[libcxx-commits] [libcxx] [libc++] Document the strengthened noexcept convention (PR #203869)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

> > > since they don't tell me how the `noexcept` is strengthened.
> > 
> > 
> > It's rarely meaningful to tell how the `noexcept` is strengthened. Exception specifications are almost always strengthened from "nothing" (implicit `noexcept(false)`) to `noexcept` or `noexcept(Cond)`.
> > The only meaningful cases I know are exception specifications relying on `allocator_traits<A>::propagate_on_container_swap::value` in the standard. We seem to be allowed to strengthen them to unconditional `noexcept` (if not, there should certainly be an LWG issue), and MSVC STL is already doing this.
> 
> I don't really know what point you're trying to make? The qualifiers you've added clearly mean that you can't assume there wasn't already something specified in the standard, so I do think it's meaningful to say how we strengthened the `noexcept`.

I'm not against saying how we strengthened the `noexcept` where the standard is already specifying `noexcept(Cond)`, and I think it would be more helpful to say so if the standard specifies `noexcept(Cond)` and we strengthen it to `noexcept(Cond || OtherCond)`.

But such cases seem to be rare, and I'm a bit against saying how we strengthened the `noexcept` elsewhere.

https://github.com/llvm/llvm-project/pull/203869