C++17 in a package with Rcpp - r

Question

#include <cmath>
#include <math.h>
#include <Rcpp.h>
// [[Rcpp::plugins("cpp17")]]
// [[Rcpp::export]]
double rcpp_hello_world(Rcpp::NumericVector x) {
double z;
z = std::cyl_bessel_i(0, x[0]);
return z ;
}
When I run the above code and call it using sourceCpp, it works as expected. However inside a R package setup I get
error: cyl_bessel_iis not a member of ‘std
11 | z = std::cyl_bessel_i[0]);
| ^~~~~~~~~~~~
make: *** [/usr/lib64/R/etc/Makeconf:177: rcpp_hello_world.o] Error 1
ERROR: compilation failed for package ‘Package’
upon running Rcpp::compileAttributes() and devtools::document(). I initialised the package with Rcpp.package.skeleton.
cyl_bessel_i was added to the standard library in C++17.
I am using gcc-12.1.0-2 and R-4.2.0-3. I am on Arch Linux.

Answer 1

The default C++ standard used in current versions of R is C++11 (or C++14 if available). Since you require C++17, you need to declare it in the src/Makevars file, using the line
CXX_STD=CXX17
In a comment you referred to the Rcpp documentation which says that Makevars is optional since Rcpp 0.11.0. I think that was written about linking to the Rcpp libs, but it is also true here, as pointed out by #MikkoMarttila: you can alternatively declare the C++ version in the SystemRequirements: field of the DESCRIPTION file, e.g.
SystemRequirements: C++17