Bizarre behavior of stanc_builder with include files

Summary:

If I #include ssm.stan and the isystem path does not exist, stanc_builder hangs indefinitely using up 100% CPU.

Description:

If I run

stanc_builder("foo.stan", isystem="/nonexistent/path/")

then, instead of getting an error, stanc_builder hangs indefinitely using 100% CPU.

Here is the file foo.stan, renamed so that github will let me upload it:
foo.stan.txt

Now for the bizarre part: If I change the line

#include ssm.stan

in foo.stan to use any other file name, I get the expected error message instead of it hanging.

This problem does not occur with stanc, but instead I get a different problem: even if the include path is correct and the include file exists, I get the error message

could not find include file: #include ssm.stan
Error in stanc("foo.stan", isystem = "/nonexistent/path/") :
  failed to parse Stan model 'foo' due to the above error.

I get this error regardless of the name of the include file.

(BTW, I can find nothing in the documentation page for stanc/stanc_builder that tells me how these two functions differ, other than having different defaults, and stanc having some additional possibilities for specifying the model code.)

RStan Version:

2.17.3

R Version:

R version 3.4.3 (2017-11-30)

Operating System:

Mac OS X 10.13.3

Reproduce the report with foo.stan, the #include ssm.stan line, and stanc_builder("foo.stan", isystem="/nonexistent/path/"); compare it with stanc and with another include filename. Trace include-path handling from the stanc_builder and stanc entry points, then verify that missing paths produce an error without hanging and that valid include paths still work.