Setting/unsetting posix mode interferes with keybindings

[Ville Skyttä]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu'
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -Wdate-time
-D_FORTIFY_SOURCE=2 -g -O2
-fdebug-prefix-map=/build/bash-N2nMjo/bash-4.4.18=.
-fstack-protector-strong -Wformat -Werror=format-security -Wall
-Wno-parentheses -Wno-format-security
uname output: Linux uncleman.localhost 4.15.0-101-generic #102-Ubuntu SMP
Mon May 11 10:07:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.4
Patch Level: 20
Release Status: release

Description:
Setting/unsetting posix mode interferes with keybindings.

This is a problem e.g. with completion functions that use
process substitution; they need to control the posixness state
in order to not run into syntax errors when they happen to be
invoked in posix mode shells.

This is not limited to the above bash version, it has been
reported to happen with much newer ones as well. More info and
background at https://github.com/scop/bash-completion/issues/190

Repeat-By:
Add these lines to ~/.inputrc:
set editing-mode vi
Tab: menu-complete
Then, in a new shell:
$ bind -q menu-complete
menu-complete can be invoked via "\C-i", "\C-n".
$ shopt -uo posix # or -so, or set [+-]o
$ bind -q menu-complete
menu-complete can be invoked via "\C-n".

Notice how \C-i was lost. It does not matter whether setting
posix actually changed its state, happens also e.g. when
unsetting when already unset.