Tests of C++20 as the default C++ standard
==========================================+

R last changed its default C++ standard to C++17 in R 4.3.0 ‘where
available’, and it is intended to change this to C++20 ‘where
available’ in R 4.6.0 around April 2026 (and hence slready in R-devel).
Several CRAN packages already require C++20, including V8 which has
hundreds of reverse dependencies.

C++20 is now used for the standard fedora-gcc, fedora-clang and M1mac
package check results, which supersede the logs formerly here.  

To test defaulting to C++20 for your own package(s) add something like

  CXX = g++ -std=gnu++20

to ~/.R/Makevars.  (Copy the local version from R_HOME/etc/Makevars
and change 17 to 20.  For Windows, use ~/.R/Makevars.win.)
Or use current R-devel.

On a Linux box with clang and libc++ installed (see
https://www.stats.ox.ac.uk/pub/bdr/Rconfig/r-devel-linux-x86_64-fedora-clang)
and R built with GCC it should be possible to use something like

  CXX = clang++ -stdlib=libc++ -std=gnu++20

to reproduce the fedora-clang results..

It will continue to be possible to force C++17 for a package by
specifying it (see ‘Writing R Extensions’).  However, this would be
unhelpful for packages which offer headers for LinkingTo as it may
force the other packages to use C++17.  See the details in

https://cran.r-project.org/doc/manuals/r-devel/R-exts.html#Using-C_002b_002b-code


GCC and GNU's libstdc++ are often slow to implement C++ deprecations
and removals so not all the issues will be reproducible there.  This includes

std::not2, which was deprecated in C++17 and removed in C++20.

std::result_of, which was deprecated in C++17 and removed in C++20.

alignof() or arithmetic on pointers to incomplete types.

Things like
call to consteval function 'fmt::fstring' is not a constant expression


OTOH, conflicts such as 'span' with Armadillo headers are only seen 
with libstdc++.   Local definitions of 'span' could be renamed or 
accessed as ::span.

If you do things like

    using namespace arma;
    using namespace std;

replace these by using fully qualified names, at least for 'span'.
Another header conflict has been seen with format: use ::format to
circumvent this. And fmt::format has been superseded by std::format.

For 

warning: implicit capture of 'this' via '[=]' is deprecated in C++20

C++20 requires this to be disambiguated to either [=, this] or [=, *this].