bug#53236: 26.1; encode-coding-string does not encode the string as expected

[Markus Triska <triska@metalevel.at>]

Dear all,

please consider the UTF-8 encoding of the Unicode codepoint 0x80, which
is formed by two bytes. In hexadecimal notation, they are: 0xC2 0x80.

We can use decode-coding-string to verify that this byte sequence is
decoded to 0x80 when specifying utf-8, which works exactly as expected:

    (decode-coding-string "\xC2\x80" 'utf-8)

This yields "\200", which is the same as "\x80", as verified via:

    (string= "\200" "\x80") --> t

Correspondingly, I expect (encode-coding-string "\200" 'utf-8) to yield
a string equivalent to "\xC2\x80", but that seems not to be the case. I get:

    (encode-coding-string "\200" 'utf-8) --> "\200"

And therefore, unexpectedly:

    (string= (encode-coding-string "\200" 'utf-8) "\xC2\x80") --> nil

It appears that encode-coding-string does not encode the string in UTF-8
as expected. Is there any way to obtain the desired encoding with
encode-coding-string, i.e., the UTF-8-encoded string "\xC2\x80"?

Thank you and all the best!
Markus

In GNU Emacs 26.1 (build 3, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2019-04-09 built on mt-laptop
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:	Ubuntu 19.04

Configured features:
XPM JPEG GIF PNG SOUND GSETTINGS NOTIFY GNUTLS LIBXML2 FREETYPE XFT ZLIB
TOOLKIT_SCROLL_BARS LUCID X11 THREADS

Important settings:
  value of $LC_MONETARY: en_GB.UTF-8
  value of $LC_NUMERIC: en_GB.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix