[libcxx-commits] [libcxx] [libc++][hardening] Check bounds on arithmetic in __bounded_iter (PR #78876)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne commented:

I just spoke with @var-const and this approach is growing on me more and more. I was uneasy about relying on "implicit" optimization hints at first, but the more I look at it the more it seems that these can actually become explicit optimization hints.

So let's say we have

```c++
template <random_access_iterator Iterator>
void algorithm(Iterator it, size_t n) {
  (void)it + n;

  // do something real
}
```

Without any hardening, we are basically telling the compiler (quite explicitly) that we expect `it + n` to be valid, and that it can assume that because otherwise we just invoked UB. With hardening, this is still true except that in addition we now validate that the assumption holds. I find this approach to be nice, generic and explicit, and I like it a lot.

I'd like @var-const to give a thumbs up too before this ships, but this LGTM with a few comments.

https://github.com/llvm/llvm-project/pull/78876