Here's the output
$ uname -a
AIX helios 3 4 0006DC2F4C00

$ grep STRTOIMAX config.h    
#define HAVE_DECL_STRTOIMAX 0
#define HAVE_STRTOIMAX 1

Been doing a retro computing project, and discovered that bash's configure script checks if strtoimax exists if the symbol is present even if the prototype is missing. This causes bash to use the system strtoimax even if its non-functional, and leads to breakage on old versions of AIX.

This results in legal_number always failing, and any error to use redirects to error out as such:

Tested on bash 5.1.16:
bash-2.05a# ./bash -c "echo test 2> /dev/null"
./bash: file descriptor out of range: A file descriptor does not refer to an open file.

Bash does have a replacement implementation for strtoimax, so this probably is just a matter of changing autoconf macros, and straight forward to fix.