On Wed, 28 Jun 2023 at 23:37, Frederick Virchanza Gotham via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
A few months ago I was thinking of submitting this to Boost, but now
I'm actually thinking maybe it should be in the Standard Library.

It won't be approved by the Parallelism & Concurrency study group, for the reasons already explained previously.

Maybe you should propose it for Boost though. The review might be useful.


#if (__cplusplus >= 201700L) || (defined(__has_cpp_attribute) &&

This is not how you test for attributes. For a start, you're assuming that if __cplusplus >= 201703 is true then the attribute is supported, which is not correct. The point of __has_cpp_attribute is to check if the attribute is supported, so that's what you should use. More importantly, if __has_cpp_attribute is not defined then this line will give an error. Using an undefined function-like macro is ill-formed.

You need to do:
#ifdef __has_cpp_attribute
# if __has_cpp_attribute(nodiscard)
#  define attrib_nodiscard [[nodiscard]]
# endif
#ifndef attrib_nodiscard
# define attrib_nodiscard [[nodiscard]]

#    define attrib_nodiscard [[nodiscard]]
#    define attrib_nodiscard /* nothing */

#if (__cplusplus >= 201907L) && defined(__cpp_concepts)

This is not how you use feature test macros. You're supposed to test the value of the macro itself, not __cplusplus, so:

#if __cpp_concepts >= 201907L

Your version would give the wrong answer for a compiler with partial C++20 support, that only supports an older implementation of concepts.