puzzling byte-compile-file message: `=' called for effect

[Jim Ottaway]

When byte-compiling a file, I get this message:

In nl-publish-markup-list:
nested-lists.el:395:19:Warning: `=' called for effect

What does this mean?  I grepped for 'for effect' in Emacs's
lisp/emacs-lisp subdirectory, and found the relevant parts of
byte-opt.el, but I am no wiser.

The code with the '=' in it is:

(when (and nl-paras-fixed-p
                     (save-excursion
                       (goto-char nl-markup-start)
                       (goto-char (muse-line-beginning-position))
                       (= (forward-line -1) 0)
                       (looking-at "^\\S-")))
            (with-current-buffer nl-temp-buffer
              (goto-char (point-min))
              (insert "\n\n")))

Any ideas? Does it mean that the byte-compiler thinks that
(forward-line -1) will always return 0 at that point? Or does 'for
effect' mean something else?

-- 
Jim Ottaway