cmdstanr compiles a model just fine for me on Windows when I run the sample code in the OpenCL vignette as long as I do not try to compile for OpenCL. So, after saving the example model as indicated in the vignette, this compiles just fine:

```
library(cmdstanr)
check_cmdstan_toolchain(fix = TRUE)
# Generate some fake data
n <- 250000
k <- 20
X <- matrix(rnorm(n * k), ncol = k)
y <- rbinom(n, size = 1, prob = plogis(3 * X[,1] - 2 * X[,2] + 1))
mdata <- list(k = k, n = n, y = y, X = X)
# Compile the model without STAN_OPENCL=TRUE
mod_cl <- cmdstan_model("bernoulli_logit_glm.stan"
# ,
# cpp_options = list(stan_opencl = TRUE)
)
```

On the other hand, this fails with the following error information:

```
> library(cmdstanr)
>
> check_cmdstan_toolchain(fix = TRUE)
The C++ toolchain required for CmdStan is setup properly!
>
> # Generate some fake data
> n <- 250000
> k <- 20
> X <- matrix(rnorm(n * k), ncol = k)
> y <- rbinom(n, size = 1, prob = plogis(3 * X[,1] - 2 * X[,2] + 1))
> mdata <- list(k = k, n = n, y = y, X = X)
>
> # Compile the model with STAN_OPENCL=TRUE
> mod_cl <- cmdstan_model("bernoulli_logit_glm.stan"
+ ,
+ cpp_options = list(stan_opencl = TRUE)
+ )
Compiling Stan program...
In file included from stan/lib/stan_math/stan/math/opencl/prim.hpp:86,
from stan/lib/stan_math/stan/math/prim.hpp:7,
from stan/src/stan/io/dump.hpp:7,
from src/cmdstan/command.hpp:24,
from src/cmdstan/main.cpp:1:
stan/lib/stan_math/stan/math/opencl/scalar_type.hpp:14:8: error: partial specialization of 'struct stan::scalar_type<T, typename std::enable_if<stan::math::conjunction<stan::is_kernel_expression_and_not_scalar<T, void> >::value, void>::type>' after instantiation of 'struct stan::scalar_type<stan::math::constant_<int>, void>' [-fpermissive]
struct scalar_type<T, require_all_kernel_expressions_and_none_scalar_t<T>> {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mingw32-make.exe: *** [make/program:14: src/cmdstan/main_opencl.o] Error 1
Error: An error occured during compilation! See the message above for more information.
```

This looks somewhat similar to the partial specialization error that was being discussed in the Linux context elsewhere on this board, but this is on Windows and seems a bit different.

Would appreciate any suggestions.

- Operating System: Windows 10
- CmdStan Version: v2.26.1
- cmdstanr Version: 0.4.0.9000
- R version: Microsoft 4.0.2 Open
- Compiler/Toolkit: mingw32 (I think, whatever RTools 4.0 installs)