This used to work:
bash-4.4$ a=0
bash-4.4$ echo $(( a[a[0]] ))
0
bash-4.4$ echo ${a[a[a[0]]]}
0
But is broken in bash-5.0:
bash-5.0$ a=0
bash-5.0$ echo ${a[a[a[0]]]}
bash: a[a[0]]: syntax error: invalid arithmetic operator (error token is "]")
bash-5.0$ echo $(( a[a[0]] ))
bash: a[a[0]] : syntax error: invalid arithmetic operator (error token is "] ")
