C can't do provenance

> C23 explicitly says that intptr_t is optional. If you don't provide it, then there is no line in the standard requiring pointer-to-integer conversions to be possible.

I believe if the implementation provides an integer type large enough to hold a pointer, it must be possible to do such a pointer-to-integer conversion. So even without specifically intptr_t (which, in the history of C is a recent-ish invention anyway as it was introduced only in C99) it can be done, and many C implementation through history have done so.

That being said, I think you're correct in that there's nothing in the standard requiring an implementation to provide such large enough integer types capable of storing a pointer. But that gets into the distinction between the standard and that a lot of C code out there is written under the assumption that such an integer type exists.

Now, CHERI C is a bit special in that they make intptr_t contain the bounds and capability tag, making it possible to do pointer<=>integer roundtrips only with that type. That's probably a good practical compromise between the purity of the capability model, standards conformance, and still allowing roundtripping with a modest porting effort.

> Please link to the proposal you are discussing, Google can't find anything by that name.

It's a typo, I meant PNVI (*sigh*). I think the latest proposal is n3005 at https://open-std.org/JTC1/SC22/WG14/www/docs/n3005.pdf . That link doesn't work for me at the moment but you can find it in the wayback machine.

> As I have repeatedly explained throughout this thread, provenance is not an optimization. It is a hardware constraint. You can't simply turn it off in difficult cases, because the dereference will trap whether the compiler wants it to or not.

Well, for CHERI it's a hardware constraint. But like it or not, non-CHERI hw will be the vast majority for the foreseeable future, and AFAIU there's no plan to make C-with-provenance (if that ever happens) non-implementable on such hardware. For mainstream environments, the practical effect of provenance is to provide compiler writers with guidance on what kinds of optimizations are allowed.