Safety is not what you think

[Basile B.]

On Monday, 5 February 2024 at 14:26:45 UTC, Basile B. wrote:
> On Tuesday, 30 January 2024 at 15:38:26 UTC, Paul Backus wrote:
>> [...]
>> 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
>
> An hypothesis is that this makes codegening the pre and the 
> post variants almost identical. The only difference is what is 
> yield. 
> [Proof](https://gitlab.com/styx-lang/styx/-/blob/master/src/styx/backend/irgen.sx?ref_type=heads#L3383).
>
> Now there's not much to say about the topic, I just thought it 
> was amusing to share that (user1234 is my noname nickname here) 
> as people might not realize how much certain expressions allows.
>
> In the same vein you have the possibility to select an lvalue 
> with a conditional expression. Pretty sure nobody knows that 
> the following is legal
>
> ```d
> int a,b;
> int c = rand();
> ((c & 3) ? a : b) = 42;
> ```

BTW, "achtung off topic", that's pretty much how the optional 
access can be an lvalue:

```d
struct S {int a};
S* s;     // null, by default init
s?.a = 0; // no access violation !! + valgrind is happy
```

You can lower that to a conditional expression:

```d
struct S {int a};
S* s;
typeof(S.a) __fallback; // compiler-generated
(s ? s.a : __fallback) = 0;
```

I've played a lot with that kind of expressions during the two 
last years. They're funny but apparently not good enough for D ;)

To conclude.