# this captures how bash and ksh93 expand indexed array subscripts in
# various contexts. if changes are ever made, or an option added to do
# this differently, the diffs will show up here

typeset -a array

index='$( echo >&2 foo )' # Literal shell code should never be evaluated unless an 'eval' is involved.

echo ${array[ $index ]} # [] expands $index, results in a literal that [] does not re-evaluate.
echo $(( $index )) # (( )) expands $index, results in a literal that (( )) does not re-evaluate.
echo $(( array[ $index ] )) # (( )) expands $index, results in a literal that  [] DOES re-evaluate.

(( array[ $index ] ))

typeset -a a

: $(( a[$index]=5 ))

#shopt -s assoc_expand_once
echo $((1+a[$index]))
echo $((1+a[\$index]))
echo "1+${a[$index]}"

# intermediate problems discovered while bash-5.0 was in testing
a=0
echo $(( a[a[0]] ))
echo ${a[a[a[0]]]}
echo $(( a[a[a[0]]] ))