failglob causes the failure of `_variable` in generating `${assoc[index]}` as a completion

[EmilyGraceSeville7cf]

Describe the bug

-S compgen option causes _variables function fail for ${array[index (lines 683-684):

        COMPREPLY+=($(compgen -W '$(printf %s\\n "${!'${BASH_REMATCH[2]}'[@]}")' \
            -P "${BASH_REMATCH[1]}${BASH_REMATCH[2]}[" -S ']}' -- "${BASH_REMATCH[3]}"))

Without -S everything works fine.

To reproduce

We can reproduce this behavior interactively inside Bash without _variables function:

1. declare -A assoc=([aa]=1 [ab]=2)
2. [[ '${assoc[a' =~ ^(\$\{[#\!]?)([A-Za-z0-9_]*)\[([^]]*)$ ]]
3. e $(compgen -W '$(printf %s\\n "${!'${BASH_REMATCH[2]}'[@]}")' -P "${BASH_REMATCH[1]}${BASH_REMATCH[2]}[" -S ']}' -- "${BASH_REMATCH[3]}")
4. See problem

Expected behavior

No error expected even with failglob enabled.

Versions (please complete the following information)

• Operating system name/distribution and version: Linux Mint 20.2 Cinnamon
• bash version, echo "$BASH_VERSION": 5.1.0(1)-release
• bash-completion version, (IFS=.; echo "${BASH_COMPLETION_VERSINFO[*]}"): 2 10

[akinomyoga]

Thanks for the report. This is actually not related to the option -S of compgen but caused by the pathname expansions applied to $(...). We may look at the following reduced test case:

$ bash --norc
$ shopt -s failglob
$ COMPREPLY=($(echo '${assoc[ab]}'))
bash: no match: ${assoc[ab]}