Backslash Bound Incorrectly (Readline)

[sunnycemetery]

Key sequences containing backslashes are bound incorrectly (and  
impossibly):

> ■ bind '"\C-\\":  undo'
> ■ LC_ALL=C bind -q undo
> undo can be invoked via "\C-x\C-u", "\C-\\\".

Is this a bug?  Perhaps there is a reason I do not see.  Thank you.

> ■ grep -i version -m 1 /usr/share/doc/readline/README
> This is the Gnu Readline library, version 8.0.
> ■ LC_ALL=C bash --version
> GNU bash, version 5.0.16(1)-release (x86_64-pc-linux-gnu)
> Copyright (C) 2019 Free Software Foundation, Inc.
> License GPLv3+:  GNU GPL version 3 or later  
> <http://gnu.org/licenses/gpl.html>
>
> This is free software; you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.
> ■ uname -srvmo
> Linux 5.5.13-arch2-1 #1 SMP PREEMPT Mon, 30 Mar 2020 20:42:41 +0000  
> x86_64 GNU/Linux