Difference of extglob between 5.0.0(1)-release and 4.4.23(1)-release

[Peng Yu]

See the following for the difference. I'd consider the behavior of
4.4.23 should be correct.

How was this bug introduced? Should there be a test case to cover this case?

$ cat main_debug.sh
#!/usr/bin/env bash
# vim: set noexpandtab tabstop=2:

echo $BASH_VERSION

declare -- null="@()"
declare -- dir="."

shopt -s extglob
set +f

matched_paths=(./$null"$dir"/main_debug.sh)
declare -p matched_paths
$ ./main_debug.sh
4.4.23(1)-release
declare -a matched_paths=([0]="././main_debug.sh")
$ ./main_debug.sh
5.0.0(1)-release
declare -a matched_paths=([0]="./@()./main_debug.sh")

-- 
Regards,
Peng