Bug#926906: libc6: range of cyrillic lowercase letters matches also uppercase

[aleksandr barakin <a@barak.in>]

Package: libc6
Version: 2.28-8
Severity: normal

Dear Maintainer, new version (2.28) of libc6 contains collation error.

expected behaviour:
$ echo АБВабв | grep -o '[а-в]'
а
б
в
$ echo АБВабв | sed 's/[а-в]/-/g'
АБВ---

observed behaviour:
$ echo АБВабв | grep -o '[а-в]'
А
Б
а
б
в
$ echo АБВабв | sed 's/[а-в]/-/g'
--В---

this error is also observed in archlinux (libc-2.28) (see discussion in
russian: https://ru.stackoverflow.com/q/968618/178576)

additional info:
$ locale
LANG=
LANGUAGE=ru
LC_CTYPE="ru_RU.UTF-8"
LC_NUMERIC="ru_RU.UTF-8"
LC_TIME="ru_RU.UTF-8"
LC_COLLATE="ru_RU.UTF-8"
LC_MONETARY="ru_RU.UTF-8"
LC_MESSAGES="ru_RU.UTF-8"
LC_PAPER="ru_RU.UTF-8"
LC_NAME="ru_RU.UTF-8"
LC_ADDRESS="ru_RU.UTF-8"
LC_TELEPHONE="ru_RU.UTF-8"
LC_MEASUREMENT="ru_RU.UTF-8"
LC_IDENTIFICATION="ru_RU.UTF-8"
LC_ALL=ru_RU.UTF-8

-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-8-amd64 (SMP w/2 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=UTF-8) (ignored: LC_ALL set to ru_RU.UTF-8), LANGUAGE=C (charmap=UTF-8) (ignored: LC_ALL set to ru_RU.UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: unable to detect

Versions of packages libc6 depends on:
ii  libgcc1  1:8.3.0-5

Versions of packages libc6 recommends:
ii  libidn2-0  2.0.5-1

Versions of packages libc6 suggests:
ii  debconf [debconf-2.0]  1.5.71
pn  glibc-doc              <none>
ii  libc-l10n              2.28-8
ii  locales                2.28-8

-- debconf information excluded