Should (p - q) be disallowed in @safe code?

[user1234]

On Thursday, 1 January 2026 at 07:27:04 UTC, Richard (Rikki) 
Andrew Cattermole wrote:
> On 01/01/2026 8:19 PM, Walter Bright wrote:
>> On 12/31/2025 10:54 PM, Richard (Rikki) Andrew Cattermole 
>> wrote:
>>> Make it ptrdiff_t not size_t, and I'm happy.
>> 
>> My bad.
>
> I wasn't correcting you, I was saying what I wanted it to do.
>
> ```d
> void func(void* a, void* b) {
> 	ptrdiff_t diff = b - a;
> 	// size_t diff = b - a; ERROR
> 	assert(diff >= 0, "ARGUMENTS BACKWARDS");
> }
> ```

Yes but this has nothing to do with the substraction. You simply 
hit the implicit corecions rules there

```d
ptrdiff_t a;
size_t b;
a = b;
b = a;
```

You need some kind of tracking/dfa/vrp to put restrictions. Same 
remark about the initial question I would say. You can imagine 
some code based on aliasing where `p - q` is finally totally 
fine. however `@safe` is good deal I would say.