Rcpp does distribute a C++14 plugin, usable with:
// [[Rcpp::plugins(cpp14)]]
Does that work in your case? If not, it's possible that RStudio needs to do some extra work to handle C++14 (and above) compilation units.
The version of libclang used by RStudio is getting somewhat old and may not understand all of the constructs used in C++14 -- we hope to upgrade it in a future release.