Warning raised if trying to set LC_ALL to a locale bearing a character type other than UTF-8 and C

[Kerin Millar]

Hello,

I was experimenting with locale variables in bash and noticed something that 
appears odd. This is with bash 5.1.16 and glibc 2.35.

$ locale -a | grep -E '^en_(US|GB)'
en_GB.utf8
en_US
en_US.iso88591
en_US.utf8

$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

$ ( LC_ALL=en_US.ISO-8859-1 )
bash: warning: setlocale: LC_ALL: cannot change locale (en_US.ISO-8859-1)

$ ( LC_ALL=en_GB.UTF-8 )
$ : setting LC_ALL to any of the UTF-8 locales does not raise a warning

Taking Perl as a counter-example, there is no problem issuing the equivalent 
setlocale call.

$ perl -MPOSIX=locale_h -E 'if (setlocale(LC_ALL, "en_US.ISO-8859-1")) { say 
"Successfully changed LC_ALL to ", setlocale(LC_ALL) }'
Successfully changed LC_ALL to en_US.ISO-8859-1

Is there a particular reason that bash appears to reject the attempt to assign 
"en_US.ISO-8859-1" to LC_CTYPE?

-- 
Kerin Millar