[MannXo]

join -i compared the first differing bytes after folding to lower case, while sort -f (ours and GNU's) and GNU join -i fold to upper case. The two orders disagree on [ \ ] ^ _ ` , which sit between Z and a, so input prepared with sort -f looked unsorted to join -i and keys present in both files were skipped as unpairable.

CaseInsensitiveSlice::cmp now folds to upper case. Equality is unchanged.

$ printf 'svc_backup mallory\nsvcabc mallory\n' | sort -f -k 1,1 > requested
$ echo SVC_BACKUP > reserved
$ join -i -v 1 requested reserved
svcabc mallory
$ join -i requested reserved
svc_backup mallory

Before, both commands reported "is not sorted" and exited 1, the first also printed svc_backup mallory, and the second printed no joined line.

case_insensitive_matches_sort_f_order fails on main.

Closes #15759