CC-ing @ben-craig.

(But we may need to reopen these issues and #2914 in future once freestanding mode is added. See #1289.)

You may want to drop a static assert somewhere checking __STDC_HOSTED__ so that these issues are noticed at that time.

It seems to me that hosted implementations are required to provide all of these macros (see "Rejected alternatives" in WG21-P2198R7). I think it makes more sense to define them in older modes when the corresponding feature/header is available.

The standard doesn't really talk about older modes, and I didn't do anything to audit which modes / standards would make sense. Sure, all the stuff in C++03 from the <utility> header is freestanding capable, but we've added stuff to <utility> since then. That could make backporting __cpp_lib_freestanding_utility weird.

Sure, all the stuff in C++03 from the <utility> header is freestanding capable, but we've added stuff to <utility> since then. That could make backporting __cpp_lib_freestanding_utility weird.

The value 202306L may be misleading for some macros, but pehaps fine for others.

I don't actually know what __cpp_lib_freestanding_feature_test_macros intentionally covers.

From the paper:

In order for users of freestanding implementations to be able to detect extensions of freestanding, the users need a way of distinguishing the C++20 macro requirements from an accurate expression of extension. This paper will provide a new macro, __cpp_lib_freestanding_feature_test_macros, so that users can distinguish between these cases. When __cpp_lib_freestanding_feature_test_macros is present, users will know that the C++20 feature-test macros aren't lying.

Example...

#if (__STDC_HOSTED__ || \
     (defined(__cpp_lib_freestanding_feature_test_macros) && \
      __cpp_lib_freestanding_feature_test_macros >= 202112))
#define TRUTHFUL_MACROS 1
#else
#define TRUTHFUL_MACROS 0
#endif

// see if filesystem happens to be supported on this possibly freestanding environment
#if TRUTHFUL_MACROS && defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L
// hosted and freestanding extension success path
#else
// fallback path
#endif

So that macro should only be important for actual freestanding implementations.

Because these feature-test macros are essentially defined unconditionally, we can accept this as a special exception to our "no C++26 PRs yet" policy.

Thanks! After fixing a copy-paste typo, looks perfect! 😻

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

Thanks for implementing all of these feature-test macros! 🎉 ⚙️ 🚀