bug#63731: [PATCH] Support Emoji Variation Sequence 16 (FE0F) where appropriate

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Steven Allen <steven@stebalien.com>,  63731@debbugs.gnu.org
> Date: Fri, 26 May 2023 10:34:02 +0200
> 
> Ugh. The following fixes it for me:
> 
> diff --git a/lisp/composite.el b/lisp/composite.el
> index fb8b76114f4..af86d1436d3 100644
> --- a/lisp/composite.el
> +++ b/lisp/composite.el
> @@ -756,7 +756,7 @@ compose-gstring-for-dotted-circle
>  ;; Allow for bootstrapping without uni-*.el.
>  (when unicode-category-table
>    (let ((elt `([,(purecopy "\\c.\\c^+") 1 compose-gstring-for-graphic]
> -	       [nil 0 compose-gstring-for-graphic])))
> +	       )))

This is unacceptable, AFAIU.  We cannot unsupported (or change) the
correct display of mark characters, can we?

> Although the following is less invasive:
> 
> diff --git a/lisp/composite.el b/lisp/composite.el
> index fb8b76114f4..333428f008a 100644
> --- a/lisp/composite.el
> +++ b/lisp/composite.el
> @@ -762,6 +762,11 @@ compose-gstring-for-dotted-circle
>  	 (if (memq val '(Mn Mc Me))
>  	     (set-char-table-range composition-function-table key elt)))
>       unicode-category-table))
> +  ;; for Emoji presentation selector
> +  (set-char-table-range
> +   composition-function-table
> +   #xFE0F
> +    `([,(purecopy "\\c.\ufe0f") 1 compose-gstring-for-graphic]))
>    ;; for dotted-circle
>    (aset composition-function-table #x25CC
>  	`([,(purecopy ".\\c^") 0 compose-gstring-for-dotted-circle]))

Can you please explain why the current setup doesn't work in this
case, even though "C-u C-x =" says the composition was done?  And how
the above patch fixes that?

> Didnʼt we conclude that composition had some issues with multiple
> entries for the same codepoint if there was a mix for forward and
> backward looking regexp?

Not sure I understand to what does this allude.  What mix of forward
and backward looking regexp do you see?