The C keyword 'register' was deprecated in C++11. Now it has been removed in C++17, clang trunk (to be released as clang 6.0.0 in a couple of months) has started warning on its use by default.
Packages
BayesXsrc DatABEL FCNN4R FNN FRESA.CAD KODAMA Matching NPBayesImpute OjaNP RANN RSNNS RSSL SnakeCharmR TAQMNGR TransferEntropy bipartite cems distances divest dpmixsim logconPH ltsk mixcat mombf msr mvPot nabor nonlinearTseries phreeqc reticulate rstpm2 smoothSurv
invoke this warning. Please remove C++ uses of 'register' at the next update.
Several months have now gone by and R-devel now defaults to C++11 if available. Packages
BDR quoth