Re: add-face-text-property destructively modified properties

[Eli Zaretskii]

> From: Lars Ingebrigtsen <address@hidden>
> Date: Wed, 09 Oct 2019 20:14:26 +0200
> 
> (progn
>   (setq str1 (propertize "foo" 'face '(underline highlight)))
>   (setq str2 (concat str1))
>   (add-face-text-property 0 3 'foobar t str2)
>   (pp str1 (current-buffer)))
> ->
> 
> #("foo" 0 3
>   (face
>    (underline highlight foobar)))
> 
> This didn't seem right to me, so I modified it to not do that, but only
> modify str2 when I told it to modify str2.
> 
> Eli wanted comments about whether anybody thinks this is a surprising
> change.

Mostly whether people think this could break something, as we had this
behavior for quite some time, I think.