bug#19645: 25.0.50; Recent EIEIO changes broke helm: Lisp error: (cl-no-next-method helm--setup-source...

[Thierry Volpiatto <thierry.volpiatto@gmail.com>]

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

> I'm still wondering why you'd use a :before method when there's no
> primary method.

To ensure this method run BEFORE the other same methods that run for
other classes:

--8<---------------cut here---------------start------------->8---
(defclass helm-test-class (helm-source-sync helm-type-file)
  ((candidates :initform (lambda () ("foo" "bar" "baz")))))
=>helm-test-class

(helm-make-source "test" 'helm-test-class)

1) With helm-type-file `helm--setup-source' method is :before
=>
((name . "test")
 (candidates lambda nil ("foo" "bar" "baz"))
 (action ("Find file" . helm-find-many-files)
         ("Find file as root" . helm-find-file-as-root)
         ("Find file other window" . find-file-other-window)
         ("Find file other frame" . find-file-other-frame)
         [...]
 (persistent-help . "Show this file")
 [...]
 (match helm-mp-exact-match helm-mp-3-match)
 (header-line . "C-j: Show this file (keeping session)")
 (dont-plug helm-compile-source--match-plugin helm-compile-source--persistent-help)
 (matchplugin))

2) With helm-type-file `helm--setup-source' method is only :primary
=>
((name . "test")
 (candidates lambda nil ("foo" "bar" "baz"))
 (action . identity)
 (match helm-mp-exact-match helm-mp-3-match)
 (dont-plug helm-compile-source--match-plugin helm-compile-source--persistent-help)
 (matchplugin))
--8<---------------cut here---------------end--------------->8---

As you can see when the helm--setup-source method of helm-type-file is
not :before, the default slots (common to both class) of helm-source-sync
take precedence. 
The helm-test-class have to inherit from two classes, and
the helm-type-file class should overhide the default slot of the
helm-source-sync class.

Maybe there is another way to achieve this ?

-- 
Thierry
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