I have managed to get cmdstanr with OpenCL up and running without any issues. However, I seem to have run into a problem while installing the development versions of StanHeaders and RStan based on the instructions here. Installing StanHeaders using the following code completes successfully.

```
remotes::install_git("https://github.com/hsbadr/rstan", subdir = "StanHeaders", ref = "develop")
```

However, when I test any of the `stanFunction`

examples (for instance, `stanFunction("log_sum_exp", x = exp(1), y = pi)`

I get the following output and associated error:

```
"C:/rtools40/mingw64/bin/"g++ -std=gnu++14 -I"C:/PROGRA~1/R/R-41~1.0/include" -DNDEBUG -I"C:/Users/ajn0093/R/win-library/4.0/Rcpp/include" -I"C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include" -I"C:/Users/ajn0093/R/win-library/4.0/RcppEigen/include" -I"C:/Users/ajn0093/R/win-library/4.0/BH/include" -I"C:/Users/ajn0093/AppData/Local/Temp/RtmpEv4RmH/sourceCpp-x86_64-w64-mingw32-1.0.6" -DUSE_STANC3 -DSTAN_OPENCL -O2 -Wall -mfpmath=sse -msse2 -mstackrealign -mtune=native -O3 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -c file2e1c85f2557.cpp -o file2e1c85f2557.o
In file included from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/core.hpp:4,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/fun/acos.hpp:4,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/fun.hpp:6,
from file2e1c85f2557.cpp:9:
C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/core/init_threadpool_tbb.hpp:9:10: fatal error: tbb/tbb_stddef.h: No such file or directory
#include <tbb/tbb_stddef.h>
^~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [C:/PROGRA~1/R/R-41~1.0/etc/x64/Makeconf:245: file2e1c85f2557.o] Error 1
Error in sourceCpp(code = code, env = env, rebuild = rebuild, cacheDir = cacheDir, :
Error 1 occurred building shared library.
```

If I try installing the development branch of StanHeaders by instead using `install.packages("StanHeaders", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))`

instead and running an emaple, I receive a similar error.

```
"C:/rtools40/mingw64/bin/"g++ -std=gnu++14 -I"C:/PROGRA~1/R/R-41~1.0/include" -DNDEBUG -I"C:/Users/ajn0093/R/win-library/4.0/Rcpp/include" -I"C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include" -I"C:/Users/ajn0093/R/win-library/4.0/RcppEigen/include" -I"C:/Users/ajn0093/R/win-library/4.0/BH/include" -I"C:/Users/ajn0093/AppData/Local/Temp/RtmpCqn3yD/sourceCpp-x86_64-w64-mingw32-1.0.6" -DUSE_STANC3 -DSTAN_OPENCL -O2 -Wall -mfpmath=sse -msse2 -mstackrealign -mtune=native -O3 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -c file1d1829a9275c.cpp -o file1d1829a9275c.o
In file included from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/core.hpp:4,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/fun/acos.hpp:4,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/fun.hpp:6,
from file1d1829a9275c.cpp:9:
C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim/core/init_threadpool_tbb.hpp:12:10: fatal error: tbb/task_scheduler_init.h: No such file or directory
#include <tbb/task_scheduler_init.h>
^~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [C:/PROGRA~1/R/R-41~1.0/etc/x64/Makeconf:245: file1d1829a9275c.o] Error 1
Error in sourceCpp(code = code, env = env, rebuild = rebuild, cacheDir = cacheDir, :
Error 1 occurred building shared library.
```

Any subsequent attempt at installing the development version of RStan with OpenCL flags fails entirely and throws the following errors in addition to several thousand lines of warnings.

```
"C:/rtools40/mingw64/bin/"g++ -std=gnu++14 -I"C:/PROGRA~1/R/R-41~1.0/include" -DNDEBUG -I"../inst/include" -I"../inst/include/boost_not_in_BH" -I"C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/src" -DBOOST_DISABLE_ASSERTS -DBOOST_PHOENIX_NO_VARIADIC_EXPRESSION -DBOOST_NO_AUTO_PTR -D_REENTRANT -DSTAN_THREADS -DUSE_STANC3 -DSTRICT_R_HEADERS -DRCPP_PARALLEL_USE_TBB=1 -I'C:/Users/ajn0093/R/win-library/4.0/Rcpp/include' -I'C:/Users/ajn0093/R/win-library/4.0/RcppEigen/include' -I'C:/Users/ajn0093/R/win-library/4.0/BH/include' -I'C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include' -I'C:/Users/ajn0093/R/win-library/4.0/RcppParallel/include' -DUSE_STANC3 -DSTAN_OPENCL -DRCPP_PARALLEL_USE_TBB=1 -DSTRICT_R_HEADERS -O2 -Wall -mfpmath=sse -msse2 -mstackrealign -mtune=native -O3 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -c sparse_extractors.cpp -o sparse_extractors.o
In file included from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/opencl/matrix_cl.hpp:8,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/opencl/kernel_generator/operation_cl.hpp:10,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/opencl/kernel_generator.hpp:106,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/opencl/prim.hpp:82,
from C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/prim.hpp:7,
from sparse_extractors.cpp:2:
C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/opencl/opencl_context.hpp:7:2: error: #error OPENCL_DEVICE_ID_NOT_SET
#error OPENCL_DEVICE_ID_NOT_SET
^~~~~
C:/Users/ajn0093/R/win-library/4.0/StanHeaders/include/stan/math/opencl/opencl_context.hpp:10:2: error: #error OPENCL_PLATFORM_ID_NOT_SET
#error OPENCL_PLATFORM_ID_NOT_SET
^~~~~
```

The system I am trying to perform the installation on is running Windows 10 Build 19042 and the output of `writeLines(readLines(file.path(Sys.getenv("HOME"), ".R/Makevars.win")))`

is

```
CXX14FLAGS += -mtune=native -O3 -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2
CPPFLAGS += -DUSE_STANC3 -DSTAN_OPENCL
STAN_OPENCL=true
OPENCL_DEVICE_ID=0
OPENCL_PLATFORM_ID=0
LDFLAGS_OPENCL= -L"${CUDA_PATH}/lib/x64" -lOpenCL
```

Please let me know if any other information is required and I appreciated any assistance with this issue.