first few C++26 core language features have been voted into the C++ Working Draft at the Varna meeting:
- P2738R1 constexpr cast from void*: towards constexpr type-erasure
- P2741R3 User-generated static_assert messages
- P2169R4 A nice placeholder with no name
I didn’t realize it was still in the plan. The paper you linked is from September, 2022. My understanding was that it was proposed for inclusion in C++26 but that the committee never adopted it. I very much hope I’m wrong, though.