(current-column) produces unexpected results with tabs when overlays have before-strings

[Michael Schierl <schierlm@gmx.de>]

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In GNU Emacs 21.3.1 (i386-msvc-windows98.3000)
 of 2003-03-28 on buffy
configured using `configure --with-msvc (12.00)'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: deu
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

Execute this example code:

(let ((buf (get-buffer-create "*new*"))
      ovl
      (str "*"))
  (set-buffer buf)
  (setq tab-width 8)
  (erase-buffer)
  (insert "1234567\t89012\n1234567        89012")
  (put-text-property 0 1 'face 'bold str)
  (setq ovl (make-overlay 0 0))
  (overlay-put ovl 'before-string str)
  (setq ovl (make-overlay 15 15))
  (overlay-put ovl 'before-string str)
  (switch-to-buffer buf))

You will see 2 lines that look the same. Each one has an 1 character
wide overlay at the beginning. So from the documentation of
current-column one expects that when point is on (before) 8, the
column number should be the same in both lines (first line has a tab,
but that one should be counted as 8 spaces as it is 8 chars wide).

However, the tab is counted as only one space (column-number will
return 8 in first line, 15 in last line). When there are no
overlays with before-string before the tab, it works as expected.

This also affects cursor movement when the cursor is right of the
tab. Set cursor before upper 8 and move up -> cursor moves left a
bit.

Recent input:
M-x r e p o r t - e m a c s - b u g <return>

Recent messages:
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