Seems like its p3125r0

Thanks! https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p31...

(is the current version of that paper, the tracking ticket insisted there's a P3125R5 and that LEWG had seen it in 2025, but it isn't listed in a mailing so it might be a mirage)

You know it's a Hana paper because it wants this to be allowed at compile time (C++ constrexpr) but joking aside this seems like a nice approach for C++ which stays agnostic about future implementation details.