On Fri, 30 Aug 2019 at 00:56, Aaron Ballman <aaron_at_[hidden]> wrote:

> On Thu, Aug 29, 2019 at 7:49 PM Barry Revzin <barry.revzin_at_[hidden]>
> wrote:
> >
> > We had two changes to [[nodiscard]] in Cologne:
> >
> > - P1301R4: [[nodiscard("should have a reason")]], for C++20
> > - P1771R1: [[nodiscard]] for constructors, as a DR
> >
> > Currently, I put both under the 201907 block (
> https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#nodiscard).
> But that means we have a [retroactive] C++17 feature whose macro has a
> value two years later. Is that fine? Alternatively, we could invent an
> earlier value for P1771R1 that's in the C++17 timeline.
> >
> > Thoughts?
>
> I raised this issue in Core and my understanding of the room was that
> this is not a serious problem for implementations. Implementations are
> expected to use the new value once they've implemented both features.
>
>
But what's the expected usage?

If I want to add a reason to my nodiscard attributes, I can do:

#if __cplusplus > 201703L && __has_cpp_attribute(nodiscard) >= 201907
# define NODISCARD(msg) [[nodiscard(#msg)]]
#elif __has_cpp_attribute(nodiscard)
# define NODISCARD(msg) [[nodiscard]]
#else
# define NODISCARD(msg)
#endif

This will not try to use a reason in C++17 code, even if the compiler
supports doing so as an extension.

If we had a separate feature test macro for P1301R4 then I could just test
for that, and be able to give a reason whenever the compiler supports it
(even in C++11/14/17 code):

#if __cpp_nodiscard_reason >= 201907
# define NODISCARD(msg) [[nodiscard(#msg)]]
#elif __has_cpp_attribute(nodiscard)
# define NODISCARD(msg) [[nodiscard]]
#else
# define NODISCARD(msg)
#endif

So I think it's OK for the __has_cpp_attribute(nodiscard) value to mean
that both new features are supported, but *also* adding a
__cpp_nodiscard_reason macro might make things easier for users. This
assumes that some implementations might choose to allow a nodiscard reason
pre-C++20.