fundamentally an operation
between two endpoints, so the range notation a..b does not work
nicely with it at the conceptual level.

When we realized that we can take advantage of the above fact, and
reuse a range notation to mean something that is generally useful in
the context of diff, such as 'one end of the comparison is the merge
base between a and b, and the other end is b', it was too late to
use "a..b", as an early adopters of Git was already used to the fact
that "a..b" happened to mean the same thing as "comparison of one
end is a, the other end is b", pretty much implemented without much
thought.

It might be _possible_ to spend a year (i.e. 4 cycles) to start
warning when two-dot notation is used for "git diff" (only, not any
plumbing like "git diff-files") and tell the user to use the more
logical two-end notation "git diff A B" and then eventually error
out when two-dot notation is used, while retaining the three-dot
notation throughout and to the eternity.  I am not sure if it is
worth the deprecation cost, though.