R stan 8schools example faile - r

Question

I am trying to run classical r-stan example from this official tutorial. But I can't because I get the following error message:
The code I use is as follows:
schools_dat <- list(J = 8,
y = c(28, 8, -3, 7, -1, 1, 18, 12),
sigma = c(15, 10, 16, 11, 9, 11, 10, 18))
fit <- stan(file = '8schools.stan', data = schools_dat,
iter = 1000, chains = 4)
Where 8schools.stan is file I have additionally created and nested with the following code:
data {
int<lower=0> J; // number of schools
real y[J]; // estimated treatment effects
real<lower=0> sigma[J]; // s.e. of effect estimates
}
parameters {
real mu;
real<lower=0> tau;
real eta[J];
}
transformed parameters {
real theta[J];
for (j in 1:J)
theta[j] = mu + tau * eta[j];
}
model {
target += normal_lpdf(eta | 0, 1);
target += normal_lpdf(y | theta, sigma);
}
I want to figure out the reasons of this error and how to avoid it.
Additionally I want mention that:
The following code works propertly and returns 10:
fx <- inline::cxxfunction( signature(x = "integer", y = "numeric" ) , '
return ScalarReal( INTEGER(x)[0] * REAL(y)[0] ) ;
' )
fx( 2L, 5 )
I have installed latest frozen version of Rtools.
Also, Rtools, R and Rstudio are located in folders without spaces.
I work on Windows (not laptop)
I have been strictly following all the steps from installiation guide.
Will be very greatfull for help!
Update
I also have set Sys.setenv(USE_CXX14=1) as Ben Goodrich has suggested but now I get the following error message

Answer 1

This is a temporary issue due to the fact that (this week) the StanHeaders R package (which requires C++14) has been accepted on CRAN but not yet the rstan R package (which uses C++14 to compile the models). So, for the time being, you first need to call
Sys.setenv(USE_CXX14=1)
But anyone reading this post much in the future is unlikely to need to do that explicitly (the rstan R package will do it internally).
On Windows you need to put
CXX14=g++ -std=c++1y
CXX14FLAGS=-O3
into the ~/.R/Makevars file but on a Mac, those lines should already be there.