Immutability and arrays

[rumbu]

I am trying to understand why in this two different cases (Simple 
and Complex), the compiler behaviour is different.

```d
struct SimpleStruct { int x;}
struct ComplexStruct { int[] x; }

void main()
{
     SimpleStruct[] buf1;
     immutable(SimpleStruct)[] ibuf1;
     buf1[0 .. 10] = ibuf1[0 .. 10];
     //this works

     ComplexStruct[] buf2;
     immutable(ComplexStruct)[] ibuf2;

     buf2[0 .. 10] = ibuf2[0 .. 10];
     //error cannot implicitly convert expression `ibuf2[0..10]` 
of type `immutable(ComplexStruct)[]` to `ComplexStruct[]`
}
```