$? is -1

[bashbug]

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-Det0cS/bash-4.4.18=. 
-fstack-protector-strong -Wformat -Werror=format-security -Wall 
-Wno-parentheses -Wno-format-security
uname output: Linux mine 4.15.0-54-generic #58-Ubuntu SMP Mon Jun 24 10:55:24 
UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.4
Patch Level: 19
Release Status: release

Description:
        Two related issues, regarding negative exit status.

        First, I always thought that the exit status, $?, was in the 0-255 
range.
        ( wait4 int argument for status masked with 0377 ) 
        But I got a -1 in the exit status of (builtin command) bind.
        Is this a bug or wanted behaviour?

        Second, when ran in a subshell, the same exit status gets mapped to 255.
        While logical, as -1 gets mapped to 255, it seems inconsistent.
        ( from the manual: "The return status is the exit status of list." )

Repeat-By:
        $ bind -x "l":ls
        bash: bind: l:ls: first non-whitespace character is not `"'
        $ echo $?
        -1

        $ ( bind -x "l":ls 2>/dev/null ); echo $?
        255

        $ # This would have worked (but is irrelevant)
        $ bind -x '"l":ls' # exit 0
        $ bind l:self-insert # undo