`#` instead of `\\` for comment is fatal in RStan 2.26

Summary:

Using # instead of \\ to begin a comment in RStan 2.26 resulted in fatal mysterious parser failed badly error.

Description:

I encountered this error in RStan 2.26. I tried to update to see if it persists in a more recent version by removing RStan and reinstalling with remotes::install_github("stan-dev/rstan", subdir = "rstan/rstan"); unfortunately this fails to install (with nothing more informative than clang: error: linker command failed with exit code 1, which I don't know how to solve). So I'm now back on RStan 2.21.0 from CRAN, for which the problem is not fatal and gives the desired Info: Comments beginning with # are deprecated. Please use // in place of # for line comments.

Reproducible Steps:

Before I downgraded from RStan 2.26, this Stan program gave the error

data {
  int<lower = 1> N;
  vector[N] y;
}

parameters {
  real mu;
  real<lower = 0> sigma;
}

# test comment
model {
  y ~ normal(mu, sigma);
}

Expected Output:

I would expect Info: Comments beginning with # are deprecated. Please use // in place of # for line comments on any version of RStan (or at least an explanation of the problem even if it's fatal).

RStan Version:

2.26 (point-something-else which I can no longer remember)

R Version:

4.2.1

Operating System:

macOS Ventura Version 13.3.1 (a)

Start by reproducing the supplied Stan program with RStan 2.26, then compare its behavior with RStan 2.21.0. Investigate the parser handling of the # comment and determine whether it can report the deprecation message instead of failing; done means the behavior is covered by a regression test and no longer produces the fatal parser error.