bug#27403: 26.0.50; Indentation misalignment with Unicode code points >65535

[Adam Niederer <adam.niederer@gmail.com>]

Hello, I believe I've found an indentation issue. To reproduce, start
emacs, create a buffer in js-mode, paste in this code, and press C-x h
TAB to indent the buffer:

let x = /* 👍 */ { foo: 0
                   bar: 0 }

let x = /* ☺ */ { foo: 0
                  bar: 0 }

Both 25.2 and 26.0.50 add one extra space before "bar" in the first
first snippet with U+1F44D THUMBS UP SIGN in the comment, whereas the
second snippet with U+263A WHITE SMILING FACE properly aligns "bar" with
"foo". This appears to happen whenever the character in the comment
needs a surrogate pair.

This issue also happens in python-mode:

"👍", {"a": 2,
       "b": 3}

"☺", {"a":2,
      "b":3}

Interestingly, pressing TAB with one's point on the second line of each
snippet to dedent the line yields a correct result for both symbols:

"👍", {"a": 2,
    "b": 3}

"☺", {"a":2,
    "b":3}

Just in case those Emoji don't make it through the mail properly, the
first snippet in each example contains U+1F44D THUMBS UP SIGN before
the map, and the second snippet contains U+263A WHITE SMILING FACE.

-Adam


In GNU Emacs 26.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.15)
of 2017-06-17 built on AdamsPC
Repository revision: 49c0ff29c2e0243ba35ec17e3e3af49369be43db
Windowing system distributor 'The X.Org Foundation', version 11.0.11903000
System Description: Arch Linux

Recent messages:
Auto-saving...
20 (#o24, #x14, ?\C-t)
21 (#o25, #x15, ?\C-u)
20 (#o24, #x14, ?\C-t) [2 times]
Undo! [3 times]
20 (#o24, #x14, ?\C-t)
Auto-saving...
mwheel-scroll: Beginning of buffer
Mark set
Auto-saving...done

Configured features:
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NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
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Important settings:
value of $LC_COLLATE: en_US.UTF-8
value of $LANG: en_US.UTF-8
locale-coding-system: utf-8-unix

Major mode: JavaScript

Minor modes in effect:
tooltip-mode: t
global-eldoc-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
tool-bar-mode: t
menu-bar-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
blink-cursor-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
line-number-mode: t
transient-mark-mode: t

Load-path shadows:
None found.

Features:
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Memory information:
((conses 16 133992 34676)
(symbols 48 23645 1)
(miscs 40 95 584)
(strings 32 30245 2336)
(string-bytes 1 974865)
(vectors 16 20847)
(vector-slots 8 721119 47152)
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