bash -declare documentation

Question

If you declare an array like this

declare -a test

you can echo the value like this

i=2
echo ${test[i]}

or

i="1+1" 
echo ${test[i]}

why the second form is accepted? i need a complex answer thanks

Answer 1

See man bash:

The subscript is treated as an arithmetic expression that must evaluate to a number.

Complex enough?