Bash 5 incorrectly overrides BASH_COMPAT env variable

[Marc-Antoine Perennou]

Hello,

After debugging why the unit tests of paludis (package manager from
Gentoo and Exherbo) were failing with bash 5, I noticed a really weird
behaviour which wasn't present in earlier versions of bash.

All the exemples I could find support the values we're currently
checking for BASH_COMPAT in several environments (namely "3.2" and
"4.2"). Everything works fine with bash 4, but with bash 5, this
variable gets overridden and the new value is respectively "32" or
"42" (notice the missing dot) when using the compat32 option.

A simple reproducer showing how... unexpected the results are:

keruspe@Lou ~ % bash
keruspe@Lou ~ $ echo $BASHOPTS
checkwinsize:cmdhist:complete_fullquote:expand_aliases:extquote:force_fignore:globasciiranges:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath
keruspe@Lou ~ $ echo $BASH_COMPAT

keruspe@Lou ~ $ BASH_COMPAT=3.2 bash
keruspe@Lou ~ $ echo $BASHOPTS # BASH_COMPAT=3.2 causes compat32 to be
added here
checkwinsize:cmdhist:compat32:complete_fullquote:expand_aliases:extquote:force_fignore:globasciiranges:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath
keruspe@Lou ~ $ echo $BASH_COMPAT
3.2
keruspe@Lou ~ $ export BASHOPTS
keruspe@Lou ~ $ bash
keruspe@Lou ~ $ echo $BASHOPTS
checkwinsize:cmdhist:compat32:complete_fullquote:expand_aliases:extquote:force_fignore:globasciiranges:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath
keruspe@Lou ~ $ echo $BASH_COMPAT # launching a new bash with compat32
in BASHOPTS has overriden the value
32


Is it intended that the value has no dot?

Regards,
Marc-Antoine