bug#63103: 30.0.50; nconc compiler optimization breaks user packages

[Drew Adams]

> The behaviour
> for improper lists needs to be specified explicitly, in particular in
> this case when the function actually overwrites arbitrary information in
> the input, not just a terminating nil.

Yes, the use of a dotted list (a term that's clearer
and more sympathetic than "improper list") for all
but the last arg needs to be documented.

(Note: *ALL* but the last.)

> Well, my assumption didn't completely stand on its own. Given that the
> last argument can be a non-list suggests that the last argument is not
> inspected or mutated, but just overwrites the cdr of the second to last
> argument. This then implies that the second to last argument could also
> be an improper list. Given that CL and Elisp have behaved like this for
> a long time, it seems better to preserve this property. I think it is
> kind of nice that `nconc' can be used as a tool to turn a proper list
> into an improper list and vice versa. It may be good to document this in
> the `nconc' docstring and the Elisp manual.

+1.

You both agree that this behavior (1) shouldn't be
lost and (2) should be documented.  That's exactly
the position taken by Common Lisp.

Please document this behavior as part of the bug
fix, if you haven't already.