I have the following .dir-locals.el for a project:

((nil . ((show-trailing-whitespace . nil)
         (eval . (c-add-style "fontforge"
                              '("stroustrup"
                                (indent-tabs-mode . t)
                                (tab-width . 8)
                                (c-offsets-alist
                                 (case-label . *)))))))
 (c-mode . ((c-file-style . "fontforge"))))

The idea is to define a new c-style and apply it to all files in that
project.

When I first visit a file under that project, I get the error: "Undefined
style: fontforge"

When I examine c-style-alist, "fontforge" is the first entry, and when I
look at c-file-style for that buffer, its value is "fontforge". However, as
the error implies, the style has not been applied to that buffer: if I
check the value of indent-tabs-mode, which I have configured globally to
"nil", it is still nil.

If I then run M-x c-set-style RET fontforge RET, the style is applied as
expected.

Hence, it appears my error has to do with the order of evaluation. I tried
putting the "eval" sexp under the "c-mode" section of the .dir-locals.el,
but that still gives the same error, even when I put it before the
c-file-style sexp.

I can't find anything about this in the manual, or online (i.e. examples
where the result of an "eval" entry are relied on by another entry). In
hack-local-variables-apply files.el I found this bit of code:

    ;; Any 'evals must run in the Right sequence.
    (setq file-local-variables-alist
      (nreverse file-local-variables-alist))

so I tried putting the "eval" part at the end of c-mode's list, but that
didn't help either. (I checked file-local-variables-alist in each case to
make sure that in one the eval was before c-file-style, and in the other,
after).

Can someone please set me straight?

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