bug#25645: 26.0.50; message.el: Fcc+handler results in write coding issues in emacs 26

[Yuri D'Elia]

On Wed, Sep 13 2017, Lars Ingebrigtsen wrote:
>> It works as expected with emacs 25.2, but fails on 26 when the body
>> contains unicode characters. Somehow when the mail is encoded in quoted
>> printable with an utf-8 charset, the buffer seem to contain raw bytes as
>> if the buffer has an incorrect buffer-file-coding-system.
>
> I'm unable to reproduce this bug.  If I put
>
> Fcc: /tmp/fcc
>
> in my headers and enter text like "föo" in the body, the file is saved
> as it should be.

Adding an Fcc is not enough, if you read the initial report, the issue
is triggered by attempting to call write-file within a
message-fcc-handler-function.