Synthesis Problem

I am going to make an assumption that this is inside a clocked process (a little bit more info would be helpful).

When you run your for loop, the array index will always follow [i], so the result is:

array[20];

..

array[19];

..

..

array[0];

but with your index as temp, you end up with

temp = 20;

..

temp = 19;

..

..

temp = 0;

In this case, temp is assigned multiple times, therefore temp will get the lower assignment (temp = 0).

so you will end up with:

Array[20] = array[0];

...

Array[19] = array[0];

...

Array[0] = array[0];

If this isn't it, more information on the code would be helpful.