Allows std::visit on types derived from std::variant.
Other papers in tuple, pair & variant
P1951R1Default Arguments for
pair's Forwarding ConstructorVirtual Jun 2021P2165R4Compatibility between tuple, pair and _tuple-like_ objectsVirtual Jul 2022P2517R1Add a conditional noexcept specification to std::applyVirtual Jul 2022P2637R3Member visitVarna 2023P2819R2Add tuple protocol to complexKona 2023P0472R3Put std::monostate in