This will let us use C++20 features that GCC 9's experimental
C++20 support did not yet cover, such as "concepts".
This reverts some commits:
- 6bcd79b997
- d5ce5329ea
- 728d14879b
Commit 63a911e657 switched
to `-std=c++20` since at the time we required GCC 10 or higher.
Commit d5ce5329ea partially reverted
to `-std=c++2a` since we once again support GCC 9, which did not
yet handle `std=c++20`.
Rangi
ISSOtm
Rangi