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bug#24176: Confusing interaction between define-derived-mode and font-lo

From: npostavs
Subject: bug#24176: Confusing interaction between define-derived-mode and font-lock-add-keywords
Date: Wed, 24 Aug 2016 20:53:15 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)

severity 24176 minor
tags 24176 confirmed
found 24176 25.1

Clément Pit--Claudel <address@hidden> writes:

> Sorry, the original report had a confusing example; here is a fixed copy.
> (define-derived-mode ~/a fundamental-mode "~/a"
>   (font-lock-add-keywords nil `(("a" 0 'font-lock-keyword-face))))
> (define-derived-mode ~/b ~/a "~/b"
>   (font-lock-add-keywords nil `(("b" 0 'font-lock-builtin-face))))
> (define-derived-mode ~/c ~/b "~/c"
>   (font-lock-add-keywords nil `(("c" 0 'font-lock-constant-face))))
> Explicitly calling (setq font-lock-major-mode major-mode) before calling 
> font-lock-add-keywords yields the expected behaviour.

Yeah, it seems `font-lock-set-defaults' (called from
`font-lock-add-keywords') deletes the parent mode's added keywords since
they were not from the "correct" mode.

    (defun font-lock-set-defaults ()
      "Set fontification defaults appropriately for this mode.
    Sets various variables using `font-lock-defaults' and
      ;; Set fontification defaults if not previously set for correct major 
      (unless (and font-lock-set-defaults
               (eq font-lock-major-mode major-mode))
        (setq font-lock-major-mode major-mode)
        (setq font-lock-set-defaults t)

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