Why is `C-x 8' limited to Latin-1 for search?

[Drew Adams]

I was thinking that one could now use `C-x 8 RET' (which is now `insert-char')
to insert any Unicode char into the Isearch search string.  Apparently not.

At least it doesn't seem to work (`C-x 8 RET' is bound to
`isearch-other-control-char'), and I couldn't find it mentioned in the doc.
Looking for it in the doc, I found only a mention that you can use `C-x 8' to
insert Latin-1 chars (and so search for them).

Is it intentional that you cannot search for other Unicode chars using `C-x 8
RET'?  Is that not possible only because it is not yet implemented, or is there
some reason why it should not be available (implemented)?

Yes, I know that `C-x 8 RET' is not necessarily an ideal way to input chars.
But it seems like you should be able to use it here anyway.

If there is a good reason why one should not be able to use `C-x 8 RET' to
directly input chars during Isearch, what about letting users at least use `C-x
8 RET' after `M-e', to input chars that way?

---

To try the latter quickly (no, this is not the way to code this; it's just a
quick way to let you try it):

(unless (fboundp 'ORIG-isearch-edit-string)
  (defalias 'ORIG-isearch-edit-string
            (symbol-function 'isearch-edit-string)))

(defun isearch-edit-string ()
  (interactive)
  (let ((enable-recursive-minibuffers  t))
    (ORIG-isearch-edit-string)))

In a buffer with some Unicode chars, e.g., `λ':

C-s M-e C-x 8 RET GREEK SMALL LETTER LAMBDA C-s

(You have completion for the character name, at least.)