about one feature in V4.2 against V4.1

[douxin]

Hello:
    i am from China and i am working on investigating the difference 
between bash V4.2 and V4.1

    according to official log there is a new feature description

     "Posix mode shells no longer exit if a variable assignment error 
occurs with an assignment preceding a command that is not a special builtin"

    i wrote a script saved as "test.sh"

    readonly a=3
    a=2
    echo "this is incorrect."

    and i wanted to demonstrate the different behavior on bash V4.1 and 
V4.2

    but i tried all below,there was no difference

    1) set -o posix then ran the script
    2) bash --posix test.sh
    3) sh test.sh

    i appreciate if you can tell me what's the problem and how to 
demonstrate the new feature against V4.1

    sorry for my disturbance

    Dou