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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: Use the Unicode replacement character for replacing unencodable
 characters into UTF-16
Date: Tue, 18 Aug 2020 19:19:52 +0300
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To: Mattias =?utf-8?Q?Engdeg=C3=A5rd?= <mattiase@acm.org>
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> Feedback-ID: mattiase@acm.or
> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 18 Aug 2020 17:36:10 +0200
> 
> The attached patch makes sure that non-Unicode characters are replaced with U+FFFD REPLACEMENT CHARACTER instead of a space when converting to UTF-16. (The space is from all evidence a historical accident.)

Can you describe under which circumstances this default-character will
be used?

The issue that bothers me is whether u+FFFD can appear in situations
where it cannot be displayed by Emacs, because then the result will be
more confusing than helping.