bug#43609: 28.0.50; eldoc-documentation-function

[martin rudalics]

When with emacs -Q I put the following snippet into *scratch*

(defun foo ()
  (ignore))

move point to somewhere on "foo" and do

M-: (funcall eldoc-documentation-function)

I get the following backtrace:

Debugger entered--Lisp error: (void-function nil)
  nil(:patient)
  eldoc--make-callback(:patient)
  eldoc-documentation-default()
  funcall(eldoc-documentation-default)
  eval((funcall eldoc-documentation-function) t)
  eval-expression((funcall eldoc-documentation-function) nil nil 127)
  funcall-interactively(eval-expression (funcall eldoc-documentation-function) 
nil nil 127)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

If I do the same thing with Emacs 27 I get

#("defun: (NAME ARGLIST &optional DOCSTRING DECL &rest BODY)" 0 5 (face 
font-lock-keyword-face) 8 12 (face eldoc-highlight-function-argument))

The doc-string of 'eldoc-documentation-function' with Emacs 27 was:

  Function to call to return doc string.
  The function of no args should return a one-line string for displaying
  doc about a function etc. appropriate to the context around point.
  It should return nil if there’s no doc appropriate for the context.
  Typically doc is returned if point is on a function-like name or in its
  arg list.

The doc-string with Emacs 28 says

  For backward compatibility to the "old" protocol, this variable
  can also be set to a function that returns nil or a doc string,
  depending whether or not there is documentation to display at
  all.

But it does not tell _how_ to do that in order to get the string
returned by Emacs 27 and I already spent a couple of hours trying.

I am using a package that displays the string produced by that
function in a tooltip near point.  This package now ceased to work
with the backtrace shown above.  Please help me to make that
package work again, either by optionally restoring the old
functionality of (funcall eldoc-documentation-function) or by
providing another function I could call instead.  The present
situation makes it currently impossible for me to use master.

Thank you, martin


In GNU Emacs 28.0.50 (build 1, i686-pc-mingw32)
 of 2020-09-16 built on NOREST
Repository revision: f028a893a552b0c38c35f949addb6a891e8586cc
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 5.1.2600
System Description: Microsoft Windows XP Service Pack 3 (v5.1.0.2600)

Configured using:
 'configure --prefix=/c/emacs-git/trunk/dbg --with-gnutls=no
 --without-pop --enable-checking=yes --enable-gcc-warnings=warn-only
 --enable-check-lisp-object-type=warn-only 'CFLAGS=-O0 -g3
 -Wno-logical-op -Wno-missing-braces''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS MODULES THREADS PDUMPER

Important settings:
  value of $LANG: DEA
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils help-fns
radix-tree cl-print debug backtrace help-mode easymenu find-func
cl-loaddefs cl-lib tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win
w32-vars term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads w32notify w32 multi-tty
make-network-process emacs)

Memory information:
((conses 8 57053 5749)
 (symbols 24 7161 1)
 (strings 16 20666 1951)
 (string-bytes 1 643171)
 (vectors 8 12211)
 (vector-slots 4 173280 17378)
 (floats 8 36 47)
 (intervals 28 304 1)
 (buffers 568 13))