# emacs --version
GNU Emacs 24.3.50.1

# emacs -q

M-x: eldoc-mode

M-: (setq | )

point ------^

Nothing shown on modeline.

C-x o

"setq ([SYM VAL]...)" appears on modeline.

Then if in `eldoc-minibuffer-message' function from eldoc.el
(force-mode-line-update) changed to (force-mode-line-update t) -- then it works as it should.