`ibuffer-switch-to-saved-filter-groups' is not known to be defined

Question

I have this line in my .emacs file.

(add-hook 'ibuffer-mode-hook
          #'(lambda ()
              (ibuffer-switch-to-saved-filter-groups "default")))

The settings work fine for me. But flycheck says:

the function `ibuffer-switch-to-saved-filter-groups' is not known to be defined.

I think if I declare the function ahead, the warning would go away. But ibuffer is a build-in package, and I cannot find a way to declare this function.

How to declare a built-in function?

Update

I found the ibuffer-switch-to-saved-filter-groups is not a built-in function, but is defined in a file called ibuf-ext.el, and locate-library can find this file from my environment path. But even after I defined declare-function like below:

(declare-function ibuffer-switch-to-saved-filters "ibuf-ext.el" nil)

I still get this warning from flycheck.

Answer 1

Your declare-function doesn't silence the warning because it doesn't declare the same function (there a "-group" missing before the final "s").

The warning basically tells you that the byte-compiler can't find anything in your code which guarantees that ibuffer-switch-to-saved-filter-groups will exist when that call is reached.

The problem is fundamentally due to the byte-compiler's much too limited understanding of things like hooks: it is not able to understand that this ibuffer-mode-hook will only be run from code inside ibuffer.el (where ibuffer-loaddefs is loaded), so the functions placed on this hook will normally only be run in a context where ibuffer-switch-to-saved-filter-groups is autoloaded ondemand).

Patches welcome.

Answer 2

Doing a (require 'ibuf-ext) will probably pacify flycheck...