SC2068 and array indices

[scop]

For bugs

• Rule Id (if any, e.g. SC1000): SC2068
• My shellcheck version (shellcheck --version or "online"): online
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on shellcheck.net and verified that this is still a problem on the latest commit

Here's a snippet or screenshot that shows the problem:

#!/bin/bash
a=(a b c)
echo ${!a[@]}

Here's what shellcheck currently says:

Line 3:
echo ${!a[@]}
     ^-- SC2068: Double quote array expansions to avoid re-splitting elements.

Here's what I wanted or expected to see:

${!a[@]} (with the exclamation mark) expands to array indices, so for normal arrays there's no danger of re-splitting. However, if the array is an associative array, then the indices can contain whitespace and the warning is good.

Improving this depends on how deeply shellcheck understands the code:

• if it knows it's dealing with a usual non-associative array, it could omit the warning, and keep emitting it for associative arrays
• if it doesn't know the above, the check can be left as is and the limitation should be documented in the Wiki

(Haven't checked if it's already there, but a unit test would be good to have to ensure desired behavior even if it's the current one.)