column numbers for non-ASCII characters in error messages

[Ben Pfaff]

The GNU coding standards say how to calculate column numbers for
ASCII characters:

    Line numbers should start from 1 at the beginning of the file, and
    column numbers should start from 1 at the beginning of the line.  (Both
    of these conventions are chosen for compatibility.)  Calculate column
    numbers assuming that space and all ASCII printing characters have
    equal width, and assuming tab stops every 8 columns.

They don't say how to calculate them for non-ASCII characters.  I
guess that this is because doing so is somewhat difficult.
However, I'd like to figure out what is the "best" way to do it.
So far, I've thought of the following ways:

        * Byte offset from beginning of line.

        * Display width from beginning of line, with double-wide
          characters counting as two positions and combining
          characters (e.g. combining accents) counting as zero
          positions.

        * Grapheme clusters (user-visible characters) from
          beginning of line, as specified in Unicode Standard
          Annex #29 "Unicode Text Segmentation".

Which one is most likely to be useful to users?  It seems that
Emacs computes column positions in terms of display width.

Thanks,

Ben.

(If there is a better place to ask this question, let me know.)
-- 
Ben Pfaff 
http://benpfaff.org