Re: undo-tree problem after updates

> I often install a new emacs git version.  After many (all? not quite
> sure) of these updates, the first start of emacs is unusable because
> as soon as I try to enter ESC-x, I get this error:
>
> Symbol’s value as variable is void: undo-tree-mode--set-explicitly

Hmm... damn... I did bump into this and I remember finding out the
likely culprit, but I got side tracked, never made it into a proper bug
report and can't quite remember what it was.

My vague memory was that it went like this:

- Start Emacs.
- Emacs loads `foo.elc` (in your case, `undo-tree.elc`).
- Sees there's no matching `.eln` file so launches a native compilation
  of `foo.eln`.
- [ So far everything works fine.  ]
- When the (async) native compilation ends, Emacs loads `foo.eln`, tho it
  does so in a special way which just overwrites the `symbol-function`
  cells of the toplevel functions defined in `foo.el(c|n)`.
- And now your Emacs is in this funny state where it will complain about
  `foo--set-explicitly`.

Exit Emacs, restart it, and the problem is gone.

Exit Emacs, delete the `foo.eln` file, restart Emacs, the problem is
gone at first but reappears as soon as the `foo.eln` is recreated and
(re)loaded.

I think this has to do with the fact to `foo.elc` was compiled with an
Emacs using an other version of the `define-globalized-minor-mode`
macro than the one used when building `foo.eln`.

[ IOW it's another corner case problem due to compiling starting from
  `foo.el` instead of starting from `foo.elc`.  🙂 ]


=== Stefan