bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Dmitry Gutov]

On 08.12.2019 5:32, Eli Zaretskii wrote:

> > > Our goal is to allow themes "inherit" the :extend attribute without
> > > having to specify it in their face specs, unlike with other
> > > attributes.  That's the only goal;
> >
> > But that's exactly what it does.
>
> It does, but the implementation is too general, and might affect other
> use cases.

Other use cases of what? face-spec-respec is about applying theme and 
default faces specs.

> > > we don't want :extend to behave
> > > differently from other face attributes in any other context.
> >
> > What other contexts do you have in mind?
>
> Any context other than a theme defining a face.
>
> > What *shouldn't* it do?
>
> What we do with any other face attribute.

Either I don't understand you here, or the patch obviously satisfies 
that criterion.

> > > If you are saying that we cannot make this change apply only to face
> > > definitions by themes,
> >
> > What other face definitions are there? There's defface, of course, which
> > we treat differently. And there are theme definitions (both third-party
> > and "user theme").
>
> All the other situations where face-spec-recalc is called.  You listed
> at least some of them up-thread.

As I've explained, the other callers are all part of the system that 
makes sure theme specs (and defaults specs, of course) are applied 
correctly in various situations.

> > > then it means we don't really understand what
> > > we could break here, and then I don't think I want this change in
> > > Emacs 27.  Sorry, it's too risky.
> >
> > What about the existing risk of breaking every theme out there by doing
> > nothing?
>
> If we don't have a safe solution, we will have to live with that risk,
> unfortunately.

We haven't even started the pretest yet. If there are bugs in this patch 
(unlikely, but always possible), we have time for people to see and 
report them.

> > > But I don't
> > > consider myself an expert on these matters, so if you say we cannot
> > > differentiate between general face definition and what themes do, so
> > > be it.)
> >
> > What's a "general face definition"?
>
> Everything except theme definition of faces.

Please give an example.