Eli Zaretskii <eliz@gnu.org> writes:

> But it produces a font different from the default face's font, which
> AFAIU was what you wanted?

When the default font is proportional; sure.  Otherwise not, I think.

> That's what happens for me: if I start Emacs with "emacs -fn FOO"
> where FOO is a variable-pitch font, then using the defface form I
> suggested makes fixed-pitch use a monospaced font.

Doesn't that happen without the ":spacing mono", too?

>> >   (defface fixed-pitch
>> >     `((t :font ,(font-spec :spacing 'M)))
>> >     "The basic fixed-pitch face."
>> >     :group 'basic-faces)
>> 
>> That leads to:
>> 
>> Don't know how to purify: #<font-spec nil nil nil nil nil nil nil
>> nil nil nil 100 nil nil>
>
> I only tried that in a running Emacs session, and with a face whose
> name is different from fixed-pitch.  I haven't tried bootstrapping
> with the above in faces.el.  I'm sure we can avoid the problem, if the
> effect is what we want: for example, we could have the defface in
> startup.el or something.

I tried it in a running Emacs, but the problem remains -- the
fixed-pitch font is different from default, which is the problem this
bug report is about.

And...  it made it use a proportional font?