Safety is not what you think

[Paul Backus]

On Tuesday, 30 January 2024 at 02:05:23 UTC, user1234 wrote:
> I want to share a stupid program to show you that D safety is 
> more complex than you might think:
>
> ```d
> module test;
>
> void test() @safe
> {
>     int i;
>     int b = (*&(*&++i))++;
> }
>
> void main() @safe
> {
>     test();
> }
> ```
>
> I'm not showing a deficiency of D, that program is undeniably 
> safe ;)

I'm surprised `&++i` even compiles in the first place, but 
looking at [the spec][1], it seems to be intentional:

> The following expressions, and no others, are called lvalue 
> expressions or lvalues:
>
> [...]
> 4. the result of the following expressions:
>    * built-in unary operators + (when applied to an lvalue), *, 
> ++ (prefix only), -- (prefix only);

Testing it out, the address you get is the same as `&i`.

This definitely isn't allowed in C or C++. I wonder what the 
rationale is for having this behavior in D?

[1]: https://dlang.org/spec/expression.html