bug#13521: `sort-lines' on the empty region

[Drew Adams <drew.adams@oracle.com>]

> > I doubt that someone who has `transient-mark-mode' off
> > would ever want commands such as `flush-lines' and
> > `sort-lines' to act on the region.  And if they did, I
> > expect they'd just narrow to it.
> 
> Please don't doubt, and please don't impose unnecessary commands on
> users who have transient-mark-mode off.

Please don't claim that I imposed any commands on anyone.

The region is nearly always present and usually nonempty.
A user with `transient-mark-mode' off would typically
(IMHO) be bothered if `flush-lines' started always acting
on the region (it would be almost always: whenever there's
a mark in the buffer and the region is nonempty). 

Do you disagree?

> > Anything that works on "the active region" is something
> > that makes sense only when `transient-mark-mode' is on
> > (IMHO).
> 
> The important point here is that sort-lines worked on the region, even
> if inactive, before the proposed changes, so restricting it now only
> to active regions would be a backward-incompatible change of behavior.

Yes, sorry; I agree about `sort-lines'.  That is not
the case for `flush-lines' and `keep-lines'.

And I think "the important point here" is that a command
that behaves differently when the region is active should
NOT act on the region when `transient-mark-mode' is off.
Why?  Because the notion of "active region" applies only
when `transient-mark-mode' is on.  Any special behavior
provided only when the region is active is, well, only
for when the region is active.  And that's never the case
when `transient-mark-mode' is on.

This was not the case for `sort-lines', as you point out.
It did NOT, and does not, behave differently when the
region is active from when it is inactive.