foo[0]=a
foo[1]=b

[[ -v foo[1] ]] && echo foo index 1: ok
[[ -v foo[2] ]] || echo foo index 2: ok

[[ -v foo ]] && echo foo: implicit reference to element 0: ok

typeset -i foo

[[ -v foo[1] ]] && echo foo: arithmetic evaluation: ok

typeset -a bar
bar[1]=set

[[ -v bar ]] || echo bar: array with element zero unset: ok
[[ -v bar[0] ]] || echo bar: element zero: ok

typeset -a qux
[[ -v qux ]] || echo qux: unset array: ok
[[ -v qux[0] ]] || echo qux: unset array element 0: ok