On Mon, Nov 25, 2019 at 3:43 PM Ville Voutilainen <ville.voutilainen@gmail.com> wrote:
On Tue, 26 Nov 2019 at 00:59, Casey Carter <cartec69@gmail.com> wrote:
> Shipping multiple feature-test macros for the same feature is trivially simple for implementors. Requiring users to check multiple feature-test macros to detect a single feature - the detection needs to work both in old and new implementations - seems hostile.

Fully agreed, but I don't see anyone suggesting such hostilities. You
can continue shipping the old spellings,
and we all can, and we can still do the policy-fixes in the standard,
and no user will be hurt the slightest.
That's what I mean by not seeing a convincing argument to deviate from
the policy; renaming
__cpp_lib_array_constexpr doesn't in any way prevent you from
continuing to ship that macro spelling.

Say an implementation has shipped __cpp_lib_meow, and WG21 chooses to rename __cpp_lib_meow to __cpp_lib_woof. The implementation continues to ship __cpp_lib_meow, and adds __cpp_lib_woof. This takes five minutes of work, the implementor is not bothered.

Users want to use this feature everywhere it's available. Since there are implementations in the wild that only define __cpp_lib_meow, and the standard only requires implementations to define __cpp_lib_woof, such users must check both __cpp_lib_meow and __cpp_lib_woof.

The harm isn't to the implementations, it's to the users that want portability and the already-shipped third-party code that uses the old name.