At this point, I'm inclined to believe that Shift_JIS is not suitable as
> a locale encoding on POSIX systems, and that we should not try to
> support it in Guile.
>
> What do you think?
>
> Can you tell me how backslash and tilde are represented in Shift JIS?
>

They aren't:  iconv is right.  Japanese Windows users are used to seeing
Windows pathnames that look like "C:¥foo¥bar", and when writing C, to
strings like "first line¥nsecond line."  So what is happening is that the
character at #\x5C is *functionally* a backslash that is *displayed* as a
yen sign.  This is reinforced by the fact that the round-trip mapping from
Shift_JIS #\x5C is U+005C BACKSLASH, whereas U+00A5 YEN SIGN is mapped only
from Unicode (or other encodings) to Shift_JIS, never the other way around.

This is the last survivor of the "national characters" concept of ISO 646,
whereby certain 7-bit characters were interpreted differently in different
countries.  For Scandinavian programmers, for example, blocks in C began
with æ and ended with å rather than { and } respectively, and the logical
OR operator was ø.  In the same way, British and Irish programmers used £
instead of # at the beginning of comments in awk and shell programs.  With
the arrival of Latin-{1,2,3,4} this concept was eventually abandoned, and
all systems converged on ISO-646-IRV (the same as US-ASCII) *except*
Japanese systems.

So I recommend that you do what everyone else does and ignore the issue in
JIS-based encodings, of which Shift_JIS is the only one in practical use
(and it _is_ heavily used in Japan, where it is almost the only encoding
for documents on desktops).   Just ignoring the encoding is not an option
in Japan: see the comments by Joel Rees, Norman Diamond, and Ryan Thompson
at the bug you pointed to.

-- 
John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
In might the Feanorians / that swore the unforgotten oath
brought war into Arvernien / with burning and with broken troth.
and Elwing from her fastness dim / then cast her in the waters wide,
but like a mew was swiftly borne, / uplifted o'er the roaring tide.
        --the Earendillinwe