Bug#1021673: Wrong __cplusplus definition when using -std=c++20

[Marco Trevisan <marco@trevi.me>]

Package: g++
Version: 4:10.2.1-1

In bullseye:

$ echo | g++ -dM -E -x c++ -std=c++20 - |grep _cplusplus
#define __cplusplus 201709L

It should instead be:

#define __cplusplus 202002L