CTFE static array error: cannot modify read-only constant

[Johan via Digitalmars-d-learn]

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