P0722R3 (wg21.link/p0722r3, just voted into the standard) does not specify a feature test macro, but I think it would benefit from one. However, it's not completely clear how we should arrange this: it needs both compiler support and library support, and is unusable without both.

Should we add two feature test macros for it (one for compiler, one for library)? Should we recommend that the library macro be defined only if the language macro is defined, so that users need only check one, or should we keep them separate, to allow the library functionality to be discovered despite the language functionality being absent? (In the latter case, a library could be built using an old compiler and a new library, and provide functionality to clients that are built using a new compiler and a new library.)

Where should such discussions occur these days (now that the feature test macros have been merged into the standard)? I'm assuming this is still the right place.