cast const pointer to non-const and change value yields neither result nor error

[Timoses]

Hey,

reading through https://dlang.org/articles/const-faq.html and 
experimenting a bit:

```
     immutable int i = 3;
     const(int)* p = &i;

     int* q = cast(int*)p;

     assert(q == p && p == &i);

     writeln(i); // 3
     *q = 1;     // Why does this have no effect at all? No error 
no nothing?!
     writeln(i); // 3
```

When changing i to non-immutable the `*q=1` sets i to 1.

There is no error message. The `*q=1` simply has no effect at 
all. Also with `const int i`.

Is that intended?