bug#49053: Error (setf seq-elt) is already defined as something else than a generic function

[Juri Linkov <juri@linkov.net>]

X-Debbugs-Cc: Stefan Monnier <monnier@iro.umontreal.ca>

I'm creating a new bug report because I was able to reproduce it.

>>> (setf (seq-elt plist ...))
>>>
>>> may help with that, though.
>>
>> BTW, often after pulling from master and recompiling,
>> Emacs fails to start with such errors:
>>
>> Debugger entered--Lisp error: (error "(setf seq-elt) is already defined as
>> something else than a generic function")
>>   error("%s is already defined as something else than a generic function" \(setf\ seq-elt\))
>>   cl-generic-ensure-function(\(setf\ seq-elt\))
>>   cl-generic-define-method(\(setf\ seq-elt\) nil (store (sequence array) n)
>> nil #f(compiled-function (store sequence n)
>
> Hmm....
>
> When you see that, could you show us the value of:
>
>     (symbol-function '\(setf\ seq-elt\))

Here is what I did to reproduce the error:

1. Checked out 2f7a115a9c
2. make bootstrap
3. Checked out 794ec934a7
4. make (without bootstrap)
5. run emacs without -Q, i.e. with the init file

It terminated after printing to stderr:

  (setf seq-elt) is already defined as something else than a generic function

6. run `emacs -Q`

  (symbol-function '\(setf\ seq-elt\))
  => nil

7. loaded the init file from ~/.emacs

It printed to the *Messages* buffer:

  Load error for /home/juri/.emacs:
  (error (setf seq-elt) is already defined as something else than a generic function)

Then evaluated symbol-function again that returned a different value:

  (symbol-function '\(setf\ seq-elt\))
  => dummy

8. run again `emacs -Q` and tried to manually evaluate each expression
   in the init file.

Then at one point, the byte-compiler compiled the file bytecomp.eln
in ~/.emacs.d/eln-cache.  And the compiled bytecomp.eln completely
fixed the problem.

I guess some change between 2f7a115a9c and 794ec934a7 (maybe 663fb3b774)
changed the file bytecomp.el, so there was the need to recompile it.