(aset UNIBYTE-STRING MULTIBYTE-CHAR)

[Kenichi Handa]

Before the unicode merge, this worked:
  (let ((str "a")) (aset str 0 (decode-char 'ucs #x100)))

In emacs-unicode-2 branch, there was a discussion about the
rightness of aset changing the multibyteness of a string,
and I changed the code to signal an error in the above case.

But, I got reports claiming that the change breaks some of
already existing Elisp packages.  Although changing the
current code again to make the above code work, it causes
another problem in this case:
  (let ((str "a")) (aset str 0 #xC0))

Currently, it changes STR to the unibyte string "\300" (that
is the same as before unicode merge), but if we allow
changing the string multibyteness, perhaps STR must be
changed to the multibyte string of A-grave "À" because the
character code of A-grave is #xC0.  But, that means we loose
a way to easily manipulate raw byte data in a unibyte
string.

What do you think is the right thing for this matter?

---
Kenichi Handa
address@hidden