R Package using Rcpp and C++20

I made a simple R Package using Rcpp but when I set
SystemRequirements: C++20
on DESCRIPTION I have the following error on build.

clang++ -arch x86_64 -std=gnu++20 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG  -I'/Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library/Rcpp/include' -I/opt/R/x86_64/include    -fPIC  -falign-functions=64 -Wall -g -O2  -UNDEBUG -Wall -pedantic -g -O0 -c RcppExports.cpp -o RcppExports.o
   In file included from RcppExports.cpp:4:
   In file included from /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library/Rcpp/include/Rcpp.h:78:
   In file included from /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library/Rcpp/include/Rcpp/sugar/sugar.h:31:
   In file included from /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library/Rcpp/include/Rcpp/sugar/functions/functions.h:42:
   /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library/Rcpp/include/Rcpp/sugar/functions/sapply.h:36:19: error: no template named 'result_of' in namespace 'std'; did you mean 'traits::result_of'?
           typedef typename ::std::result_of<Function(typename SugarExpression::stored_type)>::type type;
                            ^~~~~~~~~~~~~~~~
                            traits::result_of
   /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library/Rcpp/include/Rcpp/traits/result_of.h:30:8: note: 'traits::result_of' declared here
   struct result_of{
          ^
   1 error generated.
   make: *** [RcppExports.o] Error 1
   ERROR: compilation failed for package

Is it possible to build the package with C++20?

The last Rcpp release (version 1.0.10) happend in January. But the code necessary to support C++20 with clang++ was added in February, c.f. https://github.com/RcppCore/Rcpp/pull/1248. You can get intermediate releases from the projects drat repository:

install.packages("Rcpp", repos="https://RcppCore.github.io/drat")

Untested, since you have not provided a minimal example.

It all depends a little ... on what your compiler is. For me (in Ubuntu 22.10) the following works and demonstrates the new three-way comparison walrus operator (more here) (and I indent what is one long code line for display here)

> Rcpp::cppFunction("int threeway(int a, int b) { auto res = (a <=> b); 
     if (res < 0) return -1; else if (res > 0) return 1; else return 0; }", 
     plugin="cpp20")
> threeway(-2, 2)
[1] -1
> threeway(2, -2)
[1] 1
> threeway(2, 2)
[1] 0
> 

Now, this uses the plugin which is not needed on a package where you either use SystemRequirements: in DESCRIPTION or use CXX_STD in src/Makevars as we have for years. But that part is only about R dispatching the required flag to the compiler. What the compiler then can do with the code depends on ... what version of which compiler you have.

Now, as @Ralf correctly pointed out, the Rcpp version at CRAN has an issue with clang++ and a change from C++17 to C++20 so you need the release candidate from GitHub or the drat repo. An updated version will get to CRAN next month.