[clang] [Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (PR #95474)

[Richard Smith via cfe-commits]

zygoloid wrote:

> > If that mode doesn't already permit using unknown pointers and references in all language modes, then we should change it so that it does. `__builtin_object_size` is best-effort, but we should produce a constant value for it that's not the `-1` or `0` fallback value whenever we're able to do so.
> 
> But there isn't much we can do with unknown pointers or references, is there?
> 
> We can't have `foo` return 4 unless we know for sure that `s` isn't embedded in a larger object. In the following case, wouldn't `foo` be expected to return 12?
> 
> ```
> int bar() {
>   S a[2];
>   return foo(a[0]);
> }
> ```

It depends on the mode parameter in `__builtin_object_size`. For mode 1 and 3, we can use the access path and return 4 if `-fstrict-flex-arrays` is strict enough. For mode 0 or 2, or with lax flexible array rules, I agree that we can't conclude anything and would need to return `0` / `-1`.


https://github.com/llvm/llvm-project/pull/95474