To my surprise，the following statment:

if [ "abc" == "def" ]; then echo y; else echo n; fi

doesn't work, it says "zsh: = not found".

Then I tried it in several version of zsh (4.3.x) on different distribution.
All of them doesn't work, except the zsh 4.2.0 on sles9.

What's the problem with it, is there something wrong in my configuration?