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

[Nick Treleaven]

On Friday, 2 January 2026 at 02:00:29 UTC, Timon Gehr wrote:
> a) There does not actually seem to be any explicit 
> documentation in the D spec about pointer subtraction. If there 
> is, I have not found it.

Point 6 here: 
https://dlang.org/spec/expression.html#pointer_arithmetic

> If both operands are pointers, and the operator is -, the 
> pointers are subtracted and the result is divided by the size 
> of the type pointed to by the operands.

It sounds like we need to put in an undefined behaviour note. 
What about *RelExpression* on pointers, UB or not?

> b) In some popular languages, `p-q` is UB if `p` and `q` point 
> to different memory objects.
>
> c) It's hence possible that some D backends give UB to this 
> expression when according to your intention they should not.
>
> d) This scenario is not implausible, I think it already happens 
> for null pointer dereferences that code that the frontend says 
> is `@safe` is treated as UB by some of the backends.

Yes as of May, that requirement is in the spec:
https://dlang.org/spec/function.html#null-dereferences