CTFE static array error: cannot modify read-only constant

[Johan]

Hi all,
```
   auto foo(const int[3] x)
   {
       int[3] y = x;
       y[0] = 1; // line 4
       return y;
   }
   immutable int[3] a = [0,1,2];
   immutable int[3] b = foo(a); // line 8
```
compiles with an error:
```
4: Error: cannot modify read-only constant [0, 1, 2]
8:        called from here: foo(a)
```

What am I doing wrong?

Thanks,
   Johan