STL: Consider using P1301R4 [[nodiscard("message")]]

[StephanTLavavej]

WG21-P1301 [[nodiscard("message")]] has been implemented for VS 2019 16.5 and has also been implemented by Clang 9.

We should consider using this in the STL. We have a few criteria for marking functions as [[nodiscard]]: pure observer (most common), resource leak, incorrect behavior (e.g. remove_if()). We could have macros for each of these reasons.

[SuperWig]

Just linking #206 as you've given a much more in depth description of the goals of [[nodiscard]] there.

re:

We could have macros for each of these reasons

_NODISCARD is defined in vcruntime.h which is not in this repo so would those just go in yvals_core.h?

[StephanTLavavej]

_NODISCARD is defined in vcruntime.h which is not in this repo so would those just go in yvals_core.h?

Yep!

[StephanTLavavej]

Thinking about this a little more, we could make _NODISCARD (classic) state the "pure observer" reason, since it is overwhelmingly common, then we could have separate macros for other reasons (or maybe just one that takes a custom message). We'd need to audit the 3000+ existing _NODISCARDs to find the special ones.

[AlexGuteniev]

Please check if the approach in #2211 is right. Or maybe we can go by parts and avoid a huge PR?