Re: master 79d9f3982f: Add derived-mode test for bug#24176

[Lars Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>>     (defvar mode-a-flk '(("a" 0 'font-lock-keyword-face)))
>>>     (define-derived-mode mode-a fundamental-mode "mode-a"
>>>       (setq-local font-lock-defaults '(mode-a-flk)))
>>>     (define-derived-mode mode-b mode-a "mode-b"
>>>       (font-lock-add-keywords nil `(("b" 0 'font-lock-builtin-face))))
>>>     (defvar mode-c-flk '(("c" 0 'font-lock-keyword-face)))
>>>     (define-derived-mode mode-c mode-b "mode-c"
>>>       (setq-local font-lock-defaults '(mode-c-flk)))
>>
>> And font-lock-keywords should then be...
>> (t (("c" 0 'font-lock-keyword-face)))?
>
> I think so, yes, or maybe b+c, but it definitely should not be a+b.

Hm...  do you see any obvious way to fix this?  We could document that
you have to set font-lock-set-defaults to nil if you're setting
font-lock-defaults in a derived mode, but that's not very
satisfactory...  or we could store the value of font-lock-defaults and
check whether it's changed (in font-lock-set-defaults)?  Hm...

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