bug#7812: 23.2; compilation-error-regexp-alist extremely unfriendly to customization interface

[Glenn Morris <rgm@gnu.org>]

Dave Abrahams wrote:

[customizing compilation-error-regexp-alist]

> It was never awesome: it *should* be possible to add support for a
> couple of unique tools without losing the benefits of all the builtin
> matchers that will come along next time I upgrade emacs.  It has
> always been the case that if I customize one or two of these elements,
> I'll never see new ones that are supplied by the maintainers on
> upgrade...

That is a general issue with customize, albeit more of one for options
with long, complicated values where new ones might be added to the old
elements.

There is M-x customize-changed to point out where options have changed
since a given Emacs release. In an ideal world maybe there could be
some automatic, version-control like merging of options when you start
a new Emacs version for the first time.

> ...but now things are worse: I can't even add new alist elements
> through the customize interface, and if do I add a single new one
> using setq, all I see in the customize interface thenceforth is:
>
>   (mapcar 'car compilation-error-regexp-alist-alist)

compilation-error-regexp-alist has a bad custom :type. It does not
allow you to do anything other than turn on/off the default settings.
It is impossible to add a new element via customize, and doing so by
hand will produce a mismatch.

AFAICS, it isn't possible to have a custom type that allows you to both:

i) toggle on/off the predefined options
ii) add new elements of your own

So feature i) will probably have to go.