Unexpected behavior when casting away immutable

[Mafi via Digitalmars-d-learn]

On Wednesday, 23 September 2015 at 05:24:05 UTC, John Colvin 
wrote:
> On Wednesday, 23 September 2015 at 03:39:02 UTC, Mike Parker 
> wrote:
...
>>
>> ```
>> immutable int x = 10;
>> int* px = cast(int*)&x;
>> *px = 9;
>> writeln(x);
>> ```
>>
>> It prints 10, where I expected 9. This is on Windows. I'm 
>> curious if anyone knows why it happens.
>
> violating immutable is undefined behaviour, so the compiler is 
> technically speaking free to assume it never happens. At the 
> very least, neither snippet's result is guaranteed to show a 
> change or not. At the most, literally anything can happen.

In essence, this code snippet is even better than the OP expected 
in showing why you shouldn't cast away immutable.