Bug in `next-line' when last char in line has after-string property

[Ben North <ben@redfrontdoor.org>]

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The attached file shows a bug in 22.0.92.  To reproduce, launch "emacs
-Q".  (The bug is exhibited with or without the "-nw" option.)
`find-file' the attached file.  Make sure your window is narrow enough
that the initial line of `-' characters wraps.  Go to the end of the
buffer (M->) and do `eval-last-sexp' (C-x C-e).  The desired behaviour
is that point ends up in the first column of the second line.  It in
fact ends up in the first column of the third line.

You can carry on playing with this: go to the very start of the buffer,
and hit C-n --- point moves down two lines instead of one.  I looked
into this a bit, which is what lead to the attached test-case using only
functions implemented in C, but I don't know the C code well enough to
dig further I'm afraid.  Hope the report is useful nonetheless.

[-- Attachment #2: line-motion-bug.el --]
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; ------------------------------------------------------------------------------------------------------------------------------------

(progn (setq overlay
             (let ((end-of-first-line (save-excursion (goto-char 1) (end-of-line) (point))))
               (make-overlay (1- end-of-first-line) end-of-first-line))
             truncate-lines t)
       (overlay-put overlay 'after-string "X")
       (progn
         (goto-char 1)
         (goto-char (line-end-position))
         (vertical-motion 1)))

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