Conditionally switch between C++17 and C++20.
Conditionally switch between C++17 and C++20.

mockingbirdnest/Principia,pleroy/Principia,mockingbirdnest/Principia,pleroy/Principia,pleroy/Principia,mockingbirdnest/Principia,mockingbirdnest/Principia,pleroy/Principia