Consequences of casting away immutable from pointers

[Patrick Schluter]

On Friday, 5 January 2018 at 18:13:11 UTC, H. S. Teoh wrote:
> On Fri, Jan 05, 2018 at 05:50:34PM +0000, jmh530 via 
> Digitalmars-d-learn wrote:
>
> Be careful with that:
>
> 	class C { int x; }
> 	immutable C c = new C(5);
> 	auto i = c.x;
>
> 	C y = cast(C) c;
> 	y.x = 10;
> 	i = c.x; // <-- compiler may assume c.x is still 5
>
> Since c.x is read from an immutable object, the compiler may 
> assume that its value hasn't changed the second time you access 
> it, so it may just elide the second assignment to i completely, 
> thereby introducing a bug into the code.
>
> Basically, casting away immutable is UB, and playing with UB is 
> playing with fire. :-P
>

And these things are nasty. We had one in our C project last 
month that had us tear our hair out. It was in the end a 
documentation problem of gcc that induced  the misunderstanding 
of the purpose of __attribut__((malloc)) and its effect on 
aliased pointer.