C++14 standard requested but CXX14 is not defined

Looks like CXX14 might need an additional flag. Can you try running:

cat(c("\n CXX14FLAGS += -mtune=native -O3 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2",
      "CXX14 = $(BINPREF)g++ -std=c++1y"),
    file = "~/.R/Makevars.win", sep = "\n", append = FALSE)

Then restarting R and trying the RStan install again?

Hi. I was able to get stan to load and the example to run. For your notes, I got a warning message: In system(paste(CXX, ARGS), ignore.stdout = TRUE, ignore.stderr = TRUE) : ‘C:/Rtools/mingw_/bin/g++’ not found.

Really appreciate the help and quick support.

Is there a updated link for now? I have the same issue but the link does not work anymore.